Chandra X-ray Observatory
Chandra, previously known as the AXAF (Advanced X-ray Astrophysics Facility), is a Flagship-class space observatory of NASA. In 1976, the mission was proposed to NASA by Riccardo Giacconi and Harvey Tananbaum. Preliminary work began the following year at MSFC (Marshall Space Flight Center), Huntsville, AL and SAO (Smithsonian Astrophysical Observatory), Cambridge, MA. 1)
Prior to launch in 1999, NASA's AXAF spacecraft mission was renamed to 'Chandra X-ray Observatory' in honor of the late Indian-American Nobel laureate, Subrahmanyan Chandrasekhar. Chandrasekhar, known to the world as Chandra, which means ``moon" or ``luminous" in Sanskrit, was a popular entry in a recent NASA contest to name the spacecraft. The contest drew more than six thousand entries from fifty states and sixty-one countries. The co-winners were a tenth grade student in Laclede, Idaho, and a high school teacher in Camarillo, CA. "Chandrasekhar made fundamental contributions to the theory of black holes and other phenomena that the Chandra X-ray Observatory will study. His life and work exemplify the excellence that we can hope to achieve with this great observatory," said NASA Administrator Dan Goldin. Widely regarded as one of the foremost astrophysicists of the 20th century, Chandrasekhar won the Nobel Prize in 1983 for his theoretical studies of physical processes important to the structure and evolution of stars. He and his wife immigrated from India to the U.S. in 1935. Chandrasekhar served on the faculty of the University of Chicago until his death in 1995.
The CXO (Chandra X-ray Observatory) is part of NASA's fleet of "Great Observatories" along with the Hubble Space Telescope, the Spitzer Space Telescope and the now deorbited Compton Gamma Ray Observatory. Chandra allows scientists from around the world to obtain X-ray images of exotic environments to help understand the structure and evolution of the universe. The CXO, which was launched by Space Shuttle Columbia in 1999, can better define the hot, turbulent regions of space. This increased clarity can help scientists answer fundamental questions about the origin, evolution, and destiny of the universe. 2)
NASA's Chandra X-ray Observatory is a telescope specially designed to detect X-ray emission from very hot regions of the Universe such as exploded stars, clusters of galaxies, and matter around black holes. Because X-rays are absorbed by Earth's atmosphere, Chandra must orbit above it, up to an altitude of 139,000 km (86,500 mi, apogee) in space. The Smithsonian's Astrophysical Observatory in Cambridge, MA, hosts the Chandra X-ray Center (CXC) which operates the satellite, processes the data, and distributes it to scientists around the world for analysis. The Center maintains an extensive public web site about the science results and an education program. 3) 4)
The Chandra X-Ray Observatory provides information on the nature of objects ranging from comets in our Solar System to quasars at the edge of the observable universe. Some of the major questions addressed by Chandra are: 5)
• What and where is the "dark matter" in our universe? The largest and most massive objects in the universe are galaxy clusters — enormous collections of galaxies that include some like our own. These galaxies are bound together into a cluster by gravity. Much of the mass in the cluster is in the form of an incredibly hot, X-ray emitting gas that fills the entire space between the galaxies. Yet, neither the mass of the galaxies, nor the mass of the hot X-ray gas is enough to provide the gravity that we know holds the cluster together. Additional mass, due to a mysterious substance called dark matter, is required. X-ray observations with Chandra are mapping the location of the dark matter and helping us to identify it.
• What is the powerhouse driving the explosive activity in many distant galaxies? The centers of many distant galaxies are incredible sources of energy and radiation — especially X-rays. Scientists theorize that massive black holes are at the center of these active galaxies, gobbling up any material — even whole stars — that pass too closely. Detailed studies with Chandra are probing the faintest of these active galaxies. The research shows not only how their energy output changes with time, but also how these objects produce their intense energy emissions in the first place.
• Does the Universe contain "dark energy" and if so, how important is it? Because galaxy clusters are the largest bound structures in the Universe, they likely represent a fair sample of the matter content in the Universe. If so, the ratio of dark matter to hot gas would be the same for every cluster. Scientists used this assumption and Chandra data from galaxy clusters to show that the rate of expansion of the Universe began to accelerate about six billion years ago. This is an important confirmation of results from optical observations of supernovae in distant galaxies. Many scientists attribute the driving force behind cosmic acceleration to "dark energy," which would make it the dominant form of energy in the Universe. Although there is no accepted explanation for dark energy, most astrophysicists think that dark energy may be intimately connected with the nature of space-time itself.
The CXO (Chandra X-ray Observatory) program is managed by the Marshall Center for the Science Mission Directorate, NASA Headquarters, Washington, D.C. Northrop Grumman of Redondo Beach, Calif., formerly TRW, Inc., was the prime development contractor that assembled and tested the observatory for NASA. 6)
The Chandra X-ray Observatory has three major elements:
1) the spacecraft system,
2) the telescope system containing the X-ray optics, two X-ray transmission gratings1 that can be inserted into the X-ray path, and a 10 m long optical bench and
3) the science instruments. The SIM (Science Instrument Module) holding two focal-plane cameras, ACIS (Advanced CCD Imaging Spectrometer) and HRC (High Resolution Camera).
The spacecraft module contains computers, communication antennas and data recorders to transmit and receive information between the observatory and ground stations. The onboard computers and sensors, with ground-based control center assistance, command and control the vehicle and monitor its health.
The spacecraft module also provides reaction wheels to aim the entire observatory, a set of small momentum unloading system thrusters to control momentum buildup, an aspect camera that tells the observatory its position relative to the stars, and a Sun sensor that protects it from excessive light. Electrical power is provided by solar arrays that also charge three nickel-hydrogen batteries that provide backup power.
Figure 1: The Chandra X-ray Observatory is the world's most powerful X-ray telescope. It has eight-times greater resolution and is able to detect sources more than 20-times fainter than any previous X-ray telescope (image credit: NASA/CXC & J. Vaughan)
At the heart of the telescope system is the HRMA (High Resolution Mirror Assembly). Since high-energy X-rays would penetrate a normal mirror, special cylindrical mirrors were created. The two sets of four nested mirrors resemble tubes within tubes. Incoming X-rays graze off the highly polished mirror surfaces and are funneled to the instrument section for detection and study. 7)
The mirrors of the Chandra X-ray Observatory are the largest of their kind and the smoothest ever created. If the surface of the state of Colorado were as relatively smooth, Pike's Peak would be less than 1 inch tall. The largest of the eight mirrors is almost 4 feet in diameter and 3 feet long. Assembled, the mirror group has a mass of >1 ton.
The optical design is based on the principal that X-rays reflect efficiently only if the grazing angle between the incident ray and the reflecting surface is less than the critical angle. This angle, typically of order 0.017 radians (one degree), is approximately 10-2 (2ρ)1/2 /E, where ρ is the density of the reflecting material in g-cm-3 and E is the photon energy in keV. The X-ray optical elements for Chandra resemble shallow angle cones, and two reflections are required to provide good imaging over a useful field of view; the first reflecting surface is a paraboloid of revolution and the second a hyperboloid — the Wolter 1 configuration. The collecting area is increased by nesting concentric mirror pairs, all having the same focal plane.
Figure 2: Schematic of grazing incidence, X-ray telescope. This cross section through four nested pairs of mirrors illustrates the principle of grazing incidence reflection and focusing of X-rays. Two reflections are required to make an image. The grazing angles range from about 3.5 degrees for the outer pair to about 2 degrees for the inner pair (image credit: NASA/CXC, S. Lee)
The optical elements have four paraboloid-hyperboloid pairs with a common 10 m focal length. The element lengths are about 0.83 m, the diameters approximately 0.63, 0.85, 0.97, and 1.2 m, and wall thickness range from 16 mm for the smaller elements to 24 mm for the outer ones. Zerodur from Schott is the optical element material chosen because of its low coefficient of thermal expansion and demonstrated capability of permitting very smooth polished surfaces.
Fabrication: The major fabrication phases included coarse and fine grinding, polishing, and final smoothing. The grinding and polishing operations were done with relatively small tools under computer control. Cycles were iterative; a mirror element would be measured to yield an error map, appropriate tools selected to reduce the errors, and a polishing control file for the next cycle generated. The next cycle would cause more material removal in the high areas. The residual errors would be smaller than previously, and so the process converged to the required accuracies.
Coating: The mirror elements were coated at Optical Coating Laboratories, Inc. (OCLI) in Santa Rosa, CA, by sputtering with iridium over a binding layer of chromium. Before each coating optical witness samples were used to show that the thickness would be uniform and that the surface smoothness was not degraded. The x-ray reflectivity of these witness flats was also measured to verify coating density. Witness samples coated simultaneously with the flight elements were also tested. The final cleaning occurred at OCLI prior to coating, and subsequently stringent contamination controls were put into place to minimize particulate and molecular contamination.
Figure 3: Schematic of Grazing Incidence, X-Ray Mirror. This cutaway illustrates the design and functioning of the HRMA (High Resolution Mirror Assembly) on Chandra (image credit: NASA/CXC, D. Berry)
Assembly and Alignment: The final alignment and assembly of the mirror elements into a telescope was done by the Eastman Kodak Company (EKC). The mirror element support structure, prior to inserting the reflecting elements, is shown in Figure 4. Each mirror element was bonded near its middle to flexures attached to carbon fiber composite support sleeves. The four support sleeves and associated flexures for the paraboloids can be seen near the top of the figure. The flexures produce only very small radial forces on the mirrors, and therefore reduce support-induced axial slope errors. The thin mirror shells are also susceptible to a deformation mode in which both ends become oval, but with perpendicular major axes. Supporting the mirror elements near their centers minimized the coupling of support errors into this mode.
Figure 4: Chandra telescope mirror support structure (image credit: NASA/MSFC)
Calibration: The telescope was taken to NASA/MSFC for end-to-end ground X-ray calibration beginning in December 1996. The calibration was performed over a 6 month period at the Center's X-ray Calibration Facility. Previously, the largest paraboloid-hyperboloid pair, uncoated and uncut to its final length, had been x-ray tested at the Facility during an earlier phase of the development program. This early test showed a measured angular resolution of 0.22 arcsec (FWHM). During the activities in 1996-1997 all the flight instrumentation (ACIS, HRC, the LETG and HETG) were tested with the telescope. Moreover, additional calibrations were performed using the telescope and synchrotron-facility-calibrated non-flight detectors. The on-ground calibration results demonstrated, in advance of launch, that the Chandra X-Ray Observatory would provide the required science capabilities: high-resolution (sub-arcsec) imaging, high resolution spectrometric imaging, and high-resolution dispersive spectroscopy.
An outline drawing of the Observatory was shown in Figure 6. The spacecraft equipment panels are mounted to, and supported by, a central cylindrical structure. The rear of the spacecraft attaches to the telescope system. The spacecraft includes six subsystems :
1) Structures and Mechanical Subsystem . This subsystem includes all spacecraft structures, mechanisms (both mechanical and electro-mechanical), and structural interfaces with the Space Shuttle. Mechanisms, such as those required for the sunshade door, are also part of this subsystem.
2) TCS (Thermal Control Subsystem) . Thermal control is primarily passive, using thermal coatings and multi-layer insulation blankets. On-board-computer-controlled electrical heaters augment these passive elements to maintain sensitive items such as the HRMA at nearly constant temperature.
3) Electrical and Power Subsystem . This subsystem includes all hardware necessary to generate, condition, and store electrical energy. Power is generated by solar cells mounted on two solar array wings (three panels each), sized to provide a 15% end-of-life power margin. Electrical power is stored in three, NiH2, 30-Ampere-hour batteries. These batteries provide spacecraft power during times when either the Earth or Moon partially or completely blocks the Sun. Even so, the battery capacity requires that certain non-critical items, including science instruments, be powered down during eclipses. These eclipses occur infrequently due to the particular nature of the Chandra orbit.
4) CCDM (Communication, Command, and Data Management) subsystem . This subsystem includes all the equipment necessary to provide ranging, modulation, and demodulation of radio frequency transmission of commands and data to and from the DSN (Deep Space Network) NASA Communication System (NASCOM). The CCDM includes two low gain antennas, providing omnidirectional communications, an on-board computer (OBC), a serial digital data bus for communication with other spacecraft components, the spacecraft clock, and a telemetry formatter which provides several different formats.
5) PCAD (Pointing Control and Aspect Determination) subsystem . This subsystem includes the hardware and control algorithms for attitude determination and for attitude and solar array control. The solar arrays can be rotated about one axis. The PCAD subsystem also includes hardware for safing the observatory.
The PCAD system controls the pointing and dithering of the observatory and provides the data from which both the relative and absolute aspect are determined. Dithering is imposed to spread the instantaneous image over many different pixels of the focal-plane detector to smooth out pixel-to-pixel variations. The dither pattern is a Lissajous figure. The amplitude, phase, and velocity depend on which instrument (ACIS or HRC) is in the focal plane.
Key elements of the PCAD system are the set of redundant gyroscopes, momentum wheels, and an ACA (Aspect Camera Assembly) consisting of a four inch optical telescope with (redundant) CCD detector. The aspect camera simultaneously images a fiducial light pattern produced by light emitting diodes placed around the focal-plane instruments along with the flux from up to five bright stars that may be in the aspect camera's field-of-view. An interesting consequence is that the user may request that one of the targets of the aspect camera be at the location of the X-ray target. For bright optical counterparts, this option allows real-time optical monitoring albeit at the price of a reduced-accuracy aspect solution.
Figure 5: Photo of the aspect camera (image credit: NASA/CXC)
6) Propulsion Subsystem. This subsystem consists of the IPS (Integral Propulsion System) and the MUPS (Momentum Unloading Propulsion Subsystem ). The IPS contains the thrusters and fuel for control of the orbit and spacecraft orientation during orbit transfer. This system was disabled once the final orbit was achieved for observatory safety reasons. The MUPS provides momentum unloading during normal on-orbit operations. Given current usage rates there would be sufficient MUPS fuel to support ~50 further years of operation.
Figure 6: Illustration of the Chandra spacecraft, its main elements and its instruments (image credit: NASA)
Table 1: Chandra X-ray Observatory technical parameters (Ref. 5)
Launch: The Chandra spacecraft was launched on 23 July 1999 on the Space Shuttle Columbia (STS-93) from the Kennedy Space Center, LC-39B. Use of Boeing's IUS (Inertial Upper Stage), and Chandra's own liquid propulsion system. Two burns of the IUS took place an hour after Chandra was released.
Figure 7: Photo of the Chandra spacecraft launch on STS-93 (image credit: NASA)
Orbit: HEO (Highly Elliptical Orbit) with a perigee of ~10,000 km and an apogee of ~140,000 km, inclination = 76.72°, period of ~64 hours.
Figure 8: Animation of Chandra X-ray Observatory's orbit around Earth from August 7, 1999, to March 8, 2019 (image credit: NASA)
Chandra: Some imagery and mission status
Note: For a more detailed and complete coverage of the Chandra X-ray Observatory mission imagery and status reports, the reader is referred to the CXC (Chandra X-ray Center) homepage at http://chandra.harvard.edu/
• April 16,2019: A bright burst of X-rays has been discovered by NASA's Chandra X-ray Observatory in a galaxy 6.6 billion light years from Earth. This event likely signaled the merger of two neutron stars and could give astronomers fresh insight into how neutron stars — dense stellar objects packed mainly with neutrons — are built. 8)
- When two neutron stars merge they produce jets of high energy particles and radiation fired in opposite directions. If the jet is pointed along the line of sight to the Earth, a flash, or burst, of gamma rays can be detected. If the jet is not pointed in our direction, a different signal is needed to identify the merger.
Figure 9: These images show the location of an event, discovered by NASA's Chandra X-ray Observatory, that likely signals the merger of two neutron stars. A bright burst of X-rays in this source, dubbed XT2, could give astronomers fresh insight into how neutron stars — dense stellar objects packed mainly with neutrons — are built (image credit: X-ray: NASA/CXC/Uni. of Science and Technology of China/Y. Xue et al; Optical: NASA/STScI)
- The detection of gravitational waves — ripples in spacetime — is one such signal. Now, with the observation of a bright flare of X-rays, astronomers have found another signal, and discovered that two neutron stars likely merged to form a new, heavier and fast-spinning neutron star with an extraordinarily strong magnetic field.
- "We've found a completely new way to spot a neutron star merger," said Yongquan Xue of the University of Science and Technology of China and lead author of a paper appearing in Nature. "The behavior of this X-ray source matches what one of our team members predicted for these events." 9)
- Chandra observed the source, dubbed XT2, as it suddenly appeared and then faded away after about seven hours. The source is located in the Chandra Deep Field-South, the deepest X-ray image ever taken that contains almost 12 weeks of Chandra observing time, taken at various intervals over several years. The source appeared on March 22nd, 2015 and was discovered later in analysis of archival data.
- "The serendipitous discovery of XT2 makes another strong case that nature's fecundity repeatedly transcends human imagination,"said co-author Niel Brandt of the Pennsylvania State University and principal investigator of the relevant Chandra Deep Field-South.
- The researchers identified the likely origin of XT2 by studying how its X-ray light varied with time, and comparing this behavior with predictions made in 2013 by Bing Zhang from the University of Nevada in Las Vegas, one of the corresponding authors of the paper. The X-rays showed a characteristic signature that matched those predicted for a newly-formed magnetar — a neutron star spinning around hundreds of times per second and possessing a tremendously strong magnetic field about a quadrillion times that of Earth's.
- The team think that the magnetar lost energy in the form of an X-ray-emitting wind, slowing down its rate of spin as the source faded. The amount of X-ray emission stayed roughly constant in X-ray brightness for about 30 minutes, then decreased in brightness by more than a factor of 300 over 6.5 hours before becoming undetectable. This showed that the neutron star merger produced a new, larger neutron star and not a black hole.
- This result is important because it gives astronomers a chance to learn about the interior of neutron stars, objects that are so dense that their properties could never be replicated on Earth.
- "We can't throw neutron stars together in a lab to see what happens, so we have to wait until the Universe does it for us," said Zhang. "If two neutron stars can collide and a heavy neutron star survives, then this tells us that their structure is relatively stiff and resilient."
- Neutron star mergers have been prominent in the news since the advanced Laser Interferometer Gravitational-Wave Observatory (LIGO) detected gravitational waves from one in 2017. That source, known as GW170817, produced a burst of gamma rays and an afterglow in light detected by many other telescopes, including Chandra. Xue's team think that XT2 would also have been a source of gravitational waves, however it occurred before Advanced LIGO started its first observing run, and it was too distant to have been detected in any case.
- Xue's team also considered whether the collapse of a massive star could have caused XT2, rather than a neutron star merger. The source is in the outskirts of its host galaxy, which aligns with the idea that supernova explosions that left behind the neutron stars kicked them out of the center a few billion years earlier. The galaxy itself also has certain properties — including a low rate of star formation compared to other galaxies of a similar mass — that are much more consistent with the type of galaxy where the merger of two neutron stars is expected to occur. Massive stars are young and are associated with high rates of star formation.
- "The host-galaxy properties of XT2 indeed boost our confidence in explaining its origin,"said co-author Ye Li from Peking University.
- The team estimated the rate at which events like XT2 should occur, and found that it agrees with the rate deduced from the detection of GW170817. However, both estimates are highly uncertain because they depend on the detection of just one object each, so more examples are needed.
- "We've started looking at other Chandra data to see if similar sources are present", said co-author Xuechen Cheng, also of the University of Science and Technology of China. "Just as with this source, the data sitting in archives might contain some unexpected treasures."
• March 21, 2019: Want to take a trip to the center of the Milky Way? Check out a new immersive, ultra-high-definition visualization. This 360º-movie offers an unparalleled opportunity to look around the center of the galaxy, from the vantage point of the central supermassive black hole, in any direction the user chooses. 10)
Figure 10: A new immersive, 360º, ultra-high-definition visualization allows viewers to view the center of our Galaxy as if they were sitting in the position of the Milky Way's supermassive black hole (Sgr A*). By combining supercomputer simulations with Chandra data, the visualization shows the effects of dozens of massive stellar giants with fierce winds blowing off their surfaces in the region covering a few light years surrounding Sgr A*. Blue and cyan represent X-ray emission from hot gas with temperatures of tens of millions of degrees, while the red emission shows ultraviolet emission from moderately dense regions of cooler gas with temperatures of tens of thousands of degrees, and yellow shows the cooler gas with the highest densities (video credit: NASA/CXC/Pontifical Catholic Univ. of Chile /C. Russell et al.)
- By combining NASA Ames supercomputer simulations with data from NASA's Chandra X-ray Observatory, this visualization provides a new perspective of what is happening in and around the center of the Milky Way. It shows the effects of dozens of massive stellar giants with fierce winds blowing off their surfaces in the region a few light years away from the supermassive black hole known as Sagittarius A* (Sgr A* for short).
- These winds provide a buffet of material for the supermassive black hole to potentially feed upon. As in a previous visualization, the viewer can observe dense clumps of material streaming toward Sgr A*. These clumps formed when winds from the massive stars near Sgr A* collide. Along with watching the motion of these clumps, viewers can watch as relatively low-density gas falls toward Sgr A*. In this new visualization, the blue and cyan colors represent X-ray emission from hot gas, with temperatures of tens of millions of degrees; red shows ultraviolet emission from moderately dense regions of cooler gas, with temperatures of tens of thousands of degrees; and yellow shows of the cooler gas with the highest densities.
- A collection of X-ray-emitting gas is seen to move slowly when it is far away from Sgr A*, and then pick up speed and whip around the viewer as it comes inwards. Sometimes clumps of gas will collide with gas ejected by other stars, resulting in a flash of X-rays when the gas is heated up, and then it quickly cools down. Farther away from the viewer, the movie also shows collisions of fast stellar winds producing X-rays. These collisions are thought to provide the dominant source of hot gas that is seen by Chandra.
- When an outburst occurs from gas very near the black hole, the ejected gas collides with material flowing away from the massive stars in winds, pushing this material backwards and causing it to glow in X-rays. When the outburst dies down the winds return to normal and the X-rays fade.
- The 360-degree video of the Galactic Center is ideally viewed through virtual reality (VR) goggles, such as Samsung Gear VR or Google Cardboard. The video can also be viewed on smartphones using the YouTube app. Moving the phone around reveals a different portion of the movie, mimicking the effect in the VR goggles. Finally, most browsers on a computer also allow 360-degree videos to be shown on YouTube. To look around, either click and drag the video, or click the direction pad in the corner.
- Dr. Christopher Russell of the Pontificia Universidad Católica de Chile (Pontifical Catholic University) presented the new visualization at the 17th meeting of the High-Energy Astrophysics (HEAD) of the American Astronomical Society held in Monterey, Calif. NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program for NASA's Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, controls Chandra's science and flight operations.
- The source of the cosmic squall is a supermassive black hole buried at the center of the galaxy, officially known as SDSS 1430+1339. As matter in the central regions of the galaxy is pulled toward the black hole, it is energized by the strong gravity and magnetic fields near the black hole. The infalling material produces more radiation than all the stars in the host galaxy. This kind of actively growing black hole is known as a quasar.
- Located about 1.1 billion light years from Earth, the Teacup's host galaxy was originally discovered in visible light images by citizen scientists in 2007 as part of the Galaxy Zoo project, using data from the Sloan Digital Sky Survey. Since then, professional astronomers using space-based telescopes have gathered clues about the history of this galaxy with an eye toward forecasting how stormy it will be in the future. This new composite image contains X-ray data from Chandra (blue) along with an optical view from NASA's Hubble Space Telescope (red and green).
- Located about 1.1 billion light years from Earth, the Teacup's host galaxy was originally discovered in visible light images by citizen scientists in 2007 as part of the Galaxy Zoo project, using data from the Sloan Digital Sky Survey. Since then, professional astronomers using space-based telescopes have gathered clues about the history of this galaxy with an eye toward forecasting how stormy it will be in the future. This new composite image contains X-ray data from Chandra (blue) along with an optical view from NASA's Hubble Space Telescope (red and green).
- The "handle" of the Teacup is a ring of optical and X-ray light surrounding a giant bubble. This handle-shaped feature, which is located about 30,000 light-years from the supermassive black hole, was likely formed by one or more eruptions powered by the black hole. Radio emission — shown in a separate composite image with the optical data — also outlines this bubble, and a bubble about the same size on the other side of the black hole.
- Previously, optical telescope observations showed that atoms in the handle of the Teacup were ionized, that is, these particles became charged when some of their electrons were stripped off, presumably by the quasar's strong radiation in the past. The amount of radiation required to ionize the atoms was compared with that inferred from optical observations of the quasar. This comparison suggested that the quasar's radiation production had diminished by a factor of somewhere between 50 and 600 over the last 40,000 to 100,000 years. This inferred sharp decline led researchers to conclude that the quasar in the Teacup was fading or dying.
- New data from Chandra and ESA's XMM-Newton mission are giving astronomers an improved understanding of the history of this galactic storm. The X-ray spectra (that is, the amount of X-rays over a range of energies) show that the quasar is heavily obscured by gas. This implies that the quasar is producing much more ionizing radiation than indicated by the estimates based on the optical data alone, and that rumors of the quasar's death may have been exaggerated. Instead the quasar has dimmed by only a factor of 25 or less over the past 100,000 years.
- The Chandra data also show evidence for hotter gas within the bubble, which may imply that a wind of material is blowing away from the black hole. Such a wind, which was driven by radiation from the quasar, may have created the bubbles found in the Teacup.
- Astronomers have previously observed bubbles of various sizes in elliptical galaxies, galaxy groups and galaxy clusters that were generated by narrow jets containing particles traveling near the speed of light, that shoot away from the supermassive black holes. The energy of the jets dominates the power output of these black holes, rather than radiation.
- In these jet-driven systems, astronomers have found that the power required to generate the bubbles is proportional to their X-ray brightness. Surprisingly, the radiation-driven Teacup quasar follows this pattern. This suggests radiation-dominated quasar systems and their jet-dominated cousins can have similar effects on their galactic surroundings.
- A study describing these results was published in the March 20, 2018 issue of The Astrophysical Journal Letters and is available online. The authors are George Lansbury from the University of Cambridge in Cambridge, UK; Miranda E. Jarvis from the Max-Planck Institut für Astrophysik in Garching, Germany; Chris M. Harrison from the European Southern Observatory in Garching, Germany; David M. Alexander from Durham University in Durham, UK; Agnese Del Moro from the Max-Planck-Institut für Extraterrestrische Physik in Garching, Germany; Alastair Edge from Durham University in Durham, UK; James R. Mullaney from The University of Sheffield in Sheffield, UK and Alasdair Thomson from the University of Manchester, Manchester, UK. 13)
Figure 11: Composite optical/X-ray image of a storm raging in a cosmic tea cup(image credit: NASA/CXC/Univ. of Cambridge/G. Lansbury et al; Optical: NASA/STScI/W. Keel et al.)
Figure 12: Animation of a quasar – nicknamed the Teacup because of its shape – is causing a storm in galaxy about 1.1 billion light-years from Earth. The power source of the quasar, astronomers say, is a supermassive black hole at the galaxy's center (video credit: X-ray: NASA/CXC/Univ. of Cambridge/G. Lansbury et al; Optical: NASA/STScI/W. Keel et al.)
• February 14, 2019: New results from NASA's Chandra X-ray Observatory may have helped solve the Universe's "missing mass" problem, as reported in our latest press release. Astronomers cannot account for about a third of the normal matter — that is, hydrogen, helium, and other elements — that were created in the first billion years or so after the Big Bang. 14)
- Scientists have proposed that the missing mass could be hidden in gigantic strands or filaments of warm (temperature less than 100,000 Kelvin) and hot (temperature greater than 100,000 K) gas in intergalactic space. These filaments are known by astronomers as the WHIM (Warm-Hot Intergalactic Medium). They are invisible to optical light telescopes, but some of the warm gas in filaments has been detected in ultraviolet light. The main part of this graphic is from the Millennium simulation, which uses supercomputers to formulate how the key components of the Universe, including the WHIM, would have evolved over cosmic time.
- If these filaments exist, they could absorb certain types of light such as X-rays that pass through them. The inset in this graphic represents some of the X-ray data collected by Chandra from a distant, rapidly-growing supermassive black hole known as a quasar. The plot is a spectrum — the amount of X-rays over a range of wavelengths — from a new study of the quasar H1821+643 that is located about 3.4 billion light years from Earth.
- The latest result uses a new technique that both hones the search for the WHIM carefully and boosts the relatively weak absorption signature by combining different parts of the spectrum to find a valid signal. With this technique, researchers identified 17 possible filaments lying between the quasar and Earth, and obtained their distances.
- For each filament the spectrum was shifted in wavelength to remove the effects of cosmic expansion, and then the spectra of all the filaments were added together so that the resulting spectrum has a much stronger signal from absorption by the WHIM than in the individual spectra.
- Indeed, the team did not find absorption in the individual spectra. But by adding them together, they turned a 5.5-day-long observation into the equivalent of almost 100 days' worth (about 8 million seconds) of data. This revealed an absorption line from oxygen expected to be present in a gas with a temperature of about one million Kelvin.
- By extrapolating from these observations of oxygen to the full set of elements, and from the observed region to the local Universe, the researchers report they can account for the complete amount of missing matter. A paper describing these results was published in The Astrophysical Journal on February 13, 2019. 15)
- NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program for NASA's Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, controls Chandra's science and flight operations.
Figure 13: A quick look at where is the Universe hiding its Missing Mass? (video credit: produced by CXC; animations: ESO/M. Kornmesser; NASA/GSFC/D. Berry)
Figure 14: The Universe's "missing mass" may have been found, according to a new study using Chandra data. About a third of the "normal" matter (ie, hydrogen, helium, and other elements) created shortly after the Big Bang is not seen in the present-day Universe. One idea is that this missing mass is today in filaments of warm and hot gas known as the WHIM. Researchers suggest evidence for the WHIM is seen in absorption features in X-rays collected from a quasar billions of light years away (image credit: Illustration: Springel et al. (2005); Spectrum: NASA/CXC/CfA/Kovács et al.)
• January 29, 2019: A new study using data from NASA's Chandra X-ray Observatory and ESA's XMM-Newton suggests that dark energy may have varied over cosmic time, as reported in our latest press release. This artist's illustration helps explain how astronomers tracked the effects of dark energy to about one billion years after the Big Bang by determining the distances to quasars, rapidly growing black holes that shine extremely brightly. 16)
Figure 15: Dark energy, a proposed force or energy that permeates all space and accelerates the Universe's expansion, may vary over time. A new study combining X-rays from Chandra and XMM-Newton and ultraviolet and optical data from SDSS (Sloan Digital Sky Survey) provides distances to quasars. These quasars are observed back to times about a billion years after the Big Bang. The new result showed that dark energy's effect on the expansion rate in the early Universe may have been different from today (image credit: NASA/CXC/M.Weiss; X-ray: NASA/CXC/Univ. of Florence/G.Risaliti & E.Lusso)
- First discovered about 20 years ago by measuring the distances to exploded stars called supernovas, dark energy is a proposed type of force, or energy, that permeates all space and causes the expansion of the Universe to accelerate. Using this method, scientists tracked the effects of dark energy out to about 9 billion years ago.
- The latest result stems from the development of a new method to determine distances to about 1,598 quasars, which allows the researchers to measure dark energy's effects from the early Universe through to the present day. Two of the most distant quasars studied are shown in Chandra images in the insets.
- The new technique uses ultraviolet (UV) and X-ray data to estimate the quasar distances. In quasars, a disk of matter around the supermassive black hole in the center of a galaxy produces UV light (shown in the illustration in blue). Some of the UV photons collide with electrons in a cloud of hot gas (shown in yellow) above and below the disk, and these collisions can boost the energy of the UV light up to X-ray energies. This interaction causes a correlation between the amounts of observed UV and X-ray radiation. This correlation depends on the luminosity of the quasar, which is the amount of radiation it produces.
- Using this technique the quasars become standard candles, as implied by the artist's illustration, showing quasars with the same luminosity at different distances from Earth. Once the luminosity is known, the distance to the quasars can be calculated. This is because the observed amount of radiation from quasars converted into standard candles depend on their distance from Earth in a predictable way.
- The researchers compiled UV data for 1,598 quasars to derive a relationship between UV and X-ray fluxes, and the distances to the quasars. They then used this information to study the expansion rate of the universe back to very early times, and found evidence that the amount of dark energy is growing with time.
- Since this is a new technique, the astronomers took extra steps to show that this method gives reliable results. They showed that results from their technique match up with those from supernova measurements over the last 9 billion years, giving them confidence that their results are reliable at even earlier times. The researchers also took great care in how their quasars were selected, to minimize statistical errors and to avoid systematic errors that might depend on the distance from Earth to the object. 17)
• January 9, 2019: Astronomers have discovered behavior by a jet from a giant black hole that has never been seen before. Using NASA's Chandra X-ray Observatory they have observed a jet that bounced off a wall of gas and then punched a hole in a cloud of energetic particles. This behavior can tell scientists more about how jets from black holes interact with their surroundings. 18) 19)
- The discovery was made in Cygnus A, a large galaxy in the middle of a cluster of galaxies about 760 million light years from Earth. Chandra data show powerful jets of particles and electromagnetic energy blasting away from a rapidly growing black hole at the center of Cygnus A. After traveling more than 200,000 light years on either side of the black hole, the jets have slowed down via its interaction with multimillion-degree intergalactic gas that envelopes Cygnus A. This interaction has produced enormous clouds of energetic particles that emits X-rays and radio waves.
- In a deep observation that lasted 23 days, scientists used Chandra to create a highly detailed map of both the jet and the intergalactic gas, which they used to track the path of the jets from the black hole. The jet on the left expanded after ricocheting and created a hole in the surrounding cloud of particles that is between 50,000 and 100,000 light years deep and only 26,000 light years wide.
- "Not only did we see this black hole jet rebound off intergalactic gas, in much the same way that a pebble would skip or ricochet off the surface of a pond, it then punched a hole in a cloud of energetic particles," said Amalya Johnson of Columbia University in New York, who led the new study. "This is the first time we've seen such a cosmic hole punch."
- Chandra's sharp imaging was crucial for this discovery. "It's remarkable that Chandra can capture intricate details in X-rays about what this black hole is doing more than a billion trillion miles away from Earth," said co-author Paul Nulsen of the Center for Astrophysics Harvard and Smithsonian (CfA) in Cambridge, Mass. "Thanks to the Chandra data we can see where the jet ricocheted and follow its path before striking the gas a second time."
- While best known for pulling things toward it, black holes are also adept at blasting material away from themselves. As the black hole spins, it can produce a rotating, tightly-wound vertical tower of powerful magnetic fields. This allows the black hole to redirect some of the energy released by gas spiraling toward it, creating an energetic jet traveling at very high speeds away from the black hole. The Cygnus A jet is one of the largest and most powerful ever observed.
- The scientists are working to determine what forms of energy — kinetic energy, heat or radiation — the jet carries. The composition of the jet and the types of energy determine how the jet behaves when it ricochets, such as the size of the hole it creates. Theoretical models of the jet and its interactions with surrounding gas are needed to make conclusions about the jet's properties.
- Energy produced by jets from black holes can heat intergalactic gas in galaxy clusters and prevent it from cooling and forming large numbers of stars in a central galaxy like Cygnus A. "We know that rapidly growing black holes have a large effect on their environment," said co-author Bradford Snios, also of the CfA. "By studying Cygnus A we hope to learn more about how giant black holes affect their host galaxy over time."
Figure 16: X-ray and optical composite. A ricocheting jet blasting from a giant black hole has been captured by Chandra. These images of Cygnus A show X-rays from Chandra and an optical view from Hubble of the galaxies and stars in the same field of view. Chandra's data reveal the presence of a powerful jet of particles and electromagnetic energy that has shot out from the black hole and slammed into a wall of hot gas, then ricocheted to punch a hole in a cloud of energetic particles, before it collides with another part of the gas wall (image credit: X-ray: NASA/CXC/Columbia Univ./A. Johnson et al.; Optical: NASA/STScI)
- Figure 16 shows the location of the supermassive black hole, the jets, the point that the jet on the left ricocheted off a wall of intergalactic gas ("hotspot E"), and the point where the jet then struck the intergalactic gas a second time ("hotspot D"). The inset contains a close-up view of the hotspots on the left and the hole punched by the rebounding jet, which surrounds hotspot E. The image in the inset combines X-rays from all three energy ranges to give the greatest sensitivity to show fine structures such as the hole.
- The hole is visible because the path of the rebounding jet between hotspots E and D is almost directly along the line of sight to Earth, as shown by the schematic figure depicting the view of Cygnus A from above. A similar rebounding of the jet likely occurred between hotspots A and B but the hole is not visible because the path is not along the Earth's line of sight.
Figure 17: Black holes are notorious for pulling things toward them. But in some cases, black holes can act as powerful engines to blast material away. One of those black holes is found in Cygnus A, a large galaxy embedded within in a cluster of galaxies. Cygnus A's black hole is blasting a jet — a tightly-wound column of material — away from it at extremely high speeds. Astronomers found that his jet ricocheted off a wall of hot gas, then punched a hole in a cloud of energetic particles, leaving behind a gigantic hole (video credit: Chandra X-ray Observatory, Published on Jan 9, 2019)
• October 24, 2018: On the evening of October 21, Chandra returned to science observations after the team successfully carried out a procedure to enable a new gyroscope configuration for the spacecraft. The team initiated a set of maneuvers to change the pointing and orientation of the spacecraft to confirm that the gyroscopes were behaving as expected. During the coming week, scientists will collect spacecraft data to fine-tune the performance for the new gyroscope configuration. As a final step, the team will uplink a software patch to apply any necessary adjustments to the on-board computer. 20)
- October 15 update: The cause of Chandra's safe mode on October 10 has now been understood and the Operations team has successfully returned the spacecraft to its normal pointing mode. The safe mode was caused by a glitch in one of Chandra's gyroscopes resulting in a 3-second period of bad data that in turn led the on-board computer to calculate an incorrect value for the spacecraft momentum. The erroneous momentum indication then triggered the safe mode.
• October 16, 2018: About a year ago, astronomers excitedly reported the first detection of electromagnetic waves, or light, from a gravitational wave source. Now, a year later, researchers are announcing the existence of a cosmic relative to that historic event. 21)
- The discovery was made using data from telescopes including NASA's Chandra X-ray Observatory, Fermi Gamma-ray Space Telescope, Neil Gehrels Swift Observatory, the NASA Hubble Space Telescope (HST), and the Discovery Channel Telescope (DCT).
- The object of the new study, called GRB 150101B, was first reported as a gamma-ray burst detected by Fermi in January 2015. This detection and follow-up observations at other wavelengths show GRB 150101B shares remarkable similarities to the neutron star merger and gravitational wave source discovered by Advanced Laser Interferometer Gravitational Wave Observatory (LIGO) and its European counterpart Virgo in 2017 known as GW170817. The latest study concludes that these two separate objects may, in fact, be related.
- "It's a big step to go from one detected object to two," said Eleonora Troja, lead author of the study from NASA's Goddard Space Flight Center in Greenbelt, Maryland, and the University of Maryland at College Park (UMCP). "Our discovery tells us that events like GW170817 and GRB 150101B could represent a whole new class of erupting objects that turn on and off in X-rays and might actually be relatively common."
- Troja and her colleagues think both GRB 150101B and GW170817 were most likely produced by the same type of event: the merger of two neutron stars, a catastrophic coalescence that generated a narrow jet, or beam, of high-energy particles. The jet produced a short, intense burst of gamma rays (known as a short GRB), a high-energy flash that can last only seconds. GW170817 proved that these events may also create ripples in space-time itself called gravitational waves.
- The apparent match between GRB 150101B and GW170817 is striking: both produced an unusually faint and short-lived gamma ray burst, and both were a source of bright, blue optical light lasting a few days, and X-ray emission lasted much longer. The host galaxies are also remarkably similar, based on Hubble Space Telescope and DCT observations. Both are bright elliptical galaxies with a population of stars a few billion years old and displaying no evidence for new stars forming.
- "We have a case of cosmic lookalikes," said co-author Geoffrey Ryan of UMCP. "They look the same, act the same and come from similar neighborhoods, so the simplest explanation is that they are from the same family of objects."
Figure 18: A distant cosmic relative to the first source that astronomers detected in both gravitational waves and light may have been discovered, as reported in our latest press release. This object, called GRB 150101B, was first detected by identified as a gamma ray burst (GRB) by NASA's Fermi Gamma-ray Space Telescope in January 2015. This image shows data from NASA's Chandra X-ray Observatory (purple in the inset boxes) in context with an optical image of GRB 150101B from the Hubble Space Telescope (image credit: X-ray: NASA/CXC/GSFC/UMC/E. Troja et al.; Optical and infrared: NASA/STScI)
- In the cases of both GRB 150101B and GW170817, the slow rise in the X-ray emission compared to most GRBs implies that the explosion was likely viewed "off-axis," that is, with the jet not pointing directly towards the Earth. The discovery of GRB150101 represents only the second time astronomers have ever detected an off-axis short GRB.
- The latest study concludes that these two separate objects may, in fact, be related. The discovery suggests that events like GW170817 and GRB 150101B could represent a whole new class of erupting objects that turn on and off in X-rays and might actually be relatively common.
- The researchers think both GRB 150101B and GW170817 were most likely produced by the same type of event: the merger of two neutron stars, a catastrophic coalescence that generated a narrow jet, or beam, of high-energy particles. The jet produced a short, intense burst of gamma rays (known as a short GRB), a high-energy flash that can last only seconds. GW170817 proved that these events may also create ripples in space-time itself called gravitational waves.
- While there are many commonalities between GRB 150101B and GW170817, there are two very important differences. One is their location. GW170817 is about 130 million light years from Earth, while GRB 150101B lies about 1.7 billion light years away. Even if Advanced LIGO had been operating in early 2015, it would very likely not have detected gravitational waves from GRB 150101B because of its greater distance.
- It is possible that a few mergers like the ones seen in GW170817 and GRB 150101B had been detected as short GRBs before but had not been identified with other telescopes. Without detections at longer wavelengths like X-rays or optical light, GRB positions are not accurate enough to determine what galaxy they are located in.
- In the case of GRB 150101B, astronomers thought at first that the counterpart was an X-ray source detected by Swift in the center of the galaxy, likely from material falling into a supermassive black hole. However, follow-up observations with Chandra, with its sharp X-ray resolution, detected the true counterpart away from the center of the host galaxy. This can be seen in the Chandra images. Not only has the source dimmed dramatically, it is clearly outside the center of the galaxy, which appears as the constant brighter source to the upper right. 22)
• October 12, 2018: At approximately 13:55 GMT on 10 October 2018, NASA's Chandra X-ray Observatory entered Safe Mode, where the telescope's instruments are put into a safe configuration, critical hardware is swapped to back-up units, the spacecraft points so that the solar panels get maximum sunlight, and the mirrors point away from the Sun. Analysis of available data indicates the transition to safe mode was nominal, i.e., consistent with normal behavior for such an event. All systems functioned as expected and the scientific instruments are safe. The cause of the Safe Mode transition is currently under investigation, and we will post more information when it becomes available. 23)
- Chandra is 19 years old, which is well beyond the original design lifetime of 5 years. In 2001, NASA extended its lifetime to 10 years. It is now well into its extended mission and it is expected to continue carrying out forefront science for many years to come.
• September 25, 2018: NASA has awarded a contract extension to the Smithsonian Astrophysical Observatory (SAO) in Cambridge, Massachusetts, to continue operations and science support for the agency's Chandra X-ray Observatory. 24)
- The contract extends the agreement between NASA and SAO through Sept. 30, 2024, followed by two three-year options that would extend the contract through Sept. 30, 2030. The total potential value of the contract extension is $563.5 million.
- This contract covers continued operation of the Chandra X-ray Center (CXC) in Cambridge, which conducts key aspects of Chandra's observation, operations and research program. Core functions of the CXC include system engineering, ground system development and maintenance, mission operations, science and operations planning, science research and dissemination, and outreach support.
- Since its launch on July 23, 1999, Chandra has been NASA's flagship mission for X-ray astronomy and is among NASA's fleet of Great Observatories. The agency's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program for the agency's Science Mission Directorate in Washington.
• June 21, 2018: A new study using data from NASA's Chandra X-ray Observatory indicates that black holes have squelched star formation in small, yet massive galaxies known as "red nuggets", as reported in our latest press release. The results suggest some red nugget galaxies may have used some of the untapped stellar fuel to grow their central supermassive black holes to unusually massive proportions. 25) 26) 27)
- Red nuggets are relics of the first massive galaxies that formed within only one billion years after the Big Bang. While most red nuggets merged with other galaxies over billions of years, a small number remained solitary. These relatively pristine red nuggets allow astronomers to study how the galaxies — and the supermassive black hole at their centers — act over billions of years of isolation.
- In the latest research, astronomers used Chandra to study the hot gas in two of these isolated red nuggets, Mrk 1216, and PGC 032673. (The Chandra data, colored red, of Mrk 1216 is shown in the inset.) These two galaxies are located only 295 million and 344 million light years from Earth, respectively, rather than billions of light years for the first known red nuggets, allowing for a more detailed look. The gas in the galaxy is heated to such high temperatures that it emits brightly in X-ray light, which Chandra detects. This hot gas contains the imprint of activity generated by the supermassive black holes in each of the two galaxies.
Figure 19: An artist's illustration (main panel) shows how material falling towards black holes can be redirected outward at high speeds due to intense gravitational and magnetic fields. These high-speed jets can tamp down the formation of stars. This happens because the blasts from the vicinity of the black hole provide a powerful source of heat, preventing the galaxy's hot interstellar gas from cooling enough to allow large numbers of stars to form (image credit: X-ray: NASA/CXC/MTA-Eötvös University/N. Werner et al.; Illustration: NASA/CXC/M.Weiss)
• September 6, 2017: A new study using data from NASA's Chandra X-ray Observatory and ESA's XMM-Newton suggests X-rays emitted by a planet's host star may provide critical clues to just how hospitable a star system could be. A team of researchers looked at 24 stars similar to the Sun, each at least one billion years old, and how their X-ray brightness changed over time. 28) 29)
Figure 20: This artist's illustration depicts one of these comparatively calm, older Sun-like stars with a planet in orbit around it. The large dark area is a "coronal hole", a phenomenon associated with low levels of magnetic activity. The inset box shows the Chandra data of one of the observed objects, a two billion year old star called GJ 176, located 30 light years from Earth (image credit: X-ray: NASA/CXC/Queens Univ. of Belfast/R.Booth, et al.; Illustration: NASA/CXC/M.Weiss)
- Since stellar X-rays mirror magnetic activity, X-ray observations can tell astronomers about the high-energy environment around the star. In the new study the X-ray data from Chandra and XMM-Newton revealed that stars like the Sun and their less massive cousins calm down surprisingly quickly after a turbulent youth.
- To understand how quickly stellar magnetic activity level changes over time, astronomers need accurate ages for many different stars. This is a difficult task, but new precise age estimates have recently become available from studies of the way that a star pulsates using NASA's Kepler and ESA's CoRoT missions. These new age estimates were used for most of the 24 stars studied here.
- Astronomers have observed that most stars are very magnetically active when they are young, since the stars are rapidly rotating. As the rotating star loses energy over time, the star spins more slowly and the magnetic activity level, along with the associated X-ray emission, drops.
- Although it is not certain why older stars settle down relatively quickly, astronomers have ideas they are exploring. One possibility is that the decrease in rate of spin of the older stars occurs more quickly than it does for the younger stars. Another possibility is that the X-ray brightness declines more quickly with time for older, more slowly rotating stars than it does for younger stars.
• August 16, 2016: A new look at the debris from an exploded star in our galaxy has astronomers re-examining when the supernova actually happened. Recent observations of the supernova remnant called G11.2-0.3 with NASA's Chandra X-ray Observatory have stripped away its connection to an event recorded by the Chinese in 386 CE. 30) 31)
- Historical supernovas and their remnants can be tied to both current astronomical observations as well as historical records of the event. Since it can be difficult to determine from present observations of their remnant exactly when a supernova occurred, historical supernovas provide important information on stellar timelines. Stellar debris can tell us a great deal about the nature of the exploded star, but the interpretation is much more straightforward given a known age.
- New Chandra data on G11.2-0.3 show that dense clouds of gas lie along the line of sight from the supernova remnant to Earth. Infrared observations with the Palomar 5-meter Hale Telescope had previously indicated that parts of the remnant were heavily obscured by dust. This means that the supernova responsible for this object would simply have appeared too faint to be seen with the naked eye in 386 CE. This leaves the nature of the observed 386 CE event a mystery.
- Taking advantage of Chandra's successful operations since its launch into space in 1999, astronomers were able to compare observations of G11.2-0.3 from 2000 to those taken in 2003 and more recently in 2013. This long baseline allowed scientists to measure how fast the remnant is expanding. Using this data to extrapolate backwards, they determined that the star that created G11.2-0.3 exploded between 1,400 and 2,400 years ago as seen from Earth.
- Previous data from other observatories had shown this remnant is the product of a "core-collapse" supernova, one that is created from the collapse and explosion of a massive star. The revised timeframe for the explosion based on the recent Chandra data suggests that G11.2-0.3 is one of the youngest such supernovas in the Milky Way. The youngest, Cassiopeia A, also has an age determined from the expansion of its remnant, and like G11.2-0.3 was not seen at its estimated explosion date of 1680 CE due to dust obscuration. So far, the Crab nebula, the remnant of a supernova seen in 1054 CE, remains the only firmly identified historical remnant of a massive star explosion in our galaxy.
- This latest image of G11.2-0.3 shows low-energy X-rays in red, the medium range in green, and the high-energy X-rays detected by Chandra in blue. The X-ray data have been overlaid on an optical field from the Digitized Sky Survey, showing stars in the foreground.
- Although the Chandra image appears to show the remnant has a very circular, symmetrical shape, the details of the data indicate that the gas that the remnant is expanding into is uneven. Because of this, researchers propose that the exploded star had lost almost all of its outer regions, either in an asymmetric wind of gas blowing away from the star, or in an interaction with a companion star. They think the smaller star left behind would then have blown gas outwards at an even faster rate, sweeping up gas that was previously lost in the wind, forming the dense shell. The star would then have exploded, producing the G11.2-0.3 supernova remnant seen today.
Figure 21: New Chandra data of the supernova remnant G11.2-0.3 raise new questions about the timing of its origin (image credit: X-ray: NASA/CXC/NCSU/K. Borkowski et al; Optical: DSS)
• July 22, 2014: Fifteen years ago, NASA's Chandra X-ray Observatory was launched into space aboard the Space Shuttle Columbia. Since its deployment on July 23, 1999, Chandra has helped revolutionize our understanding of the universe through its unrivaled X-ray vision. 32) 33)
Figure 22: The images of the Tycho and G292.0+1.8 supernova remnants show how Chandra can trace the expanding debris of an exploded star and the associated shock waves that rumble through interstellar space at speeds of millions of miles per hour. The images of the Crab Nebula and 3C 58 show how extremely dense, rapidly rotating neutron stars produced when a massive star explodes can create clouds of high-energy particles light years across that glow brightly in X-rays (image credit: NASA/CXC/SAO)
- Tycho: More than four centuries after Danish astronomer Tycho Brahe first observed the supernova that bears his name, the supernova remnant it created is now a bright source of X-rays. The supersonic expansion of the exploded star produced a shock wave moving outward into the surrounding interstellar gas, and another, reverse shock wave moving back into the expanding stellar debris. This Chandra image of Tycho reveals the dynamics of the explosion in exquisite detail. The outer shock has produced a rapidly moving shell of extremely high-energy electrons (blue), and the reverse shock has heated the expanding debris to millions of degrees (red and green). There is evidence from the Chandra data that these shock waves may be responsible for some of the cosmic rays – ultra-energetic particles – that pervade the Galaxy and constantly bombard the Earth.
- G292.0+1.8: At a distance of about 20,000 light years, G292.0+1.8 is one of only three supernova remnants in the Milky Way known to contain large amounts of oxygen. These oxygen-rich supernovas are of great interest to astronomers because they are one of the primary sources of the heavy elements (that is, everything other than hydrogen and helium) necessary to form planets and people. The X-ray image from Chandra shows a rapidly expanding, intricately structured, debris field that contains, along with oxygen (yellow and orange), other elements such as magnesium (green) and silicon and sulfur (blue) that were forged in the star before it exploded.
- The Crab Nebula: In 1054 AD, Chinese astronomers and others around the world noticed a new bright object in the sky. This "new star" was, in fact, the supernova explosion that created what is now called the Crab Nebula. At the center of the Crab Nebula is an extremely dense, rapidly rotating neutron star left behind by the explosion. The neutron star, also known as a pulsar, is spewing out a blizzard of high-energy particles, producing the expanding X-ray nebula seen by Chandra. In this new image, lower-energy X-rays from Chandra are red, medium energy X-rays are green, and the highest-energy X-rays are blue.
- 3C58: 3C58 is the remnant of a supernova observed in the year 1181 AD by Chinese and Japanese astronomers. This new Chandra image shows the center of 3C58, which contains a rapidly spinning neutron star surrounded by a thick ring, or torus, of X-ray emission. The pulsar also has produced jets of X-rays blasting away from it to both the left and right, and extending trillions of miles. These jets are responsible for creating the elaborate web of loops and swirls revealed in the X-ray data. These features, similar to those found in the Crab, are evidence that 3C58 and others like it are capable of generating both swarms of high-energy particles and powerful magnetic fields. In this image, low, medium, and high-energy X-rays detected by Chandra are red, green, and blue respectively.
• March 5, 2014: Astronomers have used NASA's Chandra X-ray Observatory and the European Space Agency's (ESA's) XMM-Newton to show a supermassive black hole six billion light years from Earth is spinning extremely rapidly. This first direct measurement of the spin of such a distant black hole is an important advance for understanding how black holes grow over time. 34) 35)
- Black holes are defined by just two simple characteristics: mass and spin. While astronomers have long been able to measure black hole masses very effectively, determining their spins has been much more difficult.
- In the past decade, astronomers have devised ways of estimating spins for black holes at distances greater than several billion light-years away, meaning we see the region around black holes as they were billions of years ago. However, determining the spins of these remote black holes involves several steps that rely on one another.
Figure 23: Multiple images of a distant quasar are visible in this combined view from NASA's Chandra X-ray Observatory and the Hubble Space Telescope. The Chandra data, along with data from ESA's XMM-Newton, were used to directly measure the spin of the supermassive black hole powering this quasar. This is the most distant black hole where such a measurement has been made, as reported in our press release (image credit: X-ray: NASA/CXC/Univ of Michigan/R. C. Reis et al; Optical: NASA/STScI)
- "We want to be able to cut out the middle man, so to speak, of determining the spins of black holes across the universe," said Rubens Reis of the University of Michigan in Ann Arbor, who led a paper describing this result that was published online Wednesday in the journal Nature.
- Reis and his colleagues determined the spin of the supermassive black hole that is pulling in surrounding gas, producing an extremely luminous quasar known as RX J1131-1231 (RX J1131 for short). Because of fortuitous alignment, the distortion of space-time by the gravitational field of a giant elliptical galaxy along the line of sight to the quasar acts as a gravitational lens that magnifies the light from the quasar. Gravitational lensing, first predicted by Einstein, offers a rare opportunity to study the innermost region in distant quasars by acting as a natural telescope and magnifying the light from these sources.
- "Because of this gravitational lens, we were able to get very detailed information on the X-ray spectrum – that is, the amount of X-rays seen at different energies – from RX J1131," said co-author Mark Reynolds also of Michigan. "This in turn allowed us to get a very accurate value for how fast the black hole is spinning."
- The X-rays are produced when a swirling accretion disk of gas and dust that surrounds the black hole creates a multimillion-degree cloud, or corona near the black hole. X-rays from this corona reflect off the inner edge of the accretion disk. The strong gravitational forces near the black hole alter the reflected X-ray spectrum. The larger the change in the spectrum, the closer the inner edge of the disk must be to the black hole.
- "We estimate that the X-rays are coming from a region in the disk located only about three times the radius of the event horizon, the point of no return for infalling matter," said Jon M. Miller of Michigan, another author on the paper. "The black hole must be spinning extremely rapidly to allow a disk to survive at such a small radius."
- For example, a spinning black hole drags space around with it and allows matter to orbit closer to the black hole than is possible for a non-spinning black hole.
- By measuring the spin of distant black holes researchers discover important clues about how these objects grow over time. If black holes grow mainly from collisions and mergers between galaxies, they should accumulate material in a stable disk, and the steady supply of new material from the disk should lead to rapidly spinning black holes. In contrast, if black holes grow through many small accretion episodes, they will accumulate material from random directions. Like a merry go round that is pushed both backwards and forwards, this would make the black hole spin more slowly.
- The discovery that the black hole in RX J1131 is spinning at over half the speed of light suggests this black hole, observed at a distance of six billion light years, corresponding to an age about 7.7 billion years after the Big Bang, has grown via mergers, rather than pulling material in from different directions.
- The ability to measure black hole spin over a large range of cosmic time should make it possible to directly study whether the black hole evolves at about the same rate as its host galaxy. The measurement of the spin of the RX J1131-1231 black hole is a major step along that path and demonstrates a technique for assembling a sample of distant supermassive black holes with current X-ray observatories.
• August 15, 2012: Astronomers have found an extraordinary galaxy cluster, one of the largest objects in the universe, that is breaking several important cosmic records. Observations of the Phoenix cluster with NASA's Chandra X-ray Observatory, the National Science Foundation's South Pole Telescope, and eight other world-class observatories may force astronomers to rethink how these colossal structures and the galaxies that inhabit them evolve. 36)
- This galaxy cluster has been dubbed the "Phoenix Cluster" because it is located in the constellation of the Phoenix, and because of its remarkable properties, as explained here and in our press release. Stars are forming in the Phoenix Cluster at the highest rate ever observed for the middle of a galaxy cluster. The object is also the most powerful producer of X-rays of any known cluster, and among the most massive of clusters. The data also suggest that the rate of hot gas cooling in the central regions of the cluster is the largest ever observed.
- Like other galaxy clusters, Phoenix contains a vast reservoir of hot gas — containing more normal matter than all of the galaxies in the cluster combined — that can only be detected with X-ray telescopes like Chandra. This hot gas is giving off copious amounts of X-rays and cooling quickly over time, especially near the center of the cluster, causing gas to flow inwards and form huge numbers of stars. These features are shown in the artist's impression of the central galaxy, with hot gas in red, cooler gas in blue. The gas flows appear as the ribbon-like features and the newly formed stars are blue. An animation portrays the process of cooling and star formation in action. A close-up of the middle of the optical and UV image shows that the central galaxy has much bluer colors than the nearby galaxies in the cluster, revealing the presence of large numbers of hot, massive stars forming.
- These results are striking because most galaxy clusters have formed very few stars over the last few billion years. Astronomers think that the supermassive black hole in the central galaxy of clusters pumps energy into the system. The famous Perseus Cluster is an example of a black hole bellowing out energy and preventing the gas from cooling to form stars at a high rate. Repeated outbursts from the black hole in the center of Perseus in the form of powerful jets, created giant cavities and produced sound waves with an incredibly deep B-flat note 57 octaves below middle C. Shock waves, akin to sonic booms in Earth's atmosphere, and the very deep sound waves release energy into the gas in Perseus, preventing most of it from cooling.
Figure 24: The image on the left shows the newly discovered Phoenix Cluster, located about 5.7 billion light years from Earth. This composite includes an X-ray image from NASA's Chandra X-ray Observatory in purple, an optical image from the 4m Blanco telescope in red, green and blue, and an ultraviolet (UV) image from NASA's Galaxy Evolution Explorer (GALEX) in blue. The Chandra data show hot gas in the cluster and the optical and UV images show galaxies in the cluster and in nearby parts of the sky (image credit: X-ray: NASA/CXC/MIT/M.McDonald; UV: NASA/JPL-Caltech/M.McDonald; Optical: AURA/NOAO/CTIO/MIT/M.McDonald; Illustration: NASA/CXC/M.Weiss)
- In the case of Phoenix, jets from the giant black hole in its central galaxy are not powerful enough to prevent the cluster gas from cooling. Correspondingly, any deep notes produced by the jets must be much weaker than needed to prevent cooling and star formation.
- Based on the Chandra data and also observations at other wavelengths, the supermassive black hole in the central galaxy of Phoenix is growing very quickly, at a rate of about 60 times the mass of the Sun every year. This rate is unsustainable, because the black hole is already very large with a mass of about 20 billion times the mass of the Sun. Therefore, its growth spurt cannot last much longer than about a hundred million years or it would become much bigger than its counterparts in the nearby Universe. A similar argument applies to the growth of the central galaxy. Eventually powerful jets should be produced by the black hole in repeated outbursts, forming the deep notes seen in objects like Perseus and stopping the starburst. 37)
- The Phoenix cluster originally was detected by the National Science Foundation's South Pole Telescope, and later was observed in optical light by the Gemini Observatory, the Blanco 4-meter telescope and Magellan telescope, all in Chile. The hot gas and its rate of cooling were estimated from Chandra data. To measure the star formation rate in the Phoenix cluster, several space-based telescopes were used, including NASA's Wide-field Infrared Survey Explorer and Galaxy Evolution Explorer and ESA's Herschel.
• July 23, 2009: Ten years ago, on July 23, 1999, NASA's Chandra X-ray Observatory was launched aboard the space shuttle Columbia and deployed into orbit. Chandra has doubled its original five-year mission, ushering in an unprecedented decade of discovery for the high-energy universe. With its unrivaled ability to create high-resolution X- ray images, Chandra has enabled astronomers to investigate phenomena as diverse as comets, black holes, dark matter and dark energy. 38)
- "Chandra's discoveries are truly astonishing and have made dramatic changes to our understanding of the universe and its constituents," said Martin Weisskopf, Chandra project scientist at NASA's Marshall Space Flight Center in Huntsville, Ala.
- The science that has been generated by Chandra — both on its own and in conjunction with other telescopes in space and on the ground — has had a widespread, transformative impact on 21st century astrophysics. Chandra has provided the strongest evidence yet that dark matter must exist. It has independently confirmed the existence of dark energy and made spectacular images of titanic explosions produced by matter swirling toward supermassive black holes.
Figure 25: This image of the debris of an exploded star — known as supernova remnant 1E 0102.2-7219, or "E0102" for short — features data from NASA's Chandra X-ray Observatory. E0102 is located about 190,000 light years away in the Small Magellanic Cloud, one of the nearest galaxies to the Milky Way. It was created when a star that was much more massive than the Sun exploded, an event that would have been visible from the Southern Hemisphere of the Earth over 1000 years ago [image credit: X-ray (NASA/CXC/MIT/D. Dewey et al. & NASA/CXC/SAO/J.DePasquale); Optical (NASA/STScI)]
Legend to Figure 25: Chandra first observed E0102 shortly after its launch in 1999. New X-ray data have now been used to create this spectacular image and help celebrate the ten-year anniversary of Chandra's launch on July 23, 1999. In this latest image of E0102, the lowest-energy X-rays are colored orange, the intermediate range of X-rays is cyan, and the highest-energy X-rays Chandra detected are blue. An optical image from the Hubble Space Telescope (in red, green and blue) shows additional structure in the remnant and also reveals foreground stars in the field. — The Chandra image shows the outer blast wave produced by the supernova (blue), and an inner ring of cooler (red-orange) material. This inner ring is probably expanding ejecta from the explosion that is being heated by a shock wave traveling backwards into the ejecta. A massive star (not visible in this image) is illuminating the green cloud of gas and dust to the lower right of the image. This star may have similar properties to the one that exploded to form E0102.
- "The Great Observatories program — of which Chandra is a major part — shows how astronomers need as many tools as possible to tackle the big questions out there," said Ed Weiler, associate administrator of NASA's Science Mission Directorate at NASA Headquarters in Washington. NASA's other "Great Observatories" are the Hubble Space Telescope, Compton Gamma-Ray Observatory and Spitzer Space Telescope.
• October 5, 2005: Scientists have solved the 35-year-old mystery of the origin of powerful, split-second flashes of light known as short gamma-ray bursts. These flashes, brighter than a billion galaxies, yet lasting only a few milliseconds, have been simply too fast to catch - until now. 39) 40)
- Through the unprecedented coordination of observations from several ground-based telescopes and NASA satellites, scientists determined the flashes arise from violent collisions in space. The clashes are either between a black hole and a neutron star or between two neutron stars. In either scenario, the impact creates a new black hole.
- In at least one burst, scientists saw tantalizing, first-time evidence of a black hole eating a neutron star. The neutron star was first stretched into a crescent, then swallowed by the black hole.
- "Gamma-ray bursts in general are notoriously difficult to study, but the shortest ones have been next to impossible to pin down," said Dr. Neil Gehrels, principal investigator for the Swift satellite at NASA's Goddard Space Flight Center, Greenbelt, Md. "All that has changed. We now have the tools in place to study these events," he said.
- Gamma-ray bursts, first detected in the 1960s, are the most powerful explosions known. They are random, fleeting and can occur from any region of the sky. Two years ago, scientists discovered longer bursts, lasting more than two seconds, arise from the explosion of very massive stars. About 30 percent of bursts are short and under two seconds.
- The Swift satellite detected a short burst on May 9, and NASA's High-Energy Transient Explorer (HETE) detected another on July 9. The May 9 event marked the first time scientists identified an afterglow for a short gamma-ray burst, something commonly seen after long bursts.
- "We had a hunch that short gamma-ray bursts came from a neutron star crashing into a black hole or another neutron star, but these new detections leave no doubt," said Dr. Derek Fox, assistant professor of Astronomy & Astrophysics at Penn State University, State College, Pa. Fox is lead author of one Nature report detailing a multi-wavelength observation.
- Fox's team discovered the X-ray afterglow of the July 9 burst with NASA's Chandra X-ray Observatory. A team led by Jens Hjorth, a professor at the University of Copenhagen identified the optical afterglow using the Danish 1.5-meter telescope at the La Silla Observatory in Chile.
Figure 26: An artist's rendering (left) of GRB 050709 depicts a gamma-ray burst that was discovered on 9 July, 2005 by NASA's High-Energy Transient Explorer. The burst radiated an enormous amount of energy in gamma-rays for half a second, then faded away. Three days later, Chandra's detection of the X-ray afterglow (inset) established its position with high accuracy (image credit: X-ray: NASA/CXC/Caltech/D. Fox et al.; Illustration: NASA/D. Berry)
- A Hubble Space Telescope image showed that the burst occurred in the outskirts of a spiral galaxy about 2 billion light years from Earth. This location is outside the star-forming regions of the galaxy and evidence that the burst was not produced by the explosion of an extremely massive star.
- The most likely explanation for GRB 050709 is that it was produced by a collision of two neutron stars, or a neutron star and a black hole. Such a collision would result in the formation of a black hole (or a larger black hole), and could generate a beam of high-energy particles that could account for the powerful gamma-ray pulse as well as observed radio, optical and X-ray afterglows.
- This gamma-ray burst is one of a class of short-duration bursts that now appear to have a different origin from the more powerful, long-duration gamma-ray bursts that last more than two seconds. Long-duration bursts have been connected to black holes formed in the explosion of extremely massive stars, or hypernovas.
• August 26, 2003: NASA has awarded a contract to the SAO (Smithsonian Astrophysical Observatory) in Cambridge, Mass., to provide science and operational support for the Chandra X-ray Observatory, one of the world's most powerful tools to better understand the structure and evolution of the universe. 41)
- The contract will have a period of performance from August 31, 2003, through July 31, 2010, with an estimated value of $373 million. It is a follow-on contract to the existing contract with Smithsonian Astrophysical Observatory that has provided science and operations support to the Observatory since its launch in July 1999. At launch the intended mission life was five years.
- As a result of Chandra's success, NASA extended the mission from five to 10 years. The value of the original contract was $289 million. The follow-on contract with the Smithsonian Astrophysical Observatory will continue through the 10-year mission. The contract type is cost reimbursement with no fee.
- As a result of Chandra's success, NASA extended the mission from five to 10 years. The value of the original contract was $289 million. The follow-on contract with the Smithsonian Astrophysical Observatory will continue through the 10-year mission. The contract type is cost reimbursement with no fee.
- The science data processing tasks include the competitive selection, planning, and coordination of science observations with the general observers and processing and delivery of the resulting scientific data. There are approximately 200 to 250 observing proposals selected annually out of about 800 submitted, with a total amount of observing time of about 20 million seconds.
- Chandra has exceeded expectations of scientists, giving them unique insight into phenomena light years away, such as exotic celestial objects, matter falling into black holes, and stellar explosions.
- X-ray astronomy can only be performed from space because Earth's atmosphere blocks X-rays from reaching the surface. The Chandra Observatory travels one-third of the way to the moon during its orbit around the Earth every 64 hours. At its highest point, Chandra's highly elliptical, or egg- shaped, orbit is 200 times higher than that of its visible- light-gathering sister, the Hubble Space Telescope.
• August 26, 1999: Extraordinary first images from NASA's Chandra X-ray Observatory trace the aftermath of a gigantic stellar explosion in such stunning detail that scientists can see evidence of what may be a neutron star or black hole near the center. Another image shows a powerful X-ray jet blasting 200,000 light years into intergalactic space from a distant quasar. 42) 43) 44)
- Released today, both images confirm that NASA's newest Great Observatory is in excellent health and its instruments and optics are performing up to expectations. Chandra, the world's largest and most sensitive X-ray telescope, is still in its orbital check-out and calibration phase.
- "When I saw the first image, I knew that the dream had been realized," said Dr. Martin Weisskopf, Chandra Project Scientist, NASA's Marshall Space Flight Center, Huntsville, AL. "This observatory is ready to take its place in the history of spectacular scientific achievements."
- "We were astounded by these images," said Harvey Tananbaum, Director of the Smithsonian Astrophysical Observatory's Chandra X- ray Center, Cambridge, MA. "We see the collision of the debris from the exploded star with the matter around it, we see shock waves rushing into interstellar space at millions of miles per hour, and, as a real bonus, we see for the first time a tantalizing bright point near the center of the remnant that could possibly be a collapsed star associated with the outburst."
- After the telescope's sunshade door was opened last week, one of the first images taken was of the 320-year-old supernova remnant Cassiopeia A (Cas A), which astronomers believe was produced by the explosion of a massive star. Material blasted into space from the explosion crashed into surrounding material at 10 million miles per hour. This collision caused violent shock waves, like massive sonic booms, creating a vast 50-million degree bubble of X-ray emitting gas.
- The Cas A Supernova: A supernova occurs when a massive star has used up its nuclear fuel and the pressure drops in the central core of the star. The matter in the core is crushed by gravity to higher and higher densities, and temperatures reach billions of degrees. Under these extreme conditions, nuclear reactions occur violently and catastrophically reversing the collapse. A thermonuclear shock wave races through the now expanding stellar debris, fusing lighter elements into heavier ones and producing a brilliant visual outburst.
- About every fifty years in our galaxy, a massive star explodes. The shell of matter thrown off by the supernova creates a bubble of multi-million degree gas called a supernova remnant. Cas A is a prime example. The hot gas will expand and produce X-rays for thousands of years.
- The nature of the explosion that produced Cas A has been an enigma. Although radio, optical and x-ray observations of the remnant indicate that it was a powerful event, the visual brightness of the outburst was much less than a normal supernova. Apparently Cas A was produced by the explosion of an unusual massive star that had previously ejected most of its outer layers.
- Probing Cas A Mysteries with NASA's Chandra X-ray Observatory: Chandra's spectacularly vivid images of Cas A allow scientists to trace the dynamics of the remnant and its collision with any material ejected by the star before it exploded. Chandra detectors provide scientists with precise x-ray spectra– measurements of the energies of individual x-rays–from the Cas A remnant. These measurements make it possible to identify which heavy elements are present and in what quantities. Chandra's observations should help astronomers to resolve the long-standing mystery as to the nature and origin of Cas A.
- A related mystery is whether the explosion that produced Cas A left behind a neutron star, black hole, or nothing at all. This "First Light" Chandra image of Cas A shows a bright object near the center of the remnant! Longer observations with Chandra can determine if this is the long sought for neutron star or black hole.
Figure 27: Cas A is the remnant of a star that exploded about 300 years ago. The X-ray image shows an expanding shell of hot gas produced by the explosion. This gaseous shell is about 10 light years in diameter, and has a temperature of about 50 million degrees (image credit: NASA/CXC/SAO)
- Importance of Supernovae: The study of remnants of exploded stars, or supernovae, is essential for our understanding of the origin of life on Earth. The cloud of gas and dust that collapsed to form the sun, Earth and other planets was composed mostly of hydrogen and helium, with a small amount of heavier elements such as carbon, nitrogen, oxygen and iron. The only place where these and other heavy elements necessary for life are made, is deep in the interior of a massive star. There they remain until a catastrophic explosion spreads them throughout space.
- Supernovae are the creative flashes that renew the galaxy. They seed the interstellar gas with heavy elements, heat it with the energy of their radiation, stir it up with the force of their blast waves and cause new stars to form.
Figure 28: PKS 0637-752 is so distant that we see it as it was 6 billion years ago. It is a luminous quasar that radiates with the power of 10 trillion suns from a region smaller than our solar system. The source of this prodigious energy is believed to be a supermassive black hole.The X-ray jet observed for the first time by Chandra in PKS 0637-752, is a dramatic example of a cosmic jet. It has blasted outward from the quasar into intergalactic space for a distance of at least 200,000 light years! The jet's presence means that electromagnetic forces are continually accelerating electrons to extremely high energies over enormous distances. Chandra observations, combined with radio observations, should provide insight into this important cosmic energy conversion process (image credit: NASA/CXC/SAO)
Sensor complement (ACIS, HRC, HETG, LETG)
The SIM (Science Instrument Module) consists of the special hardware that provides mechanical and thermal interfaces to the focal-plane scientific instruments (SIs). The most critical functions from an observer's viewpoint are the capability to adjust the telescope focal length and the ability to move the instruments along an axis orthogonal to the optical axis. The SIM houses the two focal instruments - the ACIS and the HRC. Each of these have two principal components — HRC-I and -S and ACIS-I and -S. The focal-plane instrument layout is shown in Figure 30. The SIM moves in both the X-axis (focus) and the Z-axis (instrument and aimpoint selection). Note that the Y-Axis parallels the dispersion direction of the gratings. 45)
The focal-plane instruments are positioned by the SIM Z-axis translation stage with a repeatability to ±0.005 inches over a translation range of 20 inches. The SIM X-axis motion sets the focus to an accuracy of ±0.0005 inches over a range of 0.8 inches. The fine-focus adjustment step is 0.00005 inches.
Figure 29: A schematic of the SIM (Science Instrument Module), image credit: SAO
Figure 30: Arrangement of the ACIS and the HRC in the focal plane. The view is along the axis of the telescope from the direction of the mirrors. For reference, the two back-illuminated ACIS-S chips are shaded. Numbers indicate positions of chips I0-I3 and S0-S5. SIM motion can be used to place the aimpoint at any point on the vertical solid line ( image credit: SAO)
ACIS (Advanced CCD Imaging Spectrometer)
ACIS is an X-ray imager. X-ray photons hitting the camera are detected individually and their position, energy and arrival time recorded. This allows for high resolution (~1 arcsec) imaging, moderate resolution spectroscopy and timing studies. ACIS can also be used with the HETG (High Energy Transmission Grating), and less commonly the LETG (Low Energy Transmission Grating) for high resolution spectroscopy. 46)
ACIS) offers the capability to simultaneously acquire high-resolution images and moderate resolution spectra. The instrument can also be used in conjunction with the HETG or LETG to obtain higher resolution spectra. ACIS contains 10 planar, 1024 x 1024 pixel CCDs (Figure 32); four arranged in a 2×2 array (ACIS-I) used for imaging, and six arranged in a 1×6 array (ACIS-S) used either for imaging or for a grating spectrum read-out. Two CCDs are back-illuminated (BI) and eight are front-illuminated (FI). The response of the BI devices extends to energies below that accessible to the FI chips. The chip-average energy resolution of the BI devices is better than that of the FI devices.
In principle, any combination of up to 6 CCDs can be operated simultaneously. However, because of changes in the thermal environment, the CXC (Chandra X-ray Center) now recommends that fewer CCDs be selected if the science needs can be met with fewer CCDs. Some CCDs can be designated as optional, which means they may be turned off depending on thermal conditions.
The ACIS instrument was built by a team from the Massachusetts Institute of Technology's Center for Space Research and the Pennsylvania State University (PSU) for the Chandra X-ray Observatory.
Figure 31: Top view of the engineering unit of Chandra's Advanced CCD Imaging Spectrometer (ACIS), showing the 2 x 2 CCD ACIS-I and the 1 x 6 CCD ACIS-S focal planes. In the flight unit, aluminized-polyimide optical blocking filters OBF-I and OBF-S cover the I and the S focal planes, respectively. The OBFs lie within the "snoot" (with door opened since on-orbit check-out), which in turn lies within the ACIS "collimator" that envelopes the ACIS cavity (image credit: NASA & ACIS Team) 47) 48)
Figure 32: A schematic drawing of the ACIS focal plane; insight to the terminology is given in the lower left. Note the aimpoints: on S3 (the `+') and on I3 (the `x'). Note the differences in the orientation of the I and S chips, important when using subarrays. Note also the (Y, Z) coordinate system and the target offset convention as well as the SIM motion (±Z). This view is along the optical axis, from the sky toward the detectors, (-X). The numerous ways to refer to a particular CCD are indicated: chip letter+number, chip serial number, and ACIS chip number (CCD_ID), image credit: SAO
HRC (High Resolution Camera)
The HRC is comprised of two microchannel plate (MCP) imaging detectors: the HRC-I designed for wide-field imaging; and, HRC-S designed to serve as a read-out for the LETG. The HRC-I is placed at right angles to the optical axis, tangent to the focal surface. The HRC-S is made of three flat elements, the outer two of which are tilted to approximate the LETG Rowland circle. The HRC detectors have the highest spatial resolution on Chandra, matching the HRMA point spread function most closely. Under certain circumstances, the HRC-S detector also offers the fastest time resolution (16 µs). 49)
The HRC is a direct descendant of the Einstein and ROSAT mission High Resolution Imagers (HRIs). The ROSAT HRI had the same coating (CsI) as the HRC. The Instrument Principal Investigator is Dr. Ralph Kraft of the SAO (Smithsonian Astrophysical Observatory).
One purpose of the second (output) MCP is to provide additional gain. In addition, reversing the direction of the second MCP's bias angle with respect to the first removes a clear path for positive ions, and hence reduces the possibility of (positive) ion feedback - where an accelerated ion moving in the opposite direction as that of the electrons ends up causing the release of electrons and starts the process all over again.
Table 2: Common HRC Configurations: Summary of the three most common HRC operating modes
Figure 33: Photo of the HRC flight unit of Chandra (image credit: NASA)
Figure 34: A schematic of the HRC focal-plane geometry as viewed along the optical axis from the telescope towards the focal plane (image credit: SAO)
Figure 35 illustrates the features of the HRC MCP s. X-rays enter through a UV/Ion shield , necessary to reduce/avoid signals from UV light, ions, and low energy electrons. Most of these X-rays are then absorbed in the CsI-coated walls of the first (input) of two consecutive MCPs. The axes of the millions of tubes that comprise the input and output MCPs are not parallel to the optical axis but are canted ("biased") at an angle of 6° in opposite directions as shown in Figure 7.3. This bias improves the probability of an interaction. The CsI coating enhances the photoemission over that from a bare MCP. The resulting photoelectrons are then accelerated by an applied electric field. The next interaction with the walls releases several secondary electrons and so on, until a cascade of electrons is produced.
Figure 35: A schematic of the HRC Microchannel-Plate detector (image credit: SAO)
HETG (High Energy Transmission Grating)
HETG is used for high resolution spectroscopy of bright sources in the range 0.4-10 keV (31-1.2 Å). The HETG has been used to measure Doppler velocities of orbiting systems, even as low as 50 km/s, and plasma outflow velocities from a few hundred to 10's of thousands of km/s. Because the HETG can clearly resolve lines from O to Fe-K, detailed line diagnostics can be applied.
The HETG is optimized for high-resolution spectroscopy of bright sources over the energy band 0.4-10 keV. It is most commonly used with ACIS-S. The resolving power (E/ΔE) varies from ~800 at 1.5 keV to ~200 at 6 keV.
In operation with the HRMA (High Resolution Mirror Assembly) and a focal-plane imager, the complete instrument is referred to as the HETGS (High-Energy Transmission Grating Spectrometer). The HETGS provides high resolution spectra (with E/ΔE up to 1000) between 0.4 keV and 10.0 keV for point and slightly extended (few arcsec) sources. Although HETGS operation differs from proportional counter and CCD spectrometers, standard processing of an HETGS observation produces familiar spectrometer data products: PHA, ARF, and RMF files. These files can then be analyzed with standard forward-folding model fitting software, e.g., Sherpa, XSPEC, ISIS , etc. 50)
The HETG itself consists of two sets of gratings, each with different period. One set, the Medium Energy Grating (MEG), intercepts rays from the outer HRMA shells and is optimized for medium energies. The second set, the High Energy Gratings (HEG), intercepts rays from the two inner shells and is optimized for high energies. Both gratings are mounted on a single support structure and therefore are used concurrently. The two sets of gratings are mounted with their rulings at different angles so that the dispersed images from the HEG and MEG will form a shallow X centered at the undispersed (zeroth order) position; one leg of the X is from the HEG, and the other from the MEG. The HETG is designed for use with the spectroscopic array of the Chandra CCD ACIS-S (Advanced Imaging Spectrometer) although other detectors may be used for particular applications.
LETG (Low Energy Transmission Grating)
LETG ) was developed under the direction of Dr. A. C. Brinkman in the Laboratory for Space Research (SRON) in Utrecht, the Netherlands, in collaboration with the Max-Planck-Institut für Extraterrestrische Physik (MPE) in Garching, Germany. The grating was manufactured in collaboration with Heidenhaim GmbH. 51)
The LETG provides the highest spectral resolving power (E/ΔE > 1000) on Chandra at low energies (0.07 - 0.2 keV). The LETG/HRC-S combination is used extensively for high resolution spectroscopy of bright, soft sources such as stellar coronae, white dwarf atmospheres and cataclysmic variables.
LETGS (Low Energy Transmission Grating Spectrometer) comprises the LETG, a focal-plane imaging detector, and the High Resolution Mirror Assembly. The Chandra High Resolution Camera spectroscopic array (HRC-S) is the primary detector designed for use with the LETG. The spectroscopic array of the Chandra CCD Imaging Spectrometer (ACIS-S) can also be used, though with lower quantum efficiency below ~0.6 keV and a smaller detectable wavelength range than with the HRC-S. The High Energy Transmission Grating (HETG) used in combination with ACIS-S offers superior energy resolution and quantum efficiency above 0.78 keV.
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).