HaloSat (Soft X-ray Surveyor)
HaloSat is a 6U CubeSat astronomical science mission of NASA that will measure soft X-ray emissions from the halo of the Milky Way galaxy. The sum of baryons observed in the local universe falls short of the number measured at the time of the cosmic microwave background—the "missing baryon" problem. HaloSat should help determine if the missing baryons reside in the hot halos surrounding galaxies. 1) 2)
The HaloSat mission is led by the University of Iowa with Philip Kaaret as PI (Principal Investigator). The team from the University of Iowa in collaboration with NASA/GSFC ( Goddard Space Flight Center), and JHU/APL (Johns Hopkins University/Applied Physics Laboratory) will be developing the science instrument for the HaloSat mission. BCT (Blue Canyon Technologies) of Boulder, CO, will build the 6U CubeSat. Nagoya University of Japan is responsible for the Scattering measurement. 3)
Mission goal: Measuring the mass of the X-ray halo in our Galaxy.
• 6U CubeSat with a size of 10 x 20 x 34 cm3
• Observation:~75 % of the sky in 6 months
• FOV (Field of View): ~10º
• Three SDDs (Silicon Drift Detectors): ~80 eV @ 0.45 keV
• Iowa University (PI: Kaaret Philip), NASA/GSFC, Nagoya University.
Scientific Motivation: Observation of the Soft X-ray Sky.
- SXDB (Soft X-ray Diffuse Background) in the range 0.1 - 2 keV
- Soft X-ray sky is still full of mysteries
Scientific Goal (1) : Missing baryon problem
- Only ~5 % of the energy density in the Universe is in a form of baryons
- Census of baryons in the Universe today finds ~2/3 of that number
- To measure the mass of the X-ray-emitting hot gas halo in our Galaxy is important in cosmology.
Scientific Goal (2) : M band problem
- SXDB should reduce in the Galactic disk due to an absorption. However, no reduction is observed (M band problem).
- Existence of an unresolved SXDB was suggested in the Galactic disk.
Halosat allows us to study a spatial distribution of "each" SXDB component.
Table 1: Mission goal and science requirements
Figure 1: Scientific motivation: where are the missing Baryons? (image credit: University of Iowa, Ref. 2)
Operate the 6U CubeSat for 213 days (required) to 365 days (goal).
Some background: Baryonic matter makes up almost 5% of the total massenergy of the Universe today. However, observations of luminous matter fail to locate a substantial fraction of the predicted baryons. One of the possible reservoirs of the missing baryons may be halos of hot gas surrounding galaxies. The closest such hot halo is the one extending around our Milky Way galaxy. The gas within this halo is at a temperature of ~106 K. 4) Thus, it should readily emit in the X-ray band. 5)
Line emission and absorption from highly ionized species present in the gaseous halo is the primary diagnostics to study such hot gas. Using absorption lines as the diagnostic tool can only probe the properties of the halo along a limited number of lines of sight simply because the number of X-ray bright extragalactic sources is limited. In contrast, emission lines can be measured in any direction and thus provide a means to study the full geometry of the halo. In particular, emission from highly ionized oxygen, O+6 (OVII) and O+7 (OVIII), can be used as a diagnostic tool for studying the properties of the hot galactic halo. The scientific goal of the HaloSat mission is to constrain the mass and spatial distribution of the hot gas that surrounds our Milky Way by mapping the emission from OVII (561 eV) and OVIII (653 eV).
In 2016, BCT (Blue Canyon Technologies) has been awarded a contract to build and test a new 6U-class satellite. Funded by NASA's Science Mission Directorate, BCT will deliver the spacecraft bus, ready for instrumentation, to the HaloSat project, which is funded by NASA/GSFC (Goddard Space Flight Center) Wallops Flight Facility. 6)
BCT's XB spacecraft is a high-performance small satellite that includes an ultra-precise attitude control system that allows for accurate knowledge and fine-pointing of the satellite payload. This mission builds upon the success of BCT's recent performance on the MinXSS spacecraft, which is completing its 6th month on-orbit. BCT is currently working on over fifteen different spacecraft missions that use its high-performance XB spacecraft bus.
HaloSat is a CubeSat that will measure soft X-ray emissions from the halo of our Milky Way galaxy. The sum of baryons observed in the local universe falls short of the number measured at the time of the cosmic microwave background—the "missing baryon" problem. HaloSat should help determine if the missing baryons reside in the hot halos surrounding galaxies. The HaloSat mission is led by the University of Iowa Principal Investigator (PI), Philip Kaaret. The team from the University of Iowa in collaboration with NASA/GSFC, and Johns Hopkins University will be developing the science instrument for the HaloSat mission.
Located in Boulder, Colorado, the company is developing a new environmental test facility, increasing manufacturing output, and creating a satellite ground-station for single to constellation-sized missions. Working with a grant from the state of Colorado, BCT has been increasing its spacecraft production and test capabilities. BCT is now providing end-to-end mission solutions for their customers.
• Avionics system is XACT (fleXible ADCS Cubesat Technology) from Blue Canyon Technologies.
• Provides high performance pointing system in a 0.5 U package including star trackers, reaction wheels, inertial measurement unit, magnetometers, torque rods.
Figure 2: The XACT unit uses high performance components that can be used for a wide range of missions (image credit: BCT)
Table 2: Parameters of XACT
The spacecredft bus of BCT has a 6U format with with roughly 4U of the volume allocated for the science payload, 1.5U of the volume allocated for the spacecraft avionics, and 0.5U of the volume allocated for the payload-to-spacecraft interface. Power is provided by the deployable solar panels. The bus has an attitude control system with star trackers, reaction wheels, and torque rods that can be slewed at 2°/sec and point at inertial targets with an accuracy of ±0.002°(1σ). An onboard CADET radio is used to downlink telemetry to and receive commands from a ground station at NASA Wallops Flight Facility. A GlobalStar radio provides occasional housekeeping information.Figure 3 shows the spacecraft bus with the science payload integrated with the instrument cover removed.
Figure 3: Science instrument integrated with the avionics in the flight bus chassis. PCBs for analog electronics and DPUs are on the top of instrument chassis with the alignment washers and a cover over the optical alignment mirror at the front. The XB1 bus is on the left. The solar array had not been attached at this point in the integration (image credit: BCT, University of Iowa)
Figure 4: Illustration of the 6U CubeSat (image credit: BCT, University of Iowa)
Figure 5: Illustrationof the 6U CubeSat configuration (image credit: BCT)
Figure 6: Members of the HaloSat team during integration at BCT. HaloSat is visible near the center Image credit: BCT)
Figure 7: Artist's rendition of the deployed nanosatellite (image credit: BCT)
Launch: The HaloSat CubeSat was launched on 21 May 2018 (08:44 UTC) on the Cygnus CRS-9E flight of Orbital ATK (OA-9E), ELaNa-23 flight of NASA to the ISS. The launch vehicle was Antares 230 and the launch site was MARS (Mid-Atlantic Regional Spaceport) LP-0A, Wallops Island, VA, USA. 7)
Orbit: Near circular orbit, altitude of ~400 km, inclination = 51.6º.
Figure 8: The Orbital ATK Antares rocket, with the Cygnus spacecraft onboard, launched from Pad-0A, Monday, May 21, 2018 at NASA's Wallops Flight Facility in Virginia. Orbital ATK's ninth contracted cargo resupply mission with NASA to the International Space Station will deliver approximately 3352 kg of science and research, crew supplies and vehicle hardware to the orbital laboratory and its crew (image credit:NASA/Aubrey Gemignani)
The ELaNa 23 (Education Launch of Nanosatellites 23) initiative payloads of NASA on OA-9E are: 8)
• HaloSat (Soft X-ray Surveyor), a 6U CubeSat of the University of Iowa (12 kg), Iowa City, Iowa.
• TEMPEST-D1 (Temporal Experiment for Storms and Tropical Systems Technology - Demonstration 1) , a 6U CubeSat of CSU (Colorado State University), Fort Collins, CO.
• EQUISat, a 1U CubeSat of Brown University, Providence, R.I.
• MemSat, a 1U CubeSat of Rowan University, Glassboro, N.J.
• CaNOP (Canopy Near-IR Observing Project), a 3U CubeSat of Carthage College, Kenosha, WIS, USA.
• RadSat, (Radiation-tolerant SmallSat Computer System), a 3U CubeSat of MSU (Montana State University), Bozeman, Montana.
• RaInCube (Radar In a CubeSat), a 6U CubeSat of NASA/JPL (Jet Propulsion Laboratory), Pasadena, CA.
• SORTIE (Scintillation Observations and Response of the Ionosphere to Electrodynamics), a 6U CubeSat of ASTRA (Atmospheric & Space Technology Research Associates), Boulder, CO.
• CubeRTT (CubeSat Radiometer Radio Frequency Interference Technology) Validation Mission , a 6U CubeSat of OSU (Ohio State University), Columbus, Ohio.
• AeroCube-12A and -12B, a pair of 3U CubeSats of the Aerospace Corporation, El Segundo , CA, to demonstrate a the technological capability of new star-tracker imaging, a variety of nanotechnology payloads, advanced solar cells, and an electric propulsion system on on one of the two satellites (AC12-B).
• EnduroSat One, a 1U CubeSat of Bulgaria, developed by Space Challenges program and EnduroSat collaborating with the Bulgarian Federation of Radio Amateurs (BFRA) for the first Bulgarian Amateur Radio CubeSat mission.
• Lemur-2, four 3U CubeSats (4.6 kg each) of Spire Global Inc., San Francisco,CA.
On 16 July 2018, the Cygnus spacecraft of Orbital ATK (OA-9E) flight was raised to over 480 km after departing the International Space Station before further CubeSats were released. NanoRacks deployed the following satellites: 9)
- Lemur-2 (Four 3U CubeSats) of Spire Global, Inc.
- AeroCube 12A and AeroCube-12B of The Aerospace Corporation.
Figure 9: The University of Iowa HaloSat team attended the satellite's launch at NASA's Wallops Flight Facility. From left to right: Daniel LaRocca, Anna Zajczjk, Philip Kaaret, William Fuelberth, Hannah Gulick and Emily Silich. Kay Hire (center) holds the University of Iowa's tiki totem statue (image credit: Alexis Durow, Ref. 25)
• October 2019: A detailed census of the local universe fails to detect a third of the baryonic matter expected based on observations of the early universe. These missing baryons are thought to be in the form of hot (>106 K) ionized gas, which is difficult to quantify using existing observations. Large volumes of hot ionized gas are present in the halos surrounding galaxies. Measuring the mass and geometry of the halo of our Milky Way Galaxy can be used to help understand the extent to which galactic halos could account for the missing baryon problem. 10)
- The science objective of HaloSat is to help determine the baryonic content present in the halo of the Milky Way by measuring the soft X-ray line emission from highly ionized oxygen. The science instrument aboard HaloSat contains 3 co-aligned X-ray sensitive silicon drift detectors, which are responsive to the 0.4-7 keV energy band, which contains two prominent features from O VII (0.574 keV) and O VIII (0.654 keV). HaloSat is conducting an all sky survey using 333 targets (Figure 10). With the oxygen emission measurements from these targets, it is possible to determine the shape of the halo and make an estimate of the mass.
- It is necessary to observe astrophysical X-ray emission from space because the atmosphere prevents X-ray radiation from penetrating to the Earth's surface. Sounding rocket missions do not provide the required exposure time for an extensive observation of the halo, and balloon-borne missions would not be sensitive at the low energies necessary to measure oxygen emission from the halo.
- Existing X-ray observatories have been used to measure oxygen emission from the halo but are ill suited for efficient study of the diffuse emission. HaloSat was designed with a large (10° diameter circle) field-of-view and no optics to efficiently observe diffuse emission from the halo over the entire sky in a relatively short period of time. Furthermore, as a mission with a dedicated purpose, HaloSat employs observing strategies that minimize foreground contamination from magnetospheric and heliospheric solar wind charge exchange.
- HaloSat also observes low-luminosity fields at specified line-of-sight observing geometries to better model solar wind charge exchange (SWCX) emission. HaloSat is taking secondary science observations of the diffuse emission from objects such as the North Polar Spur, the Galactic Bulge, and the Cygnus Superbubble.
Figure 10: The accumulated observations from HaloSat data for the 333 targets measured in detector seconds. The projection is shown in Galactic coordinates with the Galactic center at the origin. The targets are mapped over the ROSAT all sky survey 0.75 keV map 11) (image credit: University of Iowa)
- The team at the University of Iowa acts as the Science Operations Center. We designed, built, and tested the science instrument, dictate science operations (including scheduling targets to observe), and analyze the data. BCT is serving as the Mission Operations Center. Using the Wallops Flight Facility Ground Station, BCT uplinks commands, downlinks data, and maintains the health and well-being of the spacecraft.
- HaloSat observes astrophysical targets for roughly half of each orbit. After observing, the instrument is powered off until the next orbit, and the spacecraft sun-points to recharge the batteries. For each orbit, HaloSat observes two targets for 1300 s each using all three X-ray detectors. These same two targets are observed for multiple consecutive orbits (10 consecutive orbits before 19 May 2019 and 16 consecutive orbits after). Observing the same targets for consecutive orbits helps reduce the number of commands required to operate the spacecraft.
- HaloSat completes several observations which are useful for on-orbit energy calibration and monitoring the performance of the spacecraft. Observations of the dark Earth are used to measure the instrumental lines from aluminum and silicon. To calibrate the energy scale, we use the strong emission lines from elements such as Mg, Si, S, and Ar from observations of Cassiopeia A. To check the effective area of HaloSat, we use observations of the Crab nebula.
- To verify the alignment of the detectors, we conducted a test where a bright source was centered in the expected field-of-view and the spacecraft performed eight pointing maneuvers where the boresight was offset from the source with a fixed roll angle. This allowed us to determine an offset from the expected boresight and correct it for our pointings after December 1, 2018.
- Due to its inclination, HaloSat frequently passes through the South Atlantic Anomaly (SAA), which is the region over South America where the spacecraft is bombarded by large amounts of high energy protons. This is known to cause high count rates and can damage electronic components. To reduce data rates, the HaloSat science instrument does not collect event data during transits of the SAA.
- To maximize scientific output, HaloSat observations are scheduled using strategies aimed at reducing contamination in our observations. Observations are scheduled with an Earth avoidance angle greater than 20° because the upper atmosphere can reflect and emit X-rays. The moon blocks X-ray radiation and thus all observations must be greater than 10° away from the moon. To prevent light from illuminating the front of the instrument, observations are taken with solar angles greater than 90°.
- Halo observations are contaminated by foreground oxygen emission produced by SWCX (Solar Wind Charge Exchange), where energetic particles from the solar wind interact with neutral atoms, exchange charge, and emit X-rays. When the source of neutral atoms is the geocorona of the Earth, the resulting SWCX is termed magnetospheric or MSWCX. When the source of neutral atoms is the interstellar medium, it is termed heliospheric or HSWCX. To minimize MSWCX contamination, observations are taken when targets have solar angles >110° of the antipode or the point on the sky opposite the sun. To avoid HSWCX contamination, observations are avoided in the direction with the shortest line-of-sight distance to the Parker Spiral. 12)
- To help characterize the emission from HSWCX and MSWCX, HaloSat also conducts observations of the same target with varying line-of-sight geometries expected to have different contributions from SWCX. To study the MSWCX, observations are taken down the tail (near the antipode) and through the flanks of the magnetosheath (>70° from the antipode). To study HSWCX emission, observations through varying interstellar neutral helium density sightlines are taken sporadically as the Earth transits through the helium focusing cone. 13)
- In summary, HaloSat continues to operate well in its all-sky survey, examining the diffuse X-ray emission from astrophysical sources. Analysis of the observations taken with HaloSat should scaffold our understanding of the Milky Way halo and help constrain its mass and geometry. Diffuse emission from objects such as the Galactic bulge, the North Polar Spur, and the Cygnus Superbubble has been observed and analysis is currently underway. HaloSat observations tailored to characterize the SWCX should advance our understanding of the contaminating emission in our local environment.
• September 24, 2019: The HaloSat team is excited to announce that our mission operations have been extended until June 30, 2020! HaloSat is operating well and we look to continue to observe targets in the X-ray sky. We plan to take deeper observations of the Halo of the Milky Way. We will also take more observations to characterize the X-ray emission from solar wind charge exchange as well as the diffuse X-ray emission near the galactic center. Look for more updates soon! 14)
• August 2019: HaloSat is a 6U CubeSat that is currently performing an all-sky survey of line emission from highly ionized oxygen with the goal of measuring the baryonic mass of the Milky Way's halo. As of the summer of 2019, the HaloSat spacecraft and science instrument are operating well and the primary mission of mapping soft X-ray line emission from highly ionized oxygen is underway. 15)
- Mission goals: Baryons are particles with three quarks. The only stable baryons are protons and also neutrons bound within atomic nuclei. The cosmic microwave background shows the universe when it was 400,000 years old and reveals that the universe was homogeneous with a temperature near 3000 K and that baryons constituted 4.87%±0.18% of the total mass/energy present. In the present-day universe, baryons are present at many different temperatures, complicating the task of identifying them all. The best available census avails only two thirds of the baryons seen in the early Universe. The missing baryons are thought to be gas at millions of degrees Kelvin. They may be in halos gravitationally bound to individual galaxies, in filaments stretching between galaxies, or some combination.
- The primary science goal of HaloSat is to estimate the mass of the hot halo surrounding our Milky Way galaxy. At temperatures near 106K, the most cosmically abundant elements, hydrogen and helium, are completely ionized. HaloSat is designed to measure line emission from oxygen, the third most abundant element. Oxygen at 106K is highly ionized with only one or two electrons remaining bound and produces strong emission lines near 574 eV (a triplet of lines from six times ionized oxygen denoted O VII) and 654 eV(a doublet from O VIII)in the soft X-ray range. The Milky Way's halo fills the entire sky, thus very modest angular resolution of 15° or less is required to map the emission. HaloSat will survey at least 75% of the sky with a goal of surveying the entire sky.
- The figure of merit for observing diffuse emission is a telescope's field of view times its effective area, or ‘grasp'. HaloSat uses three small detectors. Each has an effective area for X-rays of 600 eV of about 8 mm2, roughly the size of the pupil of a human eye. However, HaloSat's field of view is near 100 square degrees, enabling it to efficiently survey the sky. The grasp of HaloSat is 26 cm2deg2. This is about 20 x the grasp of the Chandra X-ray Observatory, NASA Great Observatory for the X-ray band, and about 1/12th the grasp of ESA's X-ray Multi-Mirror Mission XMM-Newton. Thus, for survey efficiency, a CubeSat can be competitive with a major space observatory.
- The accuracy of current emission line measurements of the halo is limited by foreground oxygen emission produced by SWCX (Solar-Wind Charge Exchange), when energetic particles in the solar wind exchange charge with neutral atoms within the solar system. HaloSat observes towards the anti-Sun direction during the nighttime half of its 93-minute orbit around Earth to minimize this foreground. This is not possible with XMM-Newton, because it has a fixed solar array that restricts observations to a Sun angle range of 70°-110°. Also, HaloSat has a secondary science goal to improve our understanding of SWCX emission and conducts observations specifically devoted to this goal.
- HaloSat began science operations in October 2018. The longer than expected commissioning phase was largely due to communications issues, subsequently resolved with achievement of our design downlink speed of 3 Mbit/s, and issues with the simultaneous commissioning of the three CubeSats with nearly identical orbits using a single ground station. BCT runs mission operations including command preparation and uplink, telemetry downlink, spacecraft bus state of health monitoring, and fault recovery. The University of Iowa prepares the science observing program, monitors the instrument state of health, and processes all science data. Science observations are carried out during the night-side half of each spacecraft orbit with two targets observed for about 1300 seconds each.
- The Crab is a pulsar wind nebula powered by a young pulsar with a spin period of about 33 milliseconds. The Crab has been used as a calibration target since the early days of X-ray astronomy. We used the Crab to measure the alignment between the boresights of the X-ray instruments and the coordinate system defined by the star trackers on the spacecraft bus.
- We performed a series of slew maneuvers in which the science instrument was pointed towards the Crab and then the pointing was gradually offset while the spacecraft roll angle was held fixed. Eight different maneuvers were performed corresponding to eight different roll angles at equal intervals in the spacecraft frame. The X-ray count rate versus offset data were fitted to a model matching the FoV measured on the ground with the FoV center being a fitted parameter. The count rate for DPU 54 versus radial offset from the fitted center for the best fitted model are shown in Figure 11. We found an offset of about 1.0° in the spacecraft Y direction from the nominal pre-flight instrument boresight. This correction was applied to the pointing of observations obtained after 1 December 2018. After the correction, another pointing test was performed and the fitted FoV center is consistent with the expected position within ±0.09° in the spacecraft X direction and ±0.18° in the spacecraft Y direction for all DPUs. We conclude that the pointing of the X-ray boresight of HaloSat is accurate to ±0.2°, which is a small fraction of the FoV. The X-ray pointing uncertainty is dominated by the accuracy to which we are able to measure the relative alignment between the X-ray detectors and the spacecraft reference frame. The median offset between the commanded target position during observations and the spacecraft pointing measured by the attitude control system is 0.0007°, see Figure 12.
Figure 11: X-ray count rate versus pointing offset from the Crab for DPU 54 (image credit: HaloSat Team, University of Iowa)
Figure 12: Histogram of pointing offsets in 8-second intervals (image credit: HaloSat Team, University of Iowa)
- Spectral Response: To check the on-orbit X-ray energy scale calibration, we examined spectra obtained while observing the dark side of the Earth, Cassiopeia A, and the Vela supernova. The dark Earth observations show an aluminum line likely due to fluorescence by energetic particles. The fitted centroids of that line are consistent with no shift in the energy response relative to the ground calibration with statistical accuracies of 0.4% to 1.4% for the different DPUs.
- Cassiopeia A is a young SNR (Supernova Remnant) with an age of about 300 years and has strong emission lines from heavy elements in its X-ray spectrum. X-ray emission lines from Mg, Si, S, and Ar were first detected with the solid-state spectrometer on Einstein and first mapped with ASCA (Advanced Satellite for Cosmology and Astrophysics) of NASA. Cassiopeia A has been used to calibrate the energy scale of several X-ray instruments.
- The HaloSat field centered on Cassiopeia A includes another SNR, CTB 109, and several point sources, but the emission is dominated by Cassiopeia A. We extracted spectra of the Cas A for all three DPUs, see Figure 13, and fitted them in the 1.0-3.5 keV range with a model consisting of a powerlaw and four Gaussians with line energies fixed to 1.8558 keV (Si XIII), 2.4515 keV (S XV), 2.0053 keV (S XIV), and 2.1830 keV (Si XIII). Line energies were extracted from the AtomDB database of atomic transitions and centroids for blends were calculated from their relative intensities (http://atomdb.org). The continuum X-ray spectrum of Cassiopeia A is typically described as the sum of two thermal plasma components and a powerlaw, but a single powerlaw produces an adequate fit over the limited energy band used in the fit. The line widths are consistent with the energy resolution measured during the ground calibration. Allowing the slope of the channel to energy conversion to vary reduced the χ2/DoF of the fit from 611.9/366 to 589.8/363 and resulted in gain slope correction factors of 1.0022-0.0033 +0.0009, 1.0027±0.0007, and 1.0031±0.0008 for DPU 14, 54, and 38 respectively. This may suggest a small change, 0.3% or less, in the ADC channel to energy conversion from the ground calibration to flight.
- We chose to use the temperature-averaged ground calibration for the analysis presented below.
Figure 13: X-ray spectra of the Cassiopeia A field. Data from all three detectors are shown as indicated by the DPU number in the legend, 14=black, 54=red, 38=green. Prominent emission lines are visible from Si XIII at 1.86 keV and 2.18 keV, S XV at 2.45 keV, and S XIV at 2.01 keV. These spectra use the ground energy scale calibration with no temperature correction applied (image credit: HaloSat Team)
- The Vela SNR is one of the brightest soft X-ray sources and has a diameter of 8°. We extracted spectra in the 0.5-3.0 keV band for each DPU for a field centered on Vela and including the Puppis A SNR, see Figure 8. The spectra were fitted with a model consisting of absorbed cool and hot thermal plasma components modeled using the apec and vapec models in Xspec, respectively, and an absorbed broken powerlaw for the CXB (Cosmic X-ray Background), and a powerlaw for the instrumental background. Allowing the slope of the channel to energy conversion to vary reduced the χ2/DoF of the fit from 610.9/362 to 583.0/359 and the gain slope correction factors were, again, 0.3% or less with 0.9999±0.0001, 1.0020±0.0003, and 1.0030±0.0002 for DPU 14, 54, and 38, respectively.
Figure 14: X-ray spectra of the Vela SNR (Supernova Remnant) field (image credit: HaloSat Team)
- Effective Area: We also use the Crab to calibrate the effective area of HaloSat. The Crab is often used as a ‘standard candle' in X-ray astronomy. However, it does exhibit variability of up to 7% in the 10-100 keV band on long time scales.
- We extracted Crab spectra for each detector unit and applied the flight energy calibration described in the previous section. Due to HaloSat's large FoV, the spectra also contain diffuse emission, so we extracted a background spectrum from a nearby region centered at (α, δ) = (82.64°, 34.01°) (J2000) with a similar level of diffuse emission and no bright X-ray point sources. The Crab spectrum is well modeled as a simple absorbed powerlaw. We used the response matrices and gain corrections discussed previously and the tbabs model in Xspec to describe the interstellar absorption.
- The science of HaloSat is focused on line emission in the 0.5-2.0 keV band. The Crab flux in that band depends on all of the model parameters, so we prefer to directly compare observed fluxes rather than model parameters. Most of the previously published results on Milky Way halo emission use XMM-Newton which has two imaging instruments, the EPIC-MOS and the EPIC-pn. Unfortunately, the MOS suffers from pileup during observations of the Crab and the only Crab normalizations reported for XMM are for the EPIC-pn. We adopt measurements of the Crab spectrum with the EPIC-pn, in particular the model calculated by Kirch and coauthors using Wilms abundances and Verner crosssections, giving a flux of 9.35 x10-9 erg cm-2 s-1, to calibrate the effective area of HaloSat. 16) This flux is within 4% of the Crab flux of model 3* of Weisskopf and coauthors for the EPIC-pn. 17)
- We apply a correction factor to the auxiliary response files described above to bring the HaloSat fluxes into agreement with the Crab flux given by Kirch and collaborators. We chose not to adjust any other response matrix parameters using the Crab spectrum as the strong interstellar absorption makes such tuning problematic for our energy band of interest. The fluxes for all three DPUs are consistent within the measurement error of 2.5%, so a single correction factor is used.
Initial halo science results
- The scientific goal of HaloSat is to constrain the mass and spatial distribution of hot gas associated with the Milky Way by mapping the emission in the O VII and O VIII lines. The observational goal is to reach a statistical accuracy of ±0.5 LU on the sum of the O VII and O VIII line emission for fields with a line strength near 5 LU (where LU = line unit = photon cm-2 s-1 ster-1).
- Figure 15 shows a spectrum obtained for a high Galactic latitude field at (l= 166°, b= 62°) from an observation with an exposure of 40 ks after background and data quality screening. The gain and effective area corrections described above were applied and the counts from all three detectors were summed.
- We fitted the data with a model consisting of Gaussians at 568.4 and 653.7 eV for the O VII and O VIII line emission and an absorbed thermal plasma with oxygen line emission removed.20 We added an absorbed double broken powerlaw for the CXB.21 The first broken powerlaw has a break energy of 1.2 keV, a photon index below the break of 1.54, a photon index above the break of 1.4, and a normalization at 1 keV fixed to 5.7 photon cm-2 s-1 keV-1 ster-1. The second broken powerlaw has the same parameters except the photon index below the break is 1.96 and the normalization is free. We also added a powerlaw not modified by the response matrix for particle background.
- We obtained a good fit with χ2/DoF = 188.8/172. The free CXB normalization is 5.1±2.3 photon cm-2 s-1 kev-1 ster-1 and the total CXB flux is in good agreement with previously measured values, which provides a confirmation of our flux normalization. 18) The O VII flux is 3.26±0.41 LU. The statistical accuracy meets our observational goal.
- In order to measure the properties of the halo, we must conduct similar observations over a large fraction of the sky, analyze the data, and then fit our measurements of the oxygen line intensities to models of the halo to infer its properties. Our goal is to survey the entire sky, although only fields above and below the Galactic plane will be useful in constraining the properties of the halo.
- We have selected 333 observation fields that tile the sky given HaloSat's field of view, see Figure 16.
- As of the summer 2019, we have surveyed approximately one third of the sky with deep coverage and one third with shallower coverage. We should be able to survey the entire sky with the operations currently planned and funded until October 2019. We have applied for funding for a mission extension to June 2020 that would enable us to perform deeper observations, including a second measurement of the helium focusing cone as described in the next section.
Figure 15: Spectrum of a halo field at (l = 166º, b = 62º) observed for 40 ks of good time with counts from all three detectors summed. The lowest line at 1 keV is the instrumental background, the middle curve is the astrophysical spectrum, the top curve is the sum (image credit: HaloSat Team)
Figure 16: Observations obtained to date with HaloSat. Fields already observed are shown as circles (smaller than the field of view) with color indicating the total exposure obtained. Stars mark fields that have not yet been observed. The plot is in Galactic coordinates with the center of the Milky Way at the center and Galactic North towards the top. The image is a map of diffuse soft X-ray emission from the ROSAT observatory (image credit: HaloSat Team) 19)
Solar wind charge exchange
- SWCX (Solar Wind Charge Exchange) emission occurs when a highly charged ion of the solar wind picks up an electron from a neutral atom forming an excited ion that decays by emitting an X-ray.20) SWCX emission is produced within Earth's magnetosheath and throughout the heliosphere. In the magnetosphere, the neutral targets are H atoms in the Earth's exosphere. In the heliosphere, the targets are interstellar H and He atoms flowing through interplanetary space. The SWCX line flux is the integral over the line of sight of the product of the ion density, the neutral density, the relative velocity between the two, the charge exchange cross-section, and the individual line emission probability. The magnetosheath responds rapidly to changes in the solar wind flux, so its SWCX emission is strongly time-variable and dependent on observation geometry. The heliospheric emission is integrated over a long line of sight, effectively over a month of solar wind conditions, and varies more slowly.
- SWCX emission is currently the dominant uncertainty in the oxygen line intensity measurement of the halo21).25 Our observing strategy, discussed above, should minimize SWCX contamination. We are also making observations specifically to study SWCX that should improve the accuracy with which we can model the remaining SWCX emission and, thus, improve the accuracy of our measurements of the Milky Way's halo.
- The distribution of heliospheric emission is determined by the geometry of the target gas. Neutral interstellar gas flows at ~25 km/s through the Solar System, see Figure 17. This gas, mostly hydrogen but with ~15% helium, flows from the Galactic direction (l ~ 3°, b ~ 16°), placing the Earth downstream of the Sun in early December. The flow of interstellar hydrogen is affected by both radiation pressure and gravity, and the hydrogen becomes strongly ionized through charge exchange with solar protons and photo-ionization so that the hydrogen is denser upstream than downstream. In contrast, the interstellar helium flow is not strongly ionized but is affected mainly by gravity, which focuses the flow downstream of the Sun into the "He-focusing cone." As seen from the Sun, the heliospheric SWCX emission appears roughly axisymmetric around the interstellar wind axis, save for latitudinal variations due to the anisotropy of the solar wind flux. For satellites in low Earth orbit, the change in vantage point as the Earth orbits the Sun induces parallax effects to the heliospheric intensity.
Figure 17: He-focusing cone. The figure shows emissivity within the ecliptic plane of heliospheric O VII solar wind charge exchange (SWCX). The Sun is at the center of the figure and distances are marked in astronomical units (AU) equal to the Sun-Earth distance. The ellipse shows Earth's orbit (image credit: HaloSat Team)
- The heliospheric OVII and OVIII emission are calculated from the interstellar neutral H and He distributions and measurements of the solar wind provided by solar and heliospheric observatories. A crucial input to this modeling is knowledge of the O-He interaction cross section. 22) HaloSat can perform such a measurement by observing along the H-focusing cone as the Earth passes through and then correlating the observed soft X-ray emission with the He distribution along the line of sight. We performed a series of such measurements when HaloSat (and the Earth) passed through the He-focusing cone in December 2018 with observations made at monthly intervals from two months before the passage to two months after. Our preliminary spectrum from one month before passage is shown in Figure 18.
- The spectrum was fitted with a model of the heliospheric emission which has only the normalization as a free parameter. The model also included a broken powerlaw for the CXB and a powerlaw for the instrumental background. The SWCX emission model fits very well except that the Mg XI line at 1.35 keV is stronger in the model than the data. This likely indicates that the Mg ions charge-exchanging to produce this line were less abundant in the solar wind than the standard values in the reference spectrum. The HaloSat spectrum shows that HaloSat can accurately measure SWCX emission. These observations cannot be done by any other current observatory. These HaloSat measurements will provide an accurate measurement of the O-He cross section and an absolute scale for the SWCX models.
Figure 18: HaloSat X-ray spectrum of solar wind charge exchange emission in the He-focusing cone taken one month before the Earth passed through the cone. The residuals indicate an underabundance of Mg XI ions (image credit: HaloSat Team)
- In summary, HaloSat has demonstrated that CubeSats can be the effective vehicles for astrophysics research. The commercialization of small satellite technologies enabled construction of HaloSat at a modest cost of $3.7M. This required accepting already engineered solutions with few or no modifications and adapting the instrument design to existing capabilities and interfaces of the commercial bus and components but resulted in a cost far below what would have been required to develop all of the components from scratch and also lower risk due to the heritage of the subsystems. The success of HaloSat should encourage construction of more CubeSats for astrophysics using a commercial bus that enables the science team to focus their efforts in instrument development where their expertise lies.
- One of NASA's key astrophysics science objectives is to understand the origin and destiny of the universe. HaloSat should enable a significant advance in our understanding of the geometry of the hot halo of the Milky Way by distinguishing between extended and compact halo models and constraining the baryonic mass of the Milky Way.
- NASA's key heliophysics science goal is to understand the Sun and its interactions with the Earth and the solar system. HaloSat will provide a unique data set for the study of the interaction of the solar wind with the heliosphere and the magnetosphere via solar wind charge exchange emission and enable a new measurement of the O-He interaction cross-section.
- The HaloSat program has helped train the next generation of scientists and engineers who will execute NASA's future missions. Three graduate students, two of whom are writing theses on HaloSat, and nine undergraduates in physics, astronomy, and engineering, several of whom have won Iowa Space Grant Scholarship and one of whom was named the University of Iowa's second most influential undergraduate in 2019 by College Magazine, have worked on HaloSat. HaloSat has trained two postdoctoral research scientists, one of whom has moved to a research scientist position at NASA/GSFC.
• July 13, 2019: Happy Birthday Halosat! Today marks one year since our instrument was deployed from the ISS into orbit. After one year and many offerings to our Halosat Tiki, Halosat has had 612 successful observations totaling over 11 GB of data. Plotted above is our current observation data showing the number of seconds observed per target in the X-ray sky. We've completed 270/333 targets for 81% of the sky and will look to fulfill observations in the forthcoming months. 23)
• August 18, 2018: The HaloSat science instrument was turned on for the first time on-orbit on August 18. All three detectors are working great! We turned on during a pass while the instrument was pointed at the Earth's atmosphere and the spectrum recorded by HaloSat (Figure 19) shows X-rays from Nitrogen and Oxygen in the atmosphere. 24)
Figure 19: First light image of the X-ray surveyor of Earth's atmosphere from HaloSat (image credit: University of Iowa)
• July 18,2018: HaloSat will help scientists search for the universe's missing matter by studying X-rays from hot gas surrounding our Milky Way galaxy. 25)
- CMB (Cosmic Microwave Background) is the oldest light in the universe, radiation from when it was 400,000 years old. Calculations based on CMB observations indicate the universe contains: 5 percent normal matter protons, neutrons and other subatomic particles; 25 percent dark matter, a substance that remains unknown; and 70 percent dark energy, a negative pressure accelerating the expansion of the universe.
- As the universe expanded and cooled, normal matter coalesced into gas, dust, planets, stars and galaxies. But when astronomers tally the estimated masses of these objects, they account for only about half of what cosmologists say should be present.
- "We should have all the matter today that we had back when the universe was 400,000 years old," said Philip Kaaret, HaloSat's principal investigator at the University of Iowa (UI), which leads the mission. "Where did it go? The answer to that question can help us learn how we got from the CMB's uniform state to the large-scale structures we see today."
- Researchers think the missing matter may be in hot gas located either in the space between galaxies or in galactic halos, extended components surrounding individual galaxies.
- HaloSat will study gas in the Milky Way's halo that runs about 2 million degrees Celsius. At such high temperatures, oxygen sheds most of its eight electrons and produces the X-rays HaloSat will measure.
- Other X-ray telescopes, like NASA's NICER (Neutron star Interior Composition ExploreR) and the Chandra X-ray Observatory, study individual sources by looking at small patches of the sky. HaloSat will look at the whole sky, 100 square degrees at a time, which will help determine if the diffuse galactic halo is shaped more like a fried egg or a sphere.
- "If you think of the galactic halo in the fried egg model, it will have a different distribution of brightness when you look straight up out of it from Earth than when you look at wider angles," said Keith Jahoda, a HaloSat co-investigator and astrophysicist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "If it's in some quasi-spherical shape, compared to the dimensions of the galaxy, then you expect it to be more nearly the same brightness in all directions."
- The halo's shape will determine its mass, which will help scientists understand if the universe's missing matter is in galactic halos or elsewhere.
- HaloSat will be the first astrophysics mission that minimizes the effects of X-rays produced by solar wind charge exchange. This emission occurs when the solar wind, an outflow of highly charged particles from the Sun, interacts with uncharged atoms like those in Earth's atmosphere. The solar wind particles grab electrons from the uncharged atoms and emit X-rays. These emissions exhibit a spectrum similar to what scientists expect to see from the galactic halo.
- "Every observation we make has this solar wind emission in it to some degree, but it varies with time and solar wind conditions," said Kip Kuntz, a HaloSat co-investigator at Johns Hopkins University in Baltimore. "The variations are so hard to calculate that many people just mention it and then ignore it in their observations."
- In order to minimize these solar wind X-rays, HaloSat will collect most of its data over 45 minutes on the nighttime half of its 90-minute orbit around Earth. On the daytime side, the satellite will recharge using its solar panels and transmit data to NASA's Wallops Flight Facility in Virginia, which relays the data to the mission's operations control center at Blue Canyon Technologies in Boulder, Colorado.
• July 13, 2018: NanoRacks successfully completed the 14th CubeSat Deployment mission from the Company's commercially developed platform on the International Space Station. Having released nine CubeSats into low-Earth orbit, this mission marks NanoRacks' 185th CubeSat released from the Space Station, and 217th small satellite deployed by NanoRacks overall. 26)
- The CubeSats deployed were launched to the Space Station on the ninth contracted resupply mission for Orbital ATK (now Northrop Grumman Innovation Systems) from Wallops Island, Virginia in May 2018.
- NanoRacks offered an affordable launch opportunity, payload manifesting, full safety reviews with NASA, and managed on-orbit operations in order to provide an end-to-end solution that met all customer needs.
- The satellites deployed were: CubeRRT, EQUiSat, HaloSat, MemSat, RadSat-g, RainCube, TEMPEST-D, EnduroSat One, Radix (the last two entries are commercial CubeSats).
- The CubeSats mounted externally to the Cygnus spacecraft from the May 2018 launch are scheduled to be deployed on Sunday, July 15th, pending nominal operations.
Figure 20: HaloSat, a new CubeSat mission to study the halo of hot gas surrounding the Milky Way, was released from the International Space Station over Australia on July 13, 2018 (image credit: NanoRacks/NASA)
• On May 24, 2018, the Cygnus spacecraft successfully docked to the ISS. HaloSat is scheduled to be deployed from the ISS in late July to mid-August 2018 and start collecting science data one month later.
Sensor complement: X-ray Detector
In order to achieve the scientific goals of the mission, a science instrument that meets the following requirements had to be built (Ref. 5):
• be equipped with X-ray detector(s) sensitive in an energy band of 0.3 – 2 keV with an energy resolution (defined as full width at half maximum) of ≤100 eV near 600 eV;
• achieve a statistical accuracy of ±0.5 LU (LU = photons/cm2/s/ster) for a field with an oxygen line strength of 5 LU by:
- using detectors with sufficient effective area;
- operating for 6-12 months;
- viewing large part of the sky at a time (have field of view with a diameter of 10°);
• be able to observe the whole sky.
X-ray Detector: The FAST SDDs (Silicon Drift Detectors) from Amptek Inc. are used by the HaloSat instrument to detect X-rays. The radiation sensing element with its multilayer collimator and 2-stage thermoelectric cooler is encapsulated in a TO-8 package. The detector has an active area of 17 mm2. The entrance window is a C-Series C1 window made of Si3N4 covered with a thin layer of aluminum. The C1 window has good transmission properties around 600 eV (transmission is around 40% for 600 eV X-rays) and provides sensitivity down to 0.3 keV.
Figure 21: X-ray detector assembly. SDD detector, passive shield and baseplate are indicated with yellow, semi-transparent gray and blue color, respectively (image credit: HaloSat Team)
Passive Shielding: In order to minimize the background from events resulting from cosmic ray interactions and the diffuse X-ray background, the SDD detector is surrounded on 5 sides by a passive shield made of copper-tungsten alloy electroplated with a thin layer of gold (Figure 21). The sixth side of the shield has a circular cutout to provide an unobstructed path for the X-rays from the source to reach the detector (green element in Figure 21).
Figure 22: Sensor assembly (image credit: University of Iowa)
Signal Processing Electronics: An X-ray impinging on the detector chip is converted into an electron cloud with a charge that is proportional to the energy of that X-ray. The liberated charge is then drifted down a field gradient applied between the drift rings towards centrally located anode. The charge that accumulates at the anode is converted to a voltage signal by the FET preamplifier. The signal then goes through a preamplifier and a shaping amplifier circuit, followed by lower and upper level discriminators and a resettable peak hold circuit. In the next step, the signal from the detector is digitized and sent to the spacecraft bus. The silicon drift detector, due to its method of operation, has no imaging capabilities, however, its main advantage is low noise and thus good energy resolution.
Use of X-ray detectors from Amptek Inc. with an active area of 25 mm2 behind a Si3N4 window.
• The SDDs (Silicon Drift Detectors) are inside a sealed can and cooled by a TEC (Thermoelectric Cooler)
• The power for cooling is a large fraction of the power budget
• Same detectors as used for NICER
• Lab testing with Ti-L shows ΔE ~80 eV FWHM at 451 eV.
Figure 23: Illustration of the Amptek SDD with TEC in TO-8 can (0.55" diameter), image credit: University of Iowa
• The SDDs view the sky through a 13.3 mm diameter hole that is 135 mm away (9.2º - 13.4º)
• Aspect control ±1.0º << FOV
• To veto charged particle background, SDD is enclosed in a scintillator readout with APDs (Avalanche Photo Diodes)
• Three identical detectors.
Figure 24: Functional diagram: independent electronics for each detector assembly (image credit: University of Iowa)
Figure 25: Mechanical design of the X-ray detector (image credit: University of Iowa)
Observing strategy: Once the spacecraft crosses the dusk terminator, the science payload will be turned on and pointed towards selected target. The science payload will be switched off right before the spacecraft crosses the dawn terminator. Per each orbit, during its night-side, two science targets will be observed with approximately 1300 seconds of exposure time devoted to each target. The selected pair of the targets will be observed for ten consecutive orbits after which a new pair of targets will be selected. There are 330 HaloSat targets that are evenly spread across the sky. The targets cover 98.5% of the sky. A minimum of 8000 detector-seconds will be accumulated for each target.
Figure 26: Operations of the X-ray detector (image credit: University of Iowa)
Legend to Figure 26:
• Observations on the night side, two ~1000 s exposures per orbit
• Accumulate 10,000 detector s for each of ~ 4000 targets
• Scheduled to minimize helio/magnetospheric background.
Mission operations: The CADET radio onboard the HaloSat spacecraft will be used to downlink telemetry and receive commands. The NASA Wallops Flight Facility ground station will be used to communicate with the spacecraft. Blue Canyon Technologies will run the Mission Operation Center, while the Science Operation Center will be run at the University of Iowa.
Archiving and distribution of data: All the telemetry (including X-ray event data, housekeeping, spacecraft pointing and attitude) will be captured and converted to FITS (Flexible Image Transport System) format. The data will then be archived at the HEASARC (High Energy Astrophysics Science Archive Research Center) and made publicly available within 5 months from mission completion. In addition to the telemetry data, calibration files and software required for analysis of the instrument science data will also be archived at the HEASARC.
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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 (email@example.com).