LOFAR (LOw-Frequency ARray)
LOFAR is a large radio telescope network located mainly in the Netherlands, completed in 2012 by ASTRON, the Netherlands Institute for Radio Astronomy and its international partners, and operated by ASTRON's radio observatory, of the Netherlands Organization for Scientific Research.LOFAR is a new-generation radio interferometer constructed in the north of the Netherlands and across Europe. Utilizing a novel phased-array design, LOFAR covers the largely unexplored low-frequency range from 10-240 MHz and provides a number of unique observing capabilities. 1) 2) 3)
LOFAR consists of a vast array of omnidirectional antennas using a new concept in which the signals from the separate antennas are not combined in real time as they are in most array antennas. The electronic signals from the antennas are digitized, transported to a central digital processor, and combined in software to emulate a conventional antenna. The project is based on an interferometric array of radio telescopes using about 20,000 small antennas concentrated in at least 48 stations. Forty of these stations are distributed across the Netherlands and were funded by ASTRON. The five stations in Germany, and one each in Great Britain, France, Sweden and Ireland, were funded by these countries. Further stations may also be built in other European countries. The total effective collecting area is approximately 300,000 m2, depending on frequency and antenna configuration. The data processing is performed by a Blue Gene/P supercomputer situated in the Netherlands at the University of Groningen. LOFAR is also a technology precursor for the SKA (Square Kilometer Array).
LOFAR was conceived as an innovative effort to force a breakthrough in sensitivity for astronomical observations at radio-frequencies below 250 MHz. Astronomical radio interferometers usually consist either of arrays of parabolic dishes (e.g. the One-Mile Telescope or the Very Large Array), arrays of one-dimensional antennas (e.g. the Molonglo Observatory Synthesis Telescope) or two-dimensional arrays of omnidirectional antennas (e.g. Antony Hewish's Interplanetary Scintillation Array).
LOFAR combines aspects of many of these earlier telescopes; in particular, it uses omnidirectional dipole antennas as elements of a phased array at individual stations, and combines those phased arrays using the aperture synthesis technique developed in the 1950s. Like the earlier Cambridge Low Frequency Synthesis Telescope (CLFST) low-frequency radio telescope, the design of LOFAR has concentrated on the use of large numbers of relatively cheap antennas without any moving parts, concentrated in stations, with the mapping performed using aperture synthesis software. The direction of observation ("beam") of the stations is chosen electronically by phase delays between the antennas. LOFAR can observe in several directions simultaneously, as long as their aggregated data rate remains under its cap. This in principle allows a multi-user operation.
LOFAR makes observations in the 10 MHz to 240 MHz frequency range with two types of antennas: Low Band Antenna (LBA) and High Band Antenna (HBA), optimized for 10-80 MHz and 120-240 MHz, respectively. The electric signals from the LOFAR stations are digitized, transported to a central digital processor, and combined in software in order to map the sky. Therefore, LOFAR is a "software telescope". The cost is dominated by the cost of electronics and will therefore mostly follow Moore's law, becoming cheaper with time and allowing increasingly large telescopes to be built. The antennas are simple enough, but there are about 20,000 in the LOFAR array.
The function of the LOFAR WAN (Wide-Area Network) is to transport data between the LOFAR stations and the central processor in Groningen. The main component is the streaming of observational data from the stations. A smaller part of the LOFAR datastream consists of MAC (Monitoring And Control) related data and management information of the active network equipment. Connections of the LOFAR stations in the Netherlands to Groningen run over light-paths (also referred to as managed dark fibers) that are either owned by LOFAR or leased. This ensures the required performance and security of the entire network and the equipment connected to it. Signals from all stations in the core and an area around it are first sent to a concentrator node and subsequently patched through to Groningen.
The LOFAR stations outside the Netherlands are connected via international links that often involve the local NRENs (National Research and Education Networks). In some cases, commercial providers also play a role for part of the way.
For the communication over the light-paths 10 Gbit Ethernet (GbE) technology has been adopted. The high bandwidth connection between the concentrator node in the core and Groningen uses CWDM (Course Wavelength Division Multiplexing) techniques to transfer multiple signals on a single fiber, thereby saving on costs. Since the availability requirement for LOFAR is relatively low (95%), when compared with commercial data communication networks, redundant routing has not been implemented.
Figure 1: Aerial photograph of the Superterp, the heart of the LOFAR core, on which six LOFAR stations are housed. What resembles at first an ancient earthwork in a nature reserve in the northeast of the Netherlands is actually the heart of the most advanced radio telescope in the world, spanning northwestern Europe. The LOFAR, operated by the Astron Netherlands Institute for Radio Astronomy, consists of 51 antenna stations from Sweden to Ireland and Poland tasked with studying some of the lowest frequencies that can be observed from Earth, probing the primordial era before stars and galaxies were formed (image credit: LOFAR/Astron) 6)
Figure 2: LBA antennas of the LOFAR telescope (image credit: Astron)
Figure 3: Black casings in which the HBAs (High Band Antennas) of LOFAR are housed (image credit: ASTRON)
Figure 4: Locations of the International LOFAR Telescope for radio astronomy. The Lofar, operated by the Astron Netherlands Institute for Radio Astronomy, consists of 51 antenna stations from Sweden to Ireland and Poland tasked with studying some of the lowest frequencies that can be observed from Earth, probing the primordial era before stars and galaxies were formed (image credit: LOFAR/Astron)
Some LOFAR application fields:
LOFAR is a multi-purpose sensor array. Its main application is astronomy at low frequencies (10-240 MHz). LOFAR has also applications in Geophysics and Agriculture.
KSP (Key Science Projects) are astronomical applications that helped to drive the design of LOFAR. For each KSP a team of astronomers is involved with ASTRON in realizing the required technical capabilities.
- Cosmic magnetism of the nearby universe
- Ultra high energy cosmic rays
- Epoch of reionization. Astronomers hope that LOFAR will detect the signature the EoR (Epoch of Reionization). If it does, then this will open up an area of study in astronomy that will be even bigger than the CMB (Cosmic Microwave Background). The latter looks at a single epoch in the history of the evolution of the Universe - the EoR refers to a much longer period of time - from the so-called dark ages to the generation of the light from the first stars and galaxies or whatever it was that was around in these times.
- Solar physics and space weather
- Deep extragalactic surveys
- Transients and pulsars.
Geomagnetic Halloween Storm of 29-31 October 2003
The 29-31 October 2003 space weather superstorm, also referred to as the Halloween storm, attracted wide attention both in scientific and industrial communities as well as among the general public. It is probably one of the best publicly reported storms and because of the increasing scientific instrumentation, especially in space, the best recorded storm event ever. A descriptive picture of interest in the storm is given by the number of file transfers from the NOAA Space Environment Center which peaked at 19 million hits per day on 29 October, the average being 0.5 million. The storm period caused a great number of technological impacts varying from enforced alternate airline routes due to the increased particle radiation to the loss of a Japanese US $640 million environment satellite ADEOS-II. An extensive list of effects is given in a recent report by NOAA. Despite the large GIC (Geomagnetically Induced Currents) measured in North America, GIC did not cause any large-scale failures during the storm period. Though a detailed analysis to explain this feature of the storm is lacking, it was most probably due to both the countermeasures taken by the utilities and the fact that the most intense substorms of the period occurred when North America was not in the vicinity of local midnight. 7)
The geomagnetic superstorm knocked
down a part of the high-voltage power transmission system in southern
Sweden. The blackout lasted for an hour and left about 50,000 customers
without electricity. The incident was probably the most severe GIC
(Geomagnetically Induced Current) failure observed since the well-known
March 1989 Québec blackout. The
‘‘three-phase’’ storm produced exceptionally
large geomagnetic activity at the Fennoscandian auroral region.
Although the diversity of the GIC drivers is addressed in the study,
the problems in operating the Swedish system during the storm are
attributed geophysically to substorm s, storm sudden commencement, and
enhanced ionospheric convection, all of which created large and complex
geoelectric fields capable of driving large GIC. On the basis of the
basic twofold nature of the failure-related geoelectric field
characteristics, a semi-deterministic approach for forecasting
GIC-related geomagnetic activity in which average overall activity is
supplemented with statistical estimations of the amplitudes of GIC
fluctuations is suggested. The study revealed that the primary mode of
GIC-related failures in the Swedish high-voltage power transmission
system were via harmonic distortions produced by GIC combined with too
sensitive operation of the protective relay s. The outage in Malmo¨
on 30 October 2003 was caused by a combination of
Figure5 shows an overview of the interplanetary, magnetospheric and ionospheric conditions during the 29-31 October 2003 storm. The storm period started on 29 October, at about 05:40 UT with sudden southward turning of the IMF (Interplanetary Magnetic Field) associated possibly with the sheath region of the first ICME (Iinterplanetary Coronal Mass Ejection) accompanied by extremely high speed solar wind flow of about 1900 km/s. The magnetosphere responded to this by enhancing the convection, causing the Dst index to decrease to about -180 nT. At the same time, during this ‘‘first’’ main phase of the storm, very intense ionospheric disturbances above the Fennoscandian region were observed. Shortly after northward turning of the IMF and entering the recovery phase of the first enhancement of the Dst index, probably the internal field of the ejecta itself caused another southward IMF event starting at about 14:00 UT. This caused the ‘‘second’’ main phase of the geomagnetic storm and again the minimum Dst, this time reaching about -360 nT, was accompanied by very strong ionospheric disturbances above Fennoscandia.
Figure 5: Interplanetary, magnetospheric, and ionospheric overview of the 29-30 October 2003 geomagnetic storm period: (top) z component of the interplanetary magnetic field measured by the ACE spacecraft, (middle) Dst index, and (bottom) local variant of the AL index, IL index, computed from the IMAGE magnetometer array measurements. Dashed lines indicate the times of the failures in the Swedish high-voltage power transmission system. Note that interplanetary measurements are not propagated to the magnetopause, causing about an hour lag between the top plot and the other plots (image credit: storm study team)
Figure 6: Model of the well-known 30 October 2003 Halloween solar storm produced by the MIDAS tomographic ionospheric model from the University of Bath (image credit: University of Bath, Ref. 11) 8) 9) 10)
Radio astronomers focus on ionosphere for sharper satellite navigation
• April 2018: Radio astronomers and satellite navigation engineers are focusing their attention on the same point of the sky, looking into methods of improving both satnav accuracy and radio astronomy. 11)
LOFAR, operated by the Astron Netherlands Institute for Radio Astronomy, consists of 51 antenna stations from Sweden to Ireland and Poland tasked with studying some of the lowest frequencies that can be observed from Earth, probing the primordial era before stars and galaxies were formed.
Galileo and other navigation constellations exploit higher microwave frequencies, but radio astronomers and satnav engineers have one major obstacle in common: the ionosphere, the electrically active outer layer of Earth’s atmosphere, between 80 km to several thousand kilometers in altitude.
Discovered by early-20th century radio pioneers, the ionosphere creates major satnav errors owing to its tendency to delay and bend navigation signals as they travel the many thousands of kilometers from satellites in orbit down to the ground. - In the worst cases, satnav positioning errors of tens of meters can be introduced by the ionosphere.
A key factor is that the ionosphere is continuously changing, influenced by the Sun and space weather. The astronomy network experiences similar signal interference across its different antennas, but makes up for it by observing a specific strong radio source in the sky as a reference.
For satnav, the favored solution for larger receivers is to use two frequencies – the difference in signal delay reveals the state of the ionosphere. For smaller receivers of the kind found in smartphones and other consumer devices, a simpler adjustment is applied.
Figure 7: Artist's view of a Galileo FOC (Full Operational Capability) satellite. The complete Galileo constellation will consist of 24 satellites along three orbital planes in MEO (Medium Earth Orbit, plus two spares per orbit). The result will be Europe’s largest ever fleet of satellites, operating in the new environment of MEO (image credit: ESA, Pierre Carril)
To investigate methods of improving satnav accuracy, ESA worked with Astron and the NLR Netherlands Aerospace Center to measure the ionosphere simultaneously: dual-frequency satnav receivers at Exloo and another Lofar station at Steenwijk, and radio observations were made of celestial objects on locations close to the path of European Galileo, US GPS and Russian Glonass satnav satellites.
The ionospheric error is determined by the TEC (Total Electron Count) of the signal path through space: the higher the count the greater the error. Very high TECs can lead to scintillations that can cause the loss of satnav signal lock. The free electrons making up this count are produced by high-energy particles from the Sun dislodging electrons from atoms at the top of the atmosphere.
The results from the cross-measurements have proved promising. A dual-frequency satnav receiver can give estimates to about a single ‘TEC Unit’ (equal to about 10 million billion electrons along a square meter cross section along the signal path). Vertical TEC Unit values can range from a few to several hundred TEC Units. Lofar, however, can shrink this estimate tenfold, down to an accuracy of at least 0.1 TEC Unit relative between a pair of Lofar stations, with much lower noise levels.
Analysis suggests a dedicated ionosphere observation system might be used in the future to improve overall satnav accuracy, while also supporting the calibration of Lofar and comparable radio astronomy systems.
The project was supported through ESA’s European GNSS (Global Navigation Satellite System) Evolution Program, undertaking research and development linked to satellite navigation and augmentation systems. It was run for ESA by NLR and Astron.
Some LOFAR Observations/Discoveries
• February 19, 2021: An international team of astronomers has produced the largest and sharpest map of the sky at ultra-low radio frequencies, using the Low Frequency Array (LOFAR) radio telescope. The map published in the journal Astronomy & Astrophysics reveals more than 25,000 active supermassive black holes in distant galaxies. 12)
Figure 8: At a first glance, the map looks like an image of a starry night sky. However, the map is based on data taken by LOFAR and shows the sky in the radio band. Stars are almost invisible in the radio band, but instead black holes dominate the picture. With this map, astronomers seek to discover celestial objects that only emit waves at ultra-low radio frequencies. Such objects include diffuse matter in the large scale structure of the Universe, fading jets of plasma ejected by supermassive black holes, and exoplanets whose magnetic fields are interacting with their host stars. Albeit among the largest of its kind, the published map only shows two percent of the sky. The search for these exotic phenomena will continue for several years until a map of the entire northern sky will be completed (image credit: LOFAR Team)
- The radio waves received by LOFAR and used for this work are up to six meters long which corresponds to a frequency of around 50 MHz. They are the longest radio waves ever used to observe such a wide area of the sky at this depth. “The map is the result of many years of work on incredibly difficult data. We had to invent new strategies to convert the radio signals into images of the sky, but we are proud to have opened this new window on our Universe.”, says Francesco de Gasperin, scientist at the Hamburg Observatory and leading author of the publication.
- There is a reason why the Universe at these long radio wavelengths is almost uncharted: such observations are very challenging. The ionosphere, a layer of free electrons that surrounds the Earth, acts as a lens continuously moving over the radio telescope. The effect of the ionosphere can be compared to trying to see the world while being submerged in a swimming pool. Looking upwards, the waves on the water bend the light rays and distort the view. To account for ionospheric disturbances, the scientists used supercomputers and new algorithms to reconstruct its effect every four seconds over the course of 256 hours of observation.
- LOFAR is currently the largest radio telescope operating at lowest frequencies that can be observed from Earth. It consists of 52 stations spread across nine different countries: The Netherlands, Germany, Poland, France, United Kingdom, Sweden, Ireland, Latvia, and Italy. LOFAR is a joint project of ASTRON, the Netherlands Institute of Radio Astronomy, and the universities of Amsterdam, Groningen, Leiden, Nijmegen as well as the German Long Wavelength Consortium (GLOW) to which Universität Hamburg belongs. 13)
• January 12, 2021: Astronomers have used, for the first time, the combination of LOFAR and WSRT-Apertif, the phased array upgrade of the Westerbork Synthesis Radio Telescope, to measure the life cycle of supermassive black holes emitting radio waves. This study, part of the LOFAR deep fields surveys, opens the possibility of timing this cycle for many objects in the sky and explore the impact it has on galaxy evolution. 14)
- Supermassive black holes are an important component of galaxies. When in their active phase, they eject huge amounts of energy, which eventually can expel gas and matter from galaxies and impact the entire formation of new stars.
- These ejections represent only a phase in the lifecycle of a supermassive black hole. They are believed to last from tens of millions to a few hundreds of millions of years, only a short moment in the life of a galaxy. After this, the supermassive black hole enters a quiet phase. However, astronomers think that this cycle can actually repeat multiple times in which the black hole starts a new phase of ejections. But timing this cycle is hard because the timescales involved are far too long to be directly probed: other ways to easily measure them in a large number of objects are needed.
Radio wave ejections
- Some of the energy – also called ‘flux’ – is ejected by the supermassive black hole in the form of radio waves. Both radio waves at low and high frequencies are emitted and can be detected by sensitive radio telescopes like LOFAR (low frequency radio waves) and WSRT-Apertif (high frequency radio waves). “High frequency radio waves quickly lose their energy – and, as consequence, their flux – while those in the lower frequency do so much more slowly,” Prof. Dr. Raffaella Morganti, first author of the paper called The best of both worlds: Combining LOFAR and Apertif to derive resolved radio spectral index images, says.
- Observing these supermassive black holes with both LOFAR and WSRT-Apertif, scientists have been able to say which supermassive black holes are, at present, ‘switched off’ and how long ago it happened. They also have identified a case where the ejection phase of the supermassive black hole has ‘recently’ restarted.
Dying supermassive black holes
- In a previous study, LOFAR was used to find possible supermassive black holes in the dying or restarting phase, by taking advantage of their properties at low frequencies. In this study these same sources were surveyed also using WSRT-Apertif, and thus measuring radio waves at higher frequencies. The relative strength of the emission at these two frequencies is used to derive, to first order, how old a radio source is and whether it is already in a dying phase (Figure 9).
Figure 9: LOFAR and WSRT-Apertif detection of supermassive black hole radio waves. The difference in flux at which LOFAR and WSRT-Apertif detect a supermassive black hole determines if it is in its ejection phase (a) or not (b). The lower the flux of b, the longer it has been since the supermassive black hole was in its ejection phase (image credit: Studio Eigen Merk/ASTRON)
- Morganti: “Because of our earlier studies using LOFAR, we knew the expected relative difference in flux between the lower and higher frequencies if the supermassive black holes are in the active, ejection phase. Comparing them with the, now available, Apertif data, we were able to tell, for each of them, whether the on-going activity was confirmed or whether the ejected phase had stopped.
- “Interestingly, the relative number of radio galaxies found in the ‘out’ phase is also telling for how long a supermassive black hole has been ‘switched off’. These objects are rare, therefore large surveys are necessary to collect enough data about them so that we have a large enough data size for statistical analysis.”
- With this proof of concept study Morganti and colleagues have demonstrated that a combined survey of LOFAR and WSRT-Apertif can indeed detect the phase in which a supermassive black hole currently is. Morganti: “LOFAR is unique in sensitivity and spatial resolution at the low frequencies. And while there are other radio telescopes that can observe the higher frequencies, Apertif is now covering in-depth large areas of the northern sky, instead of focusing on a single source.” That is key, because Morganti and colleagues plan to chart all detectable supermassive black holes with radio emission, in order to learn more about the birth and life cycles of galaxies.
- A next step will be to create an automated way to detect these sources over much larger areas, using the large surveys that LOFAR and Apertif are doing. This is too big a job to do manually for a small group and approaches like Radio Galaxy Zoo and machine learning will be the way forward.
Figure 10: Part of the radio sky observed by this project where many galaxies with supermassive black holes emitting radio waves can be seen. The colors give an indication of the phase in the active life of the supermassive black hole. The red colors represent emission from black holes, in the later phase, at the end of their active life. Greener colors represent black holes in their “youth” (image credit: ASTRON)
• June 30, 2020: Radio halos are giant diffuse synchrotron emission found at the center of some merging galaxy clusters. In the past, they were described as smooth and regular sources with a morphology recalling that of the X-ray emitting gas. Highly sensitive observations performed with LOFAR-HBA are changing our view of radio halos, unveiling the presence of a large diversity of surface brightness structures embedded in the diffuse emission. 15)
Figure 11: This image shows the complex radio emission from the nearby galaxy cluster Abell 2255 as observed by LOFAR in the context of the LOFAR Two-Meter Sky Survey (LoTSS, Shimwell et al. 2017). Abell 2255 is a spectacular system that in its central ~10 Mpc2 region shows a plethora of emissions on different scales, from tens of kpc to above Mpc sizes. Among the numerous interesting features observed, we note the presence of filaments in the halo, emission from radio galaxies with extended tails, and emission from bright revived fossil plasma. The sources labelled in black were known from previous observations, while those labelled in blue and have been discovered only thanks to the high resolution and high sensitivity of LOFAR to low frequency radiation (image credit: Andrea Botteon)
- Reaching noise levels below 100 µJy/beam, this LOFAR image is among the deepest and most detailed cluster images at low frequency ever obtained so far. Being one of the most intricate diffuse radio sources known to date, Abell 2255 has been chosen to be the first LOFAR galaxy cluster deep field.
- The results based on the analysis of LoTSS observations of this galaxy cluster have been recently presented and discussed in the article "The beautiful mess in Abell 2255", by Botteon et al. (2020), accepted for publication in The Astrophysical Journal (arXiv:2006.04808). 16)
- Abell 2255 is a nearby (z=0.0806) merging galaxy cluster hosting one of the first radio halos ever detected in the ICM (Intra-Cluster Medium). The deep LOFAR images at 144 MHz of the central ~10 Mpc2 region show a plethora of emission on different scales, from tens of kpc to above Mpc sizes.
• February 17, 2020: Using the Dutch-led Low Frequency Array (LOFAR) radio telescope, astronomers have discovered unusual radio waves coming from the nearby red dwarf star GJ1151. The radio waves bear the tell-tale signature of aurorae caused by an interaction between a star and its planet. The radio emission from a star-planet interaction has been predicted for over thirty-years but this is the first time astronomers have been able to discern its signature. This method, only possible with a sensitive radio telescope like LOFAR, opens the door to a new way of discovering exoplanets in the habitable zone and studying the environment they exist in. 17)
- Red dwarfs are the most abundant type of star in our Milky Way, but much smaller and cooler than our own Sun. This means for a planet to be habitable, it has to be significantly closer to its star than the Earth is to the Sun. Red dwarfs also have much stronger magnetic fields than the Sun, which means, a habitable planet around a red dwarf is exposed to intense magnetic activity. This can heat the planet and even erode its atmosphere. The radio emissions associated with this process are one of the few tools available to gauge the potency of this effect.
- "The motion of the planet through a red dwarf’s strong magnetic field acts like an electric engine much in the same way a bicycle dynamo works. This generates a huge current that powers aurorae and radio emission on the star." says Dr Harish Vedantham, the lead author of the study and a Netherlands Institute for Radio Astronomy (ASTRON) staff scientist. 18)
Figure 12: Artist's rendition of a red-dwarf star's magnetic interaction with its planet (image credit: ASTRON, Danielle Futselaar)
- Thanks to the Sun's weak magnetic field and the larger distance to the planets, similar currents are not generated in the solar system. However, the interaction of Jupiter’s moon Io with Jupiter’s magnetic field generates a similarly bright radio emission, even outshining the Sun at sufficiently low frequencies.
- "We adapted the knowledge from decades of radio observations of Jupiter to the case of this star" said Dr Joe Callingham, ASTRON postdoctoral fellow and co-author of the study. “A scaled up version of Jupiter-Io has long been predicted to exist in the form of a star-planet system, and the emission we observed fits the theory very well.”
Figure 13: Video showing the science behind the discovery (video credit: ASTRON)
- The group is now concentrating on finding similar emission from other stars. “We now know that nearly every red-dwarf hosts terrestrial planets, so there must be other stars showing similar emission. We want to know how this impacts our search for another Earth around another star” says Dr Callingham.
- The team is using images from the ongoing survey of the northern sky called the LOFAR Two Meter Sky Survey (LoTSS) of which Dr Tim Shimwell, ASTRON staff scientist and a co-author of the study, is the principal scientist. “With LOFAR’s sensitivity, we expect to find around 100 of such systems in the solar neighborhood. LOFAR will be the best game in town for such science until the Square Kilometer Array comes online.” says Dr Shimwell.
- The group expects this new method of detecting exoplanets will open up a new way of understanding the environment of exoplanets. “The long-term aim is to determine what impact the star’s magnetic activity has on an exoplanet’s habitability, and radio emissions are a big piece of that puzzle.” said Dr Vedantham. “Our work has shown that this is viable with the new generation of radio telescopes, and put us on an exciting path.”
• March 18, 2019: An international team of astrophysicists observed for the first time that the jet of a quasar is less powerful on long radio wavelengths than earlier predicted. This discovery gives new insights in the evolution of quasar jets. They made this observation using the international Low Frequency Array (LOFAR) telescope, that produced high resolution radio images of quasar 4C+19.44 located over 5 billion light-years from Earth. 19) 20)
- Supermassive black holes, many millions of times more massive than our Sun reside in the central regions of galaxies. They grow even larger by attracting and consuming nearby gas and dust. If they consume material rapidly, the infalling matter shines brightly and the source is known as a quasar. Some of this infalling matter is not digested, but instead is ejected in the form of so-called jets that punch through the surrounding galaxy and into intergalactic space for millions of light years. These jets, shining brightly at radio wavelengths, are composed of particles accelerated up to nearly the speed of light, but exactly how these particles achieve energies not attainable on the Earth is yet to be completely solved.
- The discovery on quasar 4C+19.44 gives new insights to the balance between the energy in the field surrounding the quasar and that residing in the quasar jet. This finding indicates to an intrinsic property of the source rather than due to absorption effects. It implies that the energy budget available to accelerate particles and the balance between energy stored in particles and in the magnetic field, is less than expected.
- "This is an important discovery that will be used for the years to come to improve simulations of jets. We observed for the first time a new signature of particle acceleration in the power emitted of quasar jets at long radio wavelengths. An unexpected behavior that changes our interpretation on their evolution." Said Prof. Francesco Massaro from University of Turin. "We knew that this was already discovered in other cosmic sources but it was never before observed in quasars."
- The international team of astrophysicists had observed the jet of the quasar 4C+19.44 at short radio wavelengths, in visible light, and X-ray wavelengths. The addition of the LOFAR images allowed astrophysicists to make this discovery. LOFAR is the first radio facility operating at long radio wavelengths, which produces sharp images with a resolution similar to that of the Hubble Space Telescope.
- "We have been able to perform this experiment thanks to the highest resolution ever achieved at these long radio wavelengths, made possible by LOFAR." Said Dr Adam Deller, an astrophysicist of the Swinburne University of Technology who contributed to the LOFAR data analysis and imaging of 4C +19.44 while at ASTRON in the Netherlands, heart of the LOFAR collaboration.
- Dr Raymond Oonk, an astronomer at ASTRON and Leiden University and Dr Javier Moldon, astronomer at the University of Manchester, explained that "We have developed new calibration techniques for LOFAR and this has allowed us to separate compact radio structures in the quasar jet known as radio knots, and measure their emitted light. This result was unexpected and demands to future deeper investigations. New insights and clues on particle acceleration will come soon thanks to the international stations of LOFAR."
- The observation performed on the radio jet of 4C+19.44 was designed by Dr D. E. Harris, supervisor of Prof. Francesco Massaro, while working at the Harvard-Smithsonian Center for Astrophysics, several years ago. He performed the observation in collaboration with Dr Raffaella Morganti and his friends and colleagues at ASTRON. He only got the opportunity to see preliminary results as he passed away on 2015 December 6th. This publication, published in the first March issue of the Astrophysical Journal, is in memory of a career spanned much of the history of radio astronomy.
Figure 14: The radio jet of the quasar 4C+19.44, powered by a supermassive black hole lying in the center of its host galaxy and shining at long radio wavelengths as seen by the LOFAR radio telescope (magenta). The background image shows neighboring galaxies in the visible light highlighted thanks to the Hubble Space Telescope (cyan and orange) having the radio jet passing into the dark voids of intergalactic space (Harris et al. 2019), image credit: NASA/HST/LOFAR, courtesy of J. DePasquale
• February 17, 2019: An international team of more than 200 astronomers from 18 countries has published the first phase of a major new radio sky survey at unprecedented sensitivity using the Low Frequency Array (LOFAR) telescope. The survey reveals hundreds of thousands of previously undetected galaxies, shedding new light on many research areas including the physics of black holes and how clusters of galaxies evolve. A special issue of the scientific journal Astronomy & Astrophysics is dedicated to the first twenty-six research papers describing the survey and its first results. 21)
- Radio astronomy reveals processes in the Universe that we cannot see with optical instruments. In this first part of the sky survey, LOFAR observed a quarter of the northern hemisphere at low radio frequencies. At this point, approximately ten percent of that data is now made public. It maps three hundred thousand sources, almost all of which are galaxies in the distant Universe; their radio signals have travelled billions of light years before reaching Earth.
- Black holes: Huub Röttgering, Leiden University (The Netherlands): "If we take a radio telescope and we look up at the sky, we see mainly emission from the immediate environment of massive black holes. With LOFAR we hope to answer the fascinating question: where do those black holes come from?" What we do know is that black holes are pretty messy eaters. When gas falls onto them they emit jets of material that can be seen at radio wavelengths.
- Philip Best, University of Edinburgh (UK), adds: "LOFAR has a remarkable sensitivity and that allows us to see that these jets are present in all of the most massive galaxies, which means that their black holes never stop eating."
- Clusters of galaxies: Clusters of galaxies are ensembles of hundreds to thousands of galaxies and it has been known for decades that when two clusters of galaxies merge, they can produce radio emission spanning millions of light years. This emission is thought to come from particles that are accelerated during the merger process. Amanda Wilber, University of Hamburg (Germany), elaborates: "With radio observations we can detect radiation from the tenuous medium that exists between galaxies. This radiation is generated by energetic shocks and turbulence. LOFAR allows us to detect many more of these sources and understand what is powering them."
- Annalisa Bonafede, University of Bologna and INAF (Italy), adds: "What we are beginning to see with LOFAR is that, in some cases, clusters of galaxies that are not merging can also show this emission, albeit at a very low level that was previously undetectable. This discovery tells us that, besides merger events, there are other phenomena that can trigger particle acceleration over huge scales."
- Magnetic fields: "Magnetic fields pervade the cosmos, and we want to understand how this happened. Measuring magnetic fields in intergalactic space can be difficult, because they are very weak. However, the unprecedented accuracy of the LOFAR measurements has allowed us to measure the effect of cosmic magnetic fields on radio waves from a giant radio galaxy that is 11 million light years in size. This work shows how we can use LOFAR to help us understand the origin of cosmic magnetic fields", explains Shane O'Sullivan, University of Hamburg.
Figure 15: An overview and a discussion of LOFAR experiments (video credit: ASTRON)
Figure 16: New sky map detects hundreds of thousands of unknown galaxies (image credit: ASTRON)
- High-quality images: Creating low-frequency radio sky maps takes both significant telescope and computational time and requires large teams to analyze the data. "LOFAR produces enormous amounts of data - we have to process the equivalent of ten million DVDs of data. The LOFAR surveys were recently made possible by a mathematical breakthrough in the way we understand interferometry", says Cyril Tasse, Observatoire de Paris - Station de radioastronomie à Nançay (France).
- "We have been working together with SURF in the Netherlands to efficiently transform the massive amounts of data into high-quality images. These images are now public and will allow astronomers to study the evolution of galaxies in unprecedented detail", says Timothy Shimwell, Netherlands Institute for Radio Astronomy (ASTRON) and Leiden University.
- SURF's compute and data center located at SURFsara in Amsterdam runs on 100 percent renewable energy and hosts over 20 petabytes of LOFAR data. "This is more than half of all data collected by the LOFAR telescope to date. It is the largest astronomical data collection in the world. Processing the enormous data sets is a huge challenge for scientists. What normally would have taken centuries on a regular computer was processed in less than one year using the high throughput compute cluster (Grid) and expertise", says Raymond Oonk (SURFsara).
- LOFAR: The LOFAR telescope, the Low Frequency Array, is unique in its capabilities to map the sky in fine detail at meter wavelengths. LOFAR is operated by ASTRON in The Netherlands and is considered to be the world's leading telescope of its type. "This sky map will be a wonderful scientific legacy for the future. It is a testimony to the designers of LOFAR that this telescope performs so well", says Carole Jackson, Director General of ASTRON.
- The next step: The 26 research papers in the special issue of Astronomy & Astrophysics were done with only the first two percent of the sky survey. The team aims to make sensitive high-resolution images of the whole northern sky, which will reveal 15 million radio sources in total. "Just imagine some of the discoveries we may make along the way. I certainly look forward to it", says Jackson. "And among these there will be the first massive black holes that formed when the Universe was only a ‘baby', with an age a few percent of its present age", adds Röttgering.
- Additional videos: Robert Schulz (ASTRON) created a video where we travel through the radio Universe, through the galaxies detected in the LOFAR survey.
Figure 17: Flying through the radio universe with LOFAR. Rafaël Mostert (Leiden University & ASTRON) created a movie that gives a quick tour of four galaxies in the Universe, showing the capability of LOFAR and/or the difference between detecting radio and optical waves (video credit: Robert Schulz, ASTRON)
Figure 18: LOFAR survey galaxies in radio and optical waves (video credit: Robert Schulz, ASTRON)
• June 2018: The quiet solar corona emits meter-wavelength thermal bremsstrahlung. Coronal radio emission can only propagate above that radius, Rω, where the local plasma frequency equals the observing frequency. The radio interferometer LOFAR (LOw Frequency ARray) observes in its low band (10–90 MHz) solar radio emission originating from the middle and upper corona. 22)
- Objectives: The first solar aperture synthesis imaging observations is presented in the low band of LOFAR in 12 frequencies each separated by 5 MHz. From each of these radio maps we infer Rω, and a scale height temperature, T. These results can be combined into coronal density and temperature profiles.
- Methods: Radial intensity profiles from the radio images are derived. The focus is on polar directions with simpler, radial magnetic field structure. Intensity profiles were modeled by ray-tracing simulations, following wave paths through the refractive solar corona, and including free-free emission and absorption. Model profiles were fitted to observations with Rω and T as fitting parameters.
- Results: In the low corona, Rω < 1.5 solar radii, high scale height temperatures up to 2.2 x 106 K are found, much more than the brightness temperatures usually found there. But if all Rω values are combined into a density profile, this profile can be fitted by a hydrostatic model with the same temperature, thereby confirming this with two independent methods. The density profile deviates from the hydrostatic model above 1.5 solar radii, indicating the transition into the solar wind.
- Conclusions: These results demonstrate what information can be gleaned from solar low-frequency radio images. The scale height temperatures observed are not only higher than the brightness temperatures, but also higher than temperatures derived from a coronagraph or from EUV (Extreme Ultraviolet) data. Future observations will provide continuous frequency coverage. This continuous coverage eliminates the need for local hydrostatic density models in the data analysis and enables the analysis of more complex coronal structures such as those with closed magnetic fields.
• October 3, 2017: LOFAR (Low Frequency Array) observations at 144 MHz have revealed large-scale radio sources in the unrelaxed galaxy cluster Abell 1132. The cluster hosts diffuse radio emission on scales of ~650 kpc near the cluster center and a head–tail (HT) radio galaxy, extending up to 1 Mpc (Megaparsec), south of the cluster center. The central diffuse radio emission is not seen in NRAO VLA FIRST Survey, Westerbork Northern Sky Survey, nor in C & D array VLA observations at 1.4 GHz, but is detected in our follow-up GMRT (Giant Meterwave Radio Telescope) observations at 325 MHz. Using LOFAR and GMRT data, we determine the spectral index of the central diffuse emission to be α = -1.75 ± 0.19 (S ∞ να). 23)
• September 5, 2017: By following up on mysterious high-energy sources mapped out by NASA's Fermi Gamma-ray Space Telescope, the Netherlands-based LOFAR (Low Frequency Array) radio telescope has identified a pulsar spinning at more than 42,000 revolutions per minute, making it the second-fastest known. 24) 25)
A pulsar is the core of a massive star that exploded as a supernova. In this stellar remnant, also called a neutron star, the equivalent mass of half a million Earths is crushed into a magnetized, spinning ball no larger than Washington, D.C. The rotating magnetic field powers beams of radio waves, visible light, X-rays and gamma rays. If a beam happens to sweep across Earth, astronomers observe regular pulses of emission and classify the object as a pulsar.
"Roughly a third of the gamma-ray sources found by Fermi have not been detected at other wavelengths," said Elizabeth Ferrara, a member of the discovery team at NASA's Goddard Space Center in Greenbelt, Maryland. "Many of these unassociated sources may be pulsars, but we often need follow-up from radio observatories to detect the pulses and prove it. There's a real synergy across the extreme ends of the electromagnetic spectrum in hunting for them."
The new object, named PSR J0952–0607 — or J0952 for short — is classified as a millisecond pulsar and is located between 3,200 and 5,700 light-years away in the constellation Sextans. The pulsar contains about 1.4 times the sun's mass and is orbited every 6.4 hours by a companion star that has been whittled away to less than 20 times the mass of the planet Jupiter. The scientists report their findings in a paper published in the Sept. 10 issue of The Astrophysical Journal Letters and now available online.
At some point in this system's history, matter began streaming from the companion and onto the pulsar, gradually raising its spin to 707 rotations a second, or more than 42,000 rpm, and greatly increasing its emissions. Eventually, the pulsar began evaporating its companion, and this process continues today. Because of their similarity to spiders that consume their mates, systems like J0952 are called black widow or redback pulsars, depending on how much of the companion star remains. Most of the known systems of these types were found by following up Fermi unassociated sources.
The LOFAR discovery also hints at the potential to find a new population of ultra-fast pulsars.
"LOFAR picked up pulses from J0952 at radio frequencies around 135 MHz, which is about 45 percent lower than the lowest frequencies of conventional radio searches," said lead author Cees Bassa at the Netherlands Institute for Radio Astronomy (ASTRON). "We found that J0952 has a steep radio spectrum, which means its radio pulses fade out very quickly at higher frequencies. It would have been a challenge to find it without LOFAR."
Theorists say pulsars could rotate as fast as 72,000 rpm before breaking apart, yet the fastest spin known — by PSR J1748–2446ad, reaching nearly 43,000 rpm — is just 60 percent of the theoretical maximum. Perhaps pulsars with faster periods simply can't form. But the gap between theory and observation may also result from the difficulty in detecting the fastest rotators.
"There is growing evidence that the fastest-spinning pulsars tend to have the steepest spectra," said co-author Ziggy Pleunis, a doctoral student at McGill University in Montreal. The first millisecond pulsar discovered with LOFAR, which was found by Pleunis, is J1552+5437, which spins at 25,000 rpm and also exhibits a steep spectrum. "Since LOFAR searches are more sensitive to these steep-spectrum radio pulsars, we may find that even faster pulsars do, in fact, exist and have been missed by surveys at higher frequencies," he explained.
• February 13, 2017: An international team of astronomers reports the discovery of a new GRG (Giant Radio Galaxy) associated with the galaxy triplet known as UGC 9555. The newly discovered galaxy turns out to be one of the largest GRGs so far detected. 26) 27)
Located some 820 million light years away from the Earth, UGC 9555 is a part of a larger group of galaxies designated MSPM 02158. Recently, a team of researchers led by Alex Clarke of the Jodrell Bank Centre for Astrophysics in Manchester, U.K., has combed through the data provided by the LOFAR (Low Frequency Array) and uncovered new, important information about this distant disturbed galaxy group.
The team has analyzed the data available in the LOFAR MSSS (Multifrequency Snapshot Sky Survey). It is the first northern-sky LOFAR imaging survey that covers the sky north of the celestial equator at frequencies from 119 to 158 MHz in eight separate 2.0 MHz bands. The images obtained as a part of the LOFAR MSSS allowed the scientists to distinguish a new giant radio galaxy.
GRGs are radio galaxies with an overall projected linear length exceeding 6.5 million light years. They are rare objects grown in low-density environments. GRGs are important for astronomers to study the formation and the evolution of radio sources.
The newly detected GRG which has not received any official designation yet has a projected linear size of 8.34 million light years. This makes it one of the largest GRGs known to date. Currently, with a projected size of approximately 16 million light years, the J1420-0545 holds the title of the largest giant radio galaxy discovered so far.
The team noted that the newly detected GRG has integrated flux density at 142 MHz of 1.54 Jy over the whole dual-lobe emission, including underlying background point sources, which gives a total luminosity at 142 MHz of 11.6 septillion W/Hz.
However, the available LOFAR MSSS and archival radio data are still insufficient to confirm the class of this GRG. Radio sources are divided into two classes: Fanaroff and Riley Class I (FRI), and Class II (FRII).
"We cannot clearly classify this GRG as an FR-I or FR-II source based on its morphology in the MSSS and archival radio data. There are no conclusive enhancements of emission from the resolution of the MSSS data (without the contribution from unassociated point sources) from which to use the standard Fanaroff-Riley classification," the paper reads.
Figure 19: Background SDSS (Sloan Digital Sky Survey) image (composite from bands g, r and i) overlaid with white MSSS contours of the GRG at 2, 3, 4, 6 and 8 times the RMS noise (34 mJy/beam). NVSS (NRAO VLA Sky Survey) contours are overlaid in red at 3, 5, 10 and 20 times the RMS noise (0.55 mJy/beam) revealing a bright part of the radio jet towards the north-east. The beam sizes are shown in the lower left (image credit: LOFAR study team)
• March 19, 2013: A team of astronomers led by ASTRON astronomer Dr. George Heald has discovered a previously unknown gigantic radio galaxy, using initial images from a new, ongoing all-sky radio survey. The galaxy was found using the powerful ILT (International LOFAR Telescope), built and designed by ASTRON. The team is currently performing LOFAR's first all-sky imaging survey, the MSSS (Multi-frequency Snapshot Sky Survey). While browsing the first set of MSSS images, Dr. Heald identified a new source the size of the full moon projected on the sky. The radio emission is associated with material ejected from one member of an interacting galaxy triplet system tens to hundreds of millions of years ago. The physical extent of the material is much larger than the galaxy system itself, extending millions of light years across intergalactic space. The MSSS survey is still ongoing, and is poised to discover many new sources like this one. 28)
The new galaxy is a member of a class of objects called Giant Radio Galaxies (GRGs). GRGs are a type of radio galaxy with extremely large physical size, suggesting that they are either very powerful or very old. LOFAR is an effective tool to find new GRGs like this one because of its extreme sensitivity to such large objects, combined with its operation at low frequencies that are well suited to observing old sources.
Figure 20: LOFAR discovers new giant galaxy in all-sky survey. Overlay of the new GRG (blue-white colors) on an optical image from the Digitized Sky survey. The inset shows the central galaxy triplet (image from Sloan Digital Sky Survey). The image is about 2 Mpc (Megaparsec) across (image credit: ASTRON)
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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).