 The LOFAR core near Exloo, Netherlands. The two bridges on the left give a sense of scale. | |
| Alternative names | Low-Frequency Array |
|---|---|
| Location(s) | 3km north ofExloo, the Netherlands (core) |
| Coordinates | 52°54′32″N6°52′08″E / 52.90889°N 6.86889°E /52.90889; 6.86889 |
| Organization | ASTRON |
| Wavelength | 30 to 1.3m (radio) |
| Built | 2006–2012 |
| Telescope style | Phased array of a total of ~20,000dipole antennas |
| Diameter | 1000 km or more |
| Collecting area | up to 1 km2 |
| Focal length | N/A |
| Mounting | fixed |
| Website | www |
  Location of Low-Frequency Array | |
 Related media on Commons | |
TheLow-Frequency Array (LOFAR) is a largeradio telescope, with an antenna network located mainly in theNetherlands, and spreading across 7 other European countries as of 2019. Originally designed and built byASTRON, the Netherlands Institute for Radio Astronomy, it was first opened byQueen Beatrix of The Netherlands in 2010, and has since been operated by ASTRON on behalf first of the International LOFAR Telescope (ILT) partnership and now of the LOFARERIC by ASTRON.
LOFAR consists of a vast array ofomnidirectional antennas using a modern concept, in which the signals from the separate antennas are not connected directly electrically to act as a single large antenna, as they are in mostarray antennas. Instead, the LOFAR dipole antennas (of two types) are distributed in stations, within which the antenna signals can be partly combined in analogue electronics, then digitised, then combined again across the full station. This step-wise approach provides great flexibility in setting and rapidly changing the directional sensitivity on the sky of an antenna station. The data from all stations are then transported over fiber to a central digital processor, and combined in software to emulate a conventional radio telescope dish with a resolving power corresponding to the greatest distance between the antenna stations across Europe.
LOFAR is thus aninterferometric array, using about 20,000 small antennas concentrated in 52 stations since 2019. 38 of these stations are distributed across the Netherlands, built with regional and national funding. The six stations inGermany, three inPoland, and one each inFrance,Great Britain,Ireland,Latvia, andSweden, with various national, regional, and local funding and ownership.Italy officially joined the International LOFAR Telescope (ILT) in 2018; construction at theINAF observatory site inMedicina, nearBologna, is planned as soon as upgraded (so-called LOFAR2.0) hardware becomes available.[1] Further stations in other European countries are in various stages of planning. The total effective collecting area is approximately 300,000 square meters, depending on frequency and antenna configuration.[2] Until 2014, data processing was performed by aBlue Gene/Psupercomputer situated in theNetherlands at theUniversity of Groningen. Since 2014 LOFAR uses a GPU-based correlator and beamformer, COBALT, for that task.[3] LOFAR is also a technology and science pathfinder for theSquare Kilometre 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 ofparabolic dishes (e.g. theOne-Mile Telescope or theVery Large Array), arrays of one-dimensional antennas (e.g. theMolonglo Observatory Synthesis Telescope) or two-dimensional arrays ofomnidirectional antennas (e.g.Antony Hewish'sInterplanetary Scintillation Array).
LOFAR combines aspects of many of these earlier telescopes; in particular, it uses omnidirectional dipole antennas as elements of aphased array at individual stations, and combines those phased arrays using theaperture synthesis technique developed in the 1950s. Like the earlierCambridge 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 synthesissoftware. 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 the aggregated data rate remains under its cap. This in principle allows a multi-user operation.[4]
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.[5] The electric signals from the LOFAR stations are digitised, transported to a central digital processor, and combined in software in order to map the sky. Therefore, LOFAR is a "software telescope".[6] The cost of such telescopes is dominated by the cost of electronics and will therefore mostly followMoore's law, becoming cheaper with time and allowing increasingly large telescopes to be built. Each antenna is fairly simple- but there are about 20,000 of them in the LOFAR array.[4]
To make radio surveys of the sky with adequate resolution, the antennas are arranged in clusters that are spread out over an area of more than 1000 km in diameter. The LOFAR stations in the Netherlands reach baselines of about 100 km. LOFAR currently receives data from 24 core stations (inExloo), 14 'remote' stations in The Netherlands, and 14 international stations. Each of the core and remote stations has 48 HBAs and 96 LBAs and a total of 48 digital Receiver Units (RCUs). International stations have 96 LBAs and 96 HBAs and a total of 96 digital Receiver Units (RCUs).[7]
The locations of the international LOFAR stations are:
The New Extension in Nançay Upgrading LOFAR (NenuFAR) telescope is co-located at theNançay radio telescope in central France. It is an extension of the Nançay LOFAR station (FR606), adding 96 low frequency tiles, each consisting of a "mini-array" of 19 crossed-dipole antennas, distributed in a circle with a diameter of approximately 400 m. The tiles are a hexagonal cluster with analogically phased antennas. The telescope can capture radio frequencies in the 10–85 MHz range, covering the LOFAR-Low Band (30–80 MHz) range as well. The NenuFAR array can work as a high-sensitivity LOFAR-compatible super-LBA station (LSS), operating together with rest of LOFAR to increase to array's global sensitivity by nearly a factor of two, and improve the array's imaging capabilities. It can also function as a second supercore to improve array availability. Due to its dedicated receiver, NenuFAR can also operate as a standalone instrument, known as NenuFAR/Standalone in this mode.[18][19]
Additionally, a set of LOFAR antennas is deployed at theKAIRA (Kilpisjärvi Atmospheric Imaging Receiver Array) nearKilpisjärvi,Finland. This installation functions as aVHF receiver either in stand-alone mode or part of abistatic radar system together withEISCAT transmitter inTromsø.[20]
Data transport requirements are in the range of severalgigabits per second per station and the processing power needed is several tens ofTeraFLOPS. The data from LOFAR is stored in the LOFAR long-term archive.[21] The archive is implemented as distributed storage, with data spread over theTarget data centre located in the Donald Smits Center for Information Technology at theUniversity of Groningen,SURFsara [nl] centre in Amsterdam, and the Forschungszentrum Jülich in Germany.
The mission of LOFAR is to map the Universe at radio frequencies from ~10–240MHz with greaterresolution and greater sensitivity than previous surveys, such as the7C and8C surveys, and surveys by theVery Large Array (VLA) andGiant Meterwave Radio Telescope (GMRT).
LOFAR will be the most sensitive radio observatory at its low observing frequencies until theSquare Kilometre Array (SKA) comes online in the late 2020s. Even then, the SKA will only observe at frequencies >50 MHz and LOFAR's angular resolution will remain far superior.
The sensitivities and spatial resolutions attainable with LOFAR make possible several fundamental new studies of the Universe as well as facilitating unique practical investigations of the Earth's environment. In the following list the termz is adimensionless quantity indicating theredshift of the radio sources seen by LOFAR.
One of the most exciting, but technically most challenging, applications of LOFAR will be the search for redshifted21 cm line emission from theEpoch of Reionization (EoR).[23] It is thought that the 'Dark Ages', the period after recombination when the Universe turned neutral, lasted until around z=20.WMAP polarization results appear to suggest that there may have been extended, or even multiple phases of reionisation, the start possibly being around z~15–20 and ending at z~6. Using LOFAR, the redshift range from z=11.4 (115 MHz) to z=6 (200 MHz) can be probed. The expected signal is small, and disentangling it from the much stronger foreground emission is challenging.
One of the most important applications of LOFAR will be to carry out large-sky surveys. Such surveys are well suited to the characteristics of LOFAR and have been designated as one of the key projects that have driven LOFAR since its inception. Such deep LOFAR surveys of the accessible sky at several frequencies will provide unique catalogues of radio sources for investigating several fundamental areas of astrophysics, including the formation of massiveblack holes, galaxies and clusters of galaxies. Because the LOFAR surveys will probe an unexplored parameter of the Universe, it is likely that they will discover new phenomena. In February 2021, astronomers released, for the first time, a very high-resolution image of 25,000 activesupermassive black holes, covering four percent of theNorthern celestial hemisphere, based onultra-low radio wavelengths, as detected by LOFAR.[24]
The combination of low frequencies, omnidirectional antennae, high-speed data transport and computing means that LOFAR will open a new era in the monitoring of the radio sky. It will be possible to make sensitive radio maps of the entire sky visible from The Netherlands (about 60% of the entire sky) in only one night. Transient radio phenomena, only hinted at by previous narrow-field surveys, will be discovered, rapidly localised with unprecedented accuracy, and automatically compared to data from other facilities (e.g. gamma-ray, optical, and X-ray observatories). Such transient phenomena may be associated with exploding stars, black holes, flares on Sun-like stars,radio bursts from exoplanets or evenSETI signals.[25] In addition, this key science project will make a deep survey for radiopulsars at low radio frequencies, and will attempt to detect giant radio bursts from rotatingneutron stars in distant galaxies.
LOFAR offers a unique possibility inparticle physics for studying the origin of high-energy andultra-high-energy cosmic rays (HECRs and UHECRs) at energies between 1015–1020.5 eV.[26] Both the sites and processes for accelerating particles are unknown. Possible candidate sources of these HECRs are shocks in radio lobes of powerful radio galaxies, intergalactic shocks created during the epoch of galaxy formation, so-called Hyper-novae,gamma-ray bursts, or decay products of super-massive particles from topological defects, left over from phase transitions in the early Universe. The primary observable is the intense radio pulse that is produced when a primary CR hits the atmosphere and produces an extensive air shower (EAS). An EAS is aligned along the direction of motion of the primary particle, and a substantial part of its component consists ofelectron-positron pairs which emit radio emission in the terrestrial magnetosphere (e.g., geo-synchrotron emission).
LOFAR opens the window to the so far unexplored low-energy synchrotron radio waves, emitted by cosmic-ray electrons in weak magnetic fields. Very little is known about the origin and evolution of cosmic magnetic fields. The space around galaxies and between galaxies may all be magnetic, and LOFAR may be the first to detect weak radio emission from such regions. LOFAR will also measure theFaraday effect, which is the rotation of polarization plane of low-frequency radio waves, and gives another tool to detect weak magnetic fields.[27]
The Sun is an intense radio source. The already strong thermal radiation of the 106 K hotsolar corona is superimposed by intense radio bursts that are associated with phenomena of the solar activity, likeflares andcoronal mass ejections (CMEs). Solar radio radiation in the LOFAR frequency range is emitted in the middle and upper corona. So LOFAR is an ideal instrument for studies of the launch of CMEs heading towards interplanetary space. LOFAR's imaging capabilities will yield information on whether such a CMEs might hit the Earth. This makes LOFAR is a valuable instrument forspace weather studies.
Solar observations with LOFAR will include routine monitoring of the solar activity as the root of space weather. Furthermore, LOFAR's flexibility enables rapid responses to solar radio bursts with follow-up observations. Solar flares produce energetic electrons that not only lead to the emission of non-thermal solar radio radiation. The electrons also emit X-rays and heat the ambient plasma. So joint observation campaigns with other ground- and space-based instruments, e.g.RHESSI,Hinode, theSolar Dynamics Observatory (SDO), and eventually theAdvanced Technology Solar Telescope and theSolar Orbiter provide insights into this fundamental astrophysical process.
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In the early 1990s, the study of aperture array technology for radio astronomy was being actively studied byASTRON – the Netherlands Institute for Radio Astronomy. At the same time, scientific interest in a low-frequency radio telescope began to emerge at ASTRON and at the Dutch Universities. A feasibility study was carried out and international partners sought during 1999. In 2000 the Netherlands LOFAR Steering Committee was set up by the ASTRON Board with representatives from all interested Dutch university departments and ASTRON.
In November 2003 the Dutch Government allocated 52 millioneuro to fund the infrastructure of LOFAR under the Bsik programme. In accordance with Bsik guidelines, LOFAR was funded as a multidisciplinarysensor array to facilitate research ingeophysics,computer sciences andagriculture as well asastronomy.
In December 2003 LOFAR's Initial Test Station (ITS) became operational. The ITS system consists of 60 inverse V-shaped dipoles; each dipole is connected to alow-noise amplifier (LNA), which provides enough amplification of the incoming signals to transport them over a 110 m-long (360 ft) coaxial cable to the receiver unit (RCU).
On April 26, 2005, anIBMBlue Gene/L supercomputer was installed at theUniversity of Groningen's math centre, for LOFAR'sdata processing. At that time it was the second most powerful supercomputer inEurope, after theMareNostrum inBarcelona.[28]Since 2014 an even more powerful computing cluster (correlator) called COBALT performs the correlation of signals from all individual stations.[29]
In August/September 2006 the first LOFAR station (Core Station CS001, aka. CS152°54′32″N6°52′8″E / 52.90889°N 6.86889°E /52.90889; 6.86889) was put in the field using pre-production hardware. A total of 96 dual-dipole antennas (the equivalent of a full LOFAR station) are grouped in four clusters, the central cluster with 48 dipoles and other three clusters with 16 dipoles each. Each cluster is about 100 m (330 ft) in size. The clusters are distributed over an area of ~500 m in diameter.
In November 2007 the first international LOFAR station (DE601) next to theEffelsberg 100 m radio telescope became the first operational station. The first fully complete station, (CS302) on the edge of the LOFAR core, was delivered in May 2009, with a total of 40 Dutch stations scheduled for completion in 2013. By 2014, 38 stations in the Netherlands, five stations in Germany (Effelsberg, Tautenburg, Unterweilenbach, Bornim/Potsdam, and Jülich), and one each in the UK (Chilbolton), in France (Nançay) and in Sweden (Onsala) were operational. The station atBirr in Ireland was added in 2017.[30]
LOFAR was officially opened on 12 June 2010 by Queen Beatrix of the Netherlands.[31] Regular observations started in December 2012.[citation needed]
