Low-Frequency Array

Coordinates: 52°54′32″N 6°52′08″E / 52.90889°N 6.86889°E / 52.90889; 6.86889
Source: Wikipedia, the free encyclopedia.
Not to be confused with Low-Frequency Analyzer and Recorder (LOFAR) of SOSUS.
Low-Frequency Array
The LOFAR core near Exloo, Netherlands. The two bridges on the left give a sense of scale.
Alternative namesLow-Frequency Array Edit this at Wikidata
Location(s)3 km north of Exloo, the Netherlands (core)
Coordinates52°54′32″N 6°52′08″E / 52.90889°N 6.86889°E / 52.90889; 6.86889 Edit this at Wikidata
OrganizationASTRON
Wavelength30 to 1.3 m (radio)
Built2006–2012
Telescope stylePhased array of a total of ~20,000 dipole antennas
Diameter1000 km or more
Collecting areaup to 1 km2
Focal lengthN/A
Mountingfixed
Websitewww.lofar.org
Low-Frequency Array is located in Netherlands
Low-Frequency Array
Location of Low-Frequency Array
  Related media on Commons
[edit on Wikidata]

The Low-Frequency Array (LOFAR) is a large radio telescope, with an antenna network located mainly in the Netherlands, and spreading across 7 other European countries as of 2019. Originally designed and built by ASTRON, the Netherlands Institute for Radio Astronomy, it was first opened by Queen Beatrix of The Netherlands in 2010, and has since been operated on behalf of the International LOFAR Telescope (ILT) partnership by ASTRON.

LOFAR consists of a vast array of omnidirectional radio 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 most

Blue Gene/P supercomputer situated in the Netherlands at the University 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 the Square Kilometre Array
.

Technical information

Low-band antenna with electronics cabin in the background

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 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 follow Moore'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]

LOFAR stations

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 (in Exloo), 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]

Effelsberg, next to the 100 m radio telescope (background), both run by the Max Planck Institute for Radio Astronomy
Bonn, Germany

The locations of the international LOFAR stations are:

NenuFAR

The NenuFAR telescope is co-located at the

Nançay radio telescope. 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]

Other stations

The Irish LOFAR array (I-LOFAR) in Birr, County Offaly.

Additionally, a set of LOFAR antennas is deployed at the

VHF receiver either in stand-alone mode or part of a bistatic radar system together with EISCAT transmitter in Tromsø.[20]

Data transfer

Data transport requirements are in the range of several

SURFsara [nl
] centre in Amsterdam, and the Forschungszentrum Jülich in Germany.

Sensitivity

The mission of LOFAR is to map the Universe at radio frequencies from ~10–240

Giant Meterwave Radio Telescope (GMRT)
.

LOFAR will be the most sensitive radio observatory at its low observing frequencies until the Square 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.

Science case

At low radio frequencies the sky is dominated by small bright sources (shown is a 151 MHz map of the region: 140° to 180° Galactic longitude; -5° to 5° Galactic latitude). LOFAR will have sufficient fidelity and sensitivity to see faint structure between these bright sources because of the very large number of array elements.

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 term z is a

dimensionless quantity indicating the redshift
of the radio sources seen by LOFAR.

Key projects

The epoch of reionization

One of the most exciting, but technically most challenging, applications of LOFAR will be the search for redshifted

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.

Deep extragalactic surveys

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 massive black 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 active supermassive black holes, covering four percent of the Northern celestial hemisphere, based on ultra-low radio wavelengths, as detected by LOFAR.[24]

Transient radio phenomena and pulsars

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,

SETI signals.[25] In addition, this key science project will make a deep survey for radio pulsars at low radio frequencies, and will attempt to detect giant radio bursts from rotating neutron stars
in distant galaxies.

Ultra high-energy cosmic rays

LOFAR offers a unique possibility in particle physics for studying the origin of high-energy and ultra-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 of electron-positron pairs which emit radio emission in the terrestrial magnetosphere (e.g., geo-synchrotron emission).

Cosmic magnetism

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 the Faraday effect, which is the rotation of polarization plane of low-frequency radio waves, and gives another tool to detect weak magnetic fields.[27]

Solar physics and space weather

The Sun is an intense radio source. The already strong thermal radiation of the 106 K hot

solar corona is superimposed by intense radio bursts that are associated with phenomena of the solar activity, like flares and coronal 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 for space 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.

Advanced Technology Solar Telescope and the Solar Orbiter
provide insights into this fundamental astrophysical process.

Timeline

The 'Zernikeborg' building, which houses the University of Groningen's computing centre

In the early 1990s, the study of aperture array technology for radio astronomy was being actively studied by ASTRON – 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 million

computer sciences and agriculture as well as astronomy
.

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 a low-noise amplifier (LNA), which provides enough amplification of the incoming signals to transport them over a 110 m long coaxial cable to the receiver unit (RCU).

On April 26, 2005, an

Blue Gene/L supercomputer was installed at the University of Groningen's math centre, for LOFAR's data processing. At that time it was the second most powerful supercomputer in Europe, after the MareNostrum in Barcelona.[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. CS1 52°54′32″N 6°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 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 the

Effelsberg
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.

LOFAR was officially opened on 12 June 2010 by Queen Beatrix of the Netherlands.[30] Regular observations started in December 2012.[citation needed]

See also

References

  1. ^ a b "What we look forward to in LOFAR 2.0: LOFAR expands to Italy". ASTRON. 18 June 2020.
  2. ^ "System Capabilities | ASTRON". Archived from the original on 2017-10-10. Retrieved 2011-10-04.
  3. S2CID 64719934
    .
  4. ^
    S2CID 4668405
    .
  5. ^ "Antenna Description". ASTRON. Retrieved 2015-05-12.
  6. ^ "Many-Core Processing for the LOFAR Software Telescope" (PDF).
  7. ^ "LOFAR Stations: Description and Layout". ASTRON. Retrieved 2015-05-12.
  8. ^ "German LOFAR stations". ASTRON. Retrieved 2015-05-12.
  9. ^ "Low Frequency Array (LOFAR)".
  10. ^ "LOFAR: Radioastronomie im digitalen Zeitalter".
  11. ^ "LOFAR-Station Norderstedt".
  12. ^ "LOFAR:UK". ASTRON. Retrieved 2015-05-12.
  13. ^ "LOFAR in France". ASTRON. Retrieved 2015-05-12.
  14. ^ "LOFAR at Onsala Space Observatory". Chalmers University of Technology. Retrieved 2015-05-12.
  15. ^ "Onsala Space Observatory Division | Chalmers". www.chalmers.se. Retrieved 2021-08-15.
  16. ^ "The BUild – I-LOFAR". Archived from the original on 30 June 2018. Retrieved 28 December 2017.
  17. ^ "LOFAR-BG". Retrieved 2023-05-17.
  18. ^ "From LOFAR to NenuFAR" (PDF). Retrieved 2017-06-21.
  19. ^ "NenuFAR, the LOFAR Super Station" (PDF). Retrieved 2017-06-21.
  20. doi:10.1109/TGRS.2014.2342252
    .
  21. Bibcode:2012ASPC..461..693B
    .
  22. Bibcode:2007wmdr.confE..30F
    .
  23. ^ "Epoch of Reionisation". lofar.org. Retrieved 2011-02-23.
  24. ^ Starr, Michelle (22 February 2021). "The White Dots in This Image Are Not Stars or Galaxies. They're Black Holes". ScienceAlert. Retrieved 22 February 2021.
  25. ^ Johnson, Owen (2023-12-05). "Seti: how we're searching for alien life at previously unexplored frequencies". The Conversation. Retrieved 2023-12-06.
  26. ^ LOFAR Science Case: Ultra High Energy Cosmic Rays Archived 2005-11-10 at the Wayback Machine
  27. ^ scholarpedia.org
  28. ^ TOP500 List – June 2005
  29. ^ COBALT A new correlator for LOFAR. 2013
  30. ^ "ASTRON". Archived from the original on 2011-07-24. Retrieved 2010-06-13.

External links

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