LOFAR

Technical information

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' 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 digitised, 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.

LOFAR stations

To make radio surveys of the sky with adequate resolution, the antennas are arranged in clusters (stations) 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 8 international stations. Each of the core and remote stations has 48 HBA's and 96 LBA's and a total of 48 digital Receiver Units (RCU's). International stations have 96 LBA's and 96 HBA's and a total of 96 digital Receiver Units (RCU's).

The locations of the international LOFAR stations are:

Additionally, a set of LOFAR antennas is deployed at the KAIRA (Kilpisjärvi Atmospheric Imaging Receiver Array) near Kilpisjärvi, Finland. This installation functions as a VHF receiver either in stand-alone mode or part of a bistatic radar system together with EISCAT transmitter in Tromsø.

Data transport requirements are in the range of several gigabits per second per station and the processing power needed is tens of TeraFLOPS. The data from LOFAR is stored in the LOFAR long-term archive. The archive is implemented as a distributed storage with data spread over the Target data center located in the Donald Smits Center for Information Technology at the University of Groningen, SURFsara center in Amsterdam and the Forschungszentrum Jülich in Germany.

Sensitivity

The mission of LOFAR is to map the Universe at radio frequencies from ~10–240 MHz with greater resolution and greater sensitivity than previous surveys, such as the 7C and 8C surveys, and surveys by the Very Large Array (VLA) and Giant Meterwave Radio Telescope (GMRT).

LOFAR is the most sensitive radio observatory at its low observing frequencies, until the next generation of large array radio telescope, the Square Kilometre Array (SKA), comes online around 2025.

Science case

The sensitivities and spatial resolutions attainable with LOFAR will 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.

The epoch of reionization

One of the most exciting, but technically most challenging, applications of LOFAR will be the search for redshifted 21 cm line emission from the Epoch of Reionization (EoR). 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.

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.

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, X-ray observatories). Such transient phenomena may be associated with exploding stars, black holes, flares on Sun-like stars, radio bursts from exoplanets or even SETI signals. 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 10^{15}-10^{20.5} eV. 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.

Solar physics and space weather

The Sun is an intense radio source. The already strong thermal radiation of the 10 6 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. RHESSI, Hinode, the Solar Dynamics Observatory (SDO), and eventually the Advanced Technology Solar Telescope and the Solar Orbiter provide insights into this fundamental astrophysical process.

Timeline

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 euro to fund the infrastructure of LOFAR under the Bsik programme. In accordance with Bsik guidelines, LOFAR was funded as a multidisciplinary sensor array to facilitate research in geophysics, 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 IBM Blue Gene/L supercomputer was installed at the University of Groningen's math center, for LOFAR's data processing. At that time it was the second most powerful supercomputer in Europe, after the MareNostrum in Barcelona. Since 2014 an even more powerful computing cluster (correlator) called COBALT performs the correlation of signals from all individual stations.

In August/September 2006 the first LOFAR station (Core Station CS001, aka. CS1 52°54′32″N 6°52′8″E) 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. Regular observations started in December 2012.