What is LOFAR?

LOFAR forms the world’s largest and most sensitive radio telescope and operates at the lowest radio frequencies that can be observed from Earth. Unlike classical dish telescopes, LOFAR is a multipurpose sensor network, with an innovative computer and network infrastructure that can handle extremely large data volumes. Its long-term archive already contains over 40 petabytes of astronomical data.

With its wide and flexible range of capabilities, LOFAR is having an important impact on a
broad range of astrophysics, from cosmology to Solar system studies. Much of LOFAR's highest-impact science is being carried out through ‘Key Science Projects’ (KSPs),
composed of broad teams of researchers from several European countries. Key Science
Projects include:

• The Epoch of Reionisation (EoR): the EoR is the epoch in the history of the
  Universe during which the first radiating sources (stars, galaxies) switched on. At
  these redshifts (7 < z < 11), the 21cm HI line is redshifted into the LOFAR highband.
  LOFAR has set the deepest upper limits on the redshifted 21cm signals of
  reionisation, and has strong prospects for obtaining the first EoR detection in the
  near future.
• Low frequency sky surveys: LOFAR's low frequencies open up one of the few
  poorly explored regions of the electromagnetic spectrum. Its combination of a
  dense core and long (international) baselines offers a unique combination of high
  angular resolution, high sensitivity and wide field of view combine to offering
  remarkable survey capabilities. LOFAR’s all-sky and dedicated deep-field surveys
  are greatly advancing our understanding of the formation and evolution of
  galaxies, clusters, and active galactic nuclei, as well as enabling cosmological
  studies, and will have long-lasting legacy value.

• Radio Transients and Pulsars: this is focused on the study of extreme
  astrophysical environments and events, in which physical conditions (density,
  pressure, temperature, gravity, magnetic fields, etc.) are far beyond those that
  can be reproduced on Earth. LOFAR is achieving an excellent discovery rate of
  new pulsars, including some rare examples. The recent completion of LOFAR's
  Responsive Telescope functionality is enhancing the array's capabilities in
  transient radio sources and multi-messenger astronomy, for which the wide
  field-of-view is particularly valuable.
• Cosmic Magnetism: in spite of the important role played by magnetic fields
  across a wide range of astrophysical scales, from black holes to galaxy clusters
  and beyond, there remains little empirical constraint on the origin of cosmic
  magnetic fields and a paucity of information on their structure and evolution.
  LOFAR’s high sensitivity and broad low-frequency bandwidth give it a powerful
  capability to address these fundamental issues.
• Solar and Heliospheric Physics: the Sun is a powerful emitter at radio
  wavelengths, not only during intense bursts of activity related to phenomena
  such as Solar flares and coronal mass ejections, but also during times when it is
  considered quiet at other wavelengths. LOFAR offers a highly effective Solar
  monitoring and imaging system, providing valuable insight not only into Solar
  physics, but also into the effect of Solar activity on the Earth's space
  environment.
• Ultra-High Energy Cosmic Rays: ultra-high energy cosmic rays are
  predominantly protons and heavy nuclei with energies far exceeding those
  produced in terrestrial particle accelerators. When these impact the atmosphere
  they produce a particle shower of secondary charged particles, which are then
  deflected by the Earth's magnetic field and produce coherent radiation at
  frequencies below about 100 MHz. LOFAR’s transient buffer capabilities allow it
  to study these radio showers in detail to identify the composition and origin of
  these ultra-high energy cosmic rays.

LOFAR, inaugurated in 2010, is an innovative radio telescope operating between 10 and 240 MHz, consisting of an array of many thousands of antennas distributed in fields (stations) across Europe. LOFAR does not have moving parts; steering and tracking are achieved by treating the signal from the individual antennas in each station with advanced digital beam-forming techniques that make the system agile, allowing for rapid repointing of the telescope as well as giving the potential for multiple simultaneous observations.

As of early 2021, there are 52 LOFAR stations, located in 8 countries in Europe. LOFAR has a dense core in the north of the Netherlands, close to Exloo, with 6 stations located within a 320 m-diameter island known as the Superterp, and a further 18 stations within a 2 km radius. The Superterp stations can be phased-up into a single station for increased sensitivity. Beyond the core, there are a further 14 ‘remote’ stations within the Netherlands, extending out to baselines of 90 km. LOFAR then has a further 14 international stations reaching out to over 1000km from the core. This combination of a dense core and long interferometric baselines spanning Europe allows LOFAR to achieve unparalleled sensitivity and angular resolution (approaching that of the Hubble Space Telescope at optical wavelengths) in the low-frequency radio regime.

Each LOFAR station has two separate antenna arrays: the low-band antennas (LBA)
operating below 80 MHz, and the high-band antennas (HBA) operating above 110 MHz (the FM radio band prohibits observations between ~80 and 110 MHz). The signal from the antennas is streamed to a central processing facility based in Groningen (the Netherlands). The data rate is up to 3 Gb/sec per station for the current LOFAR array, and so a dedicated wide-area network connects the Dutch LOFAR stations, and dedicated international links are used to transport the data from international stations.

The first stage of the central processing handles the real-time data operations, such as the correlation of the data streams. The correlated data is then streamed to an offline Central Processing facility, on which application-dependent offline processes are run on the data (e.g. for standard imaging, these include some or all of: flagging of corrupted data; averaging; calibration; self-calibration; image creation). The data are then transported to the Long-Term Archive (LTA), where they are stored for further distribution to the scientific community. As of early 2021, there are three LTA data centres, located in Amsterdam (the Netherlands), Jülich (Germany), and Poznan (Poland).