LOFAR (Low Frequency Array) is currently the largest radio telescope operating at the lowest frequencies that can be observed from Earth. Unlike single-dish telescope, LOFAR is a multipurpose sensor network, with an innovative computer and network infrastructure that can handle extremely large data volumes.
The core of LOFAR consists of a network of thirty eight stations concentrated in the north-east of the Netherlands. In addition, LOFAR is extended with 12 stations in Germany, France, Sweden, the United Kingdom and Poland. An ongoing project (NenuFAR) aims to build a separate super-station in Nancay which will greatly increasing the sensitivity of LOFAR.
LOFAR is also a recognised science and technology pathfinder facility for the next-generation radio telescope, the Square Kilometre Array (SKA). The SKA is being prepared by a global collaboration of countries and institutes, in which ASTRON has been an initiator and principal player throughout. ASTRON is now taking many concepts from LOFAR, in particular the (real-time and off-line) handling of huge data streams, to the next level in preparation for the SKA.
The LOFAR radio telescope has facilities in several countries, that are owned by various parties (each with their own funding sources), and that are collectively operated by the International LOFAR Telescope (ILT) foundation under a joint scientific policy.
Next to its two types of ultra-modern phased array antenna stations for the flagship radio astronomy application, the LOFAR network currently has stations with seismic sensors (geophones) and infrasound detectors for atmospheric studies.
Science with LOFAR
The revolutionary multi-beaming capabilities of the LOFAR telescope allow astronomers to engage in multiple lines of research at once: they can look back billions of years to a time before the first stars and galaxies were formed (the so-called ‘Dark Ages’), they can survey vast areas of the low-frequency radio sky, and they can be constantly on the lookout for radio transients originating from some of the most energetic explosions in the universe.
- Epoch of Reionization
The Epoch of Reionization (EoR) is the period during which the neutral gas in the Universe was completely ionised by the first stars and galaxies. This important event occurred when the Universe was a few hundred million years old (about a twentieth of its current age) and is critical in understanding how the first stars and galaxies formed. Observations of the 21cm (1420 MHz) line, red-shifted to frequencies between 70 and 200 MHz, are a unique probe of this era. LOFAR is the key pathfinder to investigate how to solve the calibration and imaging challenges at these low frequencies. Once these are solved, several thousand hours of LOFAR observations (a few peta-bytes of data) are needed to detect the very faint hydrogen signals.
Image: A high-resolution image of a small (25x35 arc-minute) area close to the true north celestial pole (i.e. declination 90) made from a deep integration using a bandwidth of 60 MHz in the HBA-band. It has a resolution of about 6 arc-seconds and a noise level of about 40 microJy
© Sarod Yatawatta, Ger de Bruyn and the EoR team
- Deep Extragalactic Surveys
The International LOFAR Telescope offers a transformational increase in radio survey speed compared to existing radio telescopes, as well as opening up one of the few poorly explored regions of the electromagnetic spectrum. For these reasons, an important goal that has driven the development of LOFAR since its inception is to explore the low-frequency radio sky through surveys, in order to advance our understanding of the formation and evolution of galaxies, clusters and active galactic nuclei (AGN).
The LOFAR Surveys Key Science Project has planned a wedding-cake survey strategy, with three tiers of observations, to be carried out over the next 5 years. Tier-1 is the widest tier, and includes low-band (LBA) and high-band (HBA) observations across the whole northern sky. Deeper Tier-2 and Tier-3 observations cover smaller areas, focusing on fields with the highest quality multi-wavelength datasets available across a broad range of the electromagnetic spectrum.
Reducing LOFAR radio data is a major challenge. This is due to the enormous data rates, bright sources far away from the pointing centre dominating the signal, radio frequency interference from radio, TV and planes and the corrupting influence of the ionosphere. The breakthrough idea was ‘facet-calibration’ and enables us to produce, for the first time ever, thermal noise limited maps at low frequencies with an angular resolution of ~5 arc seconds from 8 hours of data.
A key aim of the project is to study the properties of shocks and turbulence in merging, low-redshift galaxy clusters. The facet calibration technique has resulting in some of the most sensitive and high-resolution low-frequency images that have ever been produced. These images are highlighting new insights whilst revealing many new questions into how particles are accelerated within the intra-cluster medium (ICM). An excellent example is the LOFAR image of the `Sausage cluster’, enabling the most precise characterisation of a cluster shock ever.
Image: The most sensitive image of the Sausage cluster observed with the LOFAR HBA antennas at 150 MHz. The cluster shows two opposing giant radio relics created through galaxy cluster mergers. The merger heats up the intra-cluster gas to extremely hot plasma emitting X-rays (green).
Here, particles (e.g. electrons) are accelerated to very high energies. The rest of the resolved sources are mostly radio galaxies hosting an active super massive black hole.
© Duy Hoang (Leiden), Tim Shimwell (Leiden), Andra Stroe (ESO), Reinout van Weeren (Harvard), Georgina Ogrean (ESO) and Huub Rottgering (Leiden) for the LOFAR surveys team
- Transient sources and pulsars
The cosmic lighthouses known as ‘pulsars’ shine brightest in long-wavelength radio light, making LOFAR an ideal telescope for studying them. With its huge sensitivity and enormous computational power, LOFAR has already discovered 50 new pulsars, including a pair of millisecond pulsars, as part of its all sky survey, LOFAR Tied-Array All-Sky Survey (LOTAAS), and dedicated searches.
The exceptional sensitivity and wide bandwidth have also enabled LOFAR to make detailed studies of the emission properties of the largest ever samples of normal and millisecond pulsars. Pulsars also provide us with excellent probes of the ionised interstellar medium and magnetic fields and at LOFAR frequencies it is revealing new information on how they vary as a function of space and time through detailed studies of individual objects and precision timing observations.
LOFAR has also been used in combination with the XMM-Newton X-ray telescope to reveal the perplexing relationship between the radio and X-ray emission from two pulsars which show variable radio emission. High-energy astrophysical sources such as pulsars and black holes can also be associated with ejections of matter and energy at close to the speed of light. Such relativistic outbursts have a characteristic signature in the radio band, and LOFAR has been used to both search for new events, utilising its enormous field of view, and to follow-up events first detected with other facilities such as orbiting X-ray telescopes and Advanced LIGO (Laser Interferometer Gravitational-Wave Observatory).
In the past year astronomers have discovered LOFAR’s first ‘blind’ (previously unknown) transient, in the direction of the north celestial pole, and measured the low-frequency spectrum of radio emission from a nearby black hole, V404 Cyg, in outburst and followed-up the first detection of gravitational waves.
Image: The figure shows radio pulsar B0329+54: on the left-hand side, the radio sky around this pulsar is pictured; on the right-hand side, the individual pulses of this lighthouse-like source are shown, stacked onto on another. LOFAR is unique in allowing for simultaneous imaging and high-time resolution recording!
© LOFAR Pulsar Working Group
- Ultra-high energy cosmic rays
Astroparticle physicists have known for a long time that there are extremely energetic particles (ionised atomic nuclei, i.e. atomic nuclei without surrounding electrons) that travel through our Universe. These particles carry macroscopic energies of tens of Joules, concentrated in one elementary particle. These particles have energies many thousand times bigger than what can be achieved in man-made accelerators.
How and where nature accelerates these highest-energy particles in the Universe is an open question. The favourite explanation is that they are being accelerated by exploding stars (in so-called supernova explosions) in our Milky Way and by supermassive black holes at the centre of other galaxies.
In these active galaxies, a black hole is swallowing material and ejecting jets at velocities close to the speed of light. It is commonly assumed that up to a certain energy cosmic rays are accelerated in our Milky Way and at the highest energies they are originating in other galaxies, i.e. they are of an extragalactic origin.
LOFAR offers the unique possibility to study the origin of high-energy cosmic rays through the detection of sharp pulses of radio emission that are associated with air showers, caused by the interaction of cosmic rays with the Earth’s atmosphere.
A key method to explore the origin of cosmic rays is to measure their properties at Earth, such as their arrival direction, their (kinetic) energy, and the type of particle, often expressed in terms of the atomic mass. In recent publications, it has been shown that these properties are measured with unprecedented precision with LOFAR. The technique to measure the properties of cosmic rays through the recording of MHz radio pulses has been established by LOFAR and is now routinely used to explore the origin of cosmic rays. First LOFAR data are already constraining astrophysical models of the origin of cosmic rays. The data give new insights into the transition region from an origin of cosmic rays in our Milky Way to extragalactic sources.
Image: A LOFAR low-band antenna measurement of an air shower above the LOFAR Superterp. Good agreement between the data (shown by the circles) and simulations (shown by the background colours) is illustrated.
© A. Nelles & the LOFAR Cosmic Rays KSP
- Solar science and space weather
The Sun’s activity appears not only in the well-known 11-year Sunspot cycle but also in short duration eruptions as flares and coronal mass ejections (CMEs). Such eruptive events can harmfully influence our Earth’s environment and technical civilisation and are usually called Space Weather. These events are accompanied with an enhanced radio emission of the Sun especially in the frequency range (30-240 MHz) covered by LOFAR.
Hence, LOFAR is of great interest for solar physicists, since LOFAR with its spectroscopic and imaging capabilities is well suited for studying active processes in the Sun’s corona. This is the reason why the Key Science Project “Solar Physics and Space Weather with LOFAR” was founded. During LOFAR’s commissioning phase and the first cycles of regular observations, the solar KSP performed observations of the Sun together with ASTRON.
An example is presented in the figure below, showing the propagation of a type III burst source along the coronal magnetic field (white lines) through the corona. This example impressively demonstrates that LOFAR can really work as a dynamic spectroscopic radio imager of the Sun and is able to track fast motions of radio sources in the Sun’s corona.
Image: LOFAR’s imaging spectroscopy of a solar type III burst occurred in the frequency range 30-60 MHz at 11:00 UT on June 23, 2012. The radio source at 55 MHz is enclosed by a yellow contour line at a 50% flux density level. The white dots mark the source centers at seven frequencies in the range 30-60 MH to show the propagation path of the radio source.
The propagation appears outwards and along the coronal magnetic-field lines (white lines). The white dashed line indicates the distance of two solar radii from the center of the Sun. The blue dashed ellipse (top right) represents the width of the beam. The corresponding extreme ultraviolet image of the Sun (left) was simultaneously recorded by the SWAP instrument onboard the spacecraft PROBA2. It shows the structure of the lower corona.
- Cosmic magnetism
Magnetic fields exist throughout the Universe. They permeate the interstellar medium of our own Galaxy, trace the large-scale structure of external galaxies and fill even the voids and filaments of the vast cosmic web that follows cosmological structure formation. Observations at low radio frequencies with the LOFAR telescope allow us to measure these fields at high accuracy, revealing some of the weakest magnetic structures in the Universe.
Recent observations with LOFAR of the region around the bright quasar 3C196 (Jelic et al. 2015) have uncovered a rich morphology of polarised emission probing the ordered magnetic field in our own Galaxy (see image below). As well as complex filamentary structures, these images also show a network of dark linear features known as depolarisation canals. One explanation for how these canals are created is that they may be trails left by close-by fast-moving stars. To test this theory, further observations at higher frequencies are now in progress and, when combined with the results of simulations, they may be able to answer this question.
As well as revealing new structures in our own Galaxy, other recent LOFAR observations (Van Eck et al. 2016) along the plane of the Milky Way show extremely rich polarisation structures emanating from previously known objects. These data focus on nearby clouds of neutral hydrogen gas and have enabled us for the first time to determine the strength of the magnetic field in these clouds at high precision. Being able to measure such fields is important for understanding how stars form and galaxies evolve.
Beyond our own Galaxy, LOFAR is now revealing incredible detail within other nearby galaxies where individual star forming regions are being resolved. Never before have these regions been observed in external galaxies at such low frequencies. From these new data, LOFAR will help us understand the composition and astrophysics of galaxies similar to our own and violently energetic systems known as radio galaxies, which produce huge radio jets powered by super-massive black holes. Polarised radio emission has been observed in the jets and lobes of several such radio galaxies with LOFAR allowing us to obtain accurate magnetic field strengths for these distant objects.
Image: A colour composite image of polarised radio emission observed in 3C196 field is shown in figure.
Different colours represent emission detected at different Faraday depths. Straight and long filamentary structures are visible parallel to the Galactic plane. They are located somewhere within the Local Bubble. A system of linear depolarisation canals (black stripes) is also conspicuous in the image.
© Jelic et al.
The Multi-frequency Snapshot Sky Survey (MSSS) is a radio survey, imaging the northern celestial hemisphere using the LOFAR low and high frequency bands. The high band survey is complete with the results to be published soon and made accessible to the general public. MSSS is complementary to other surveys and its large frequency coverage is unique. It is the first survey of the northern sky performed with an interferometer based on aperture arrays. The MSSS effort is on going, as the full survey potential is being exploited, resulting in the first publications using MSSS data.
Image: Part of the radio sky as LOFAR sees it. This is a multi-frequency (false colour) image produced from data taken for the LOFAR MSSS survey. The image has a resolution of 45 arc-seconds. The nearby galaxies M81 and M82 are visible, along with the giant radio galaxy 4C 73.08.
© Riseley, Gurkan, Heald and the MSSS team.