Full list of publications with 'LOFAR' in abstract (ADS search)
A full publication list of LOFAR-related papers can be retrieved from the NASA SAO/NASA Astrophysics Data System (ADS). You can use this link to directly query for LOFAR publications in ADS. Note, here we search for all articles which have the acronym LOFAR in the abstract.
Recent LOFAR Highlights
The LOFAR telescope allows to study the low frequency radio sky with unprecedented resolution and sensitivity. Here we feature recent results with significant contribution from GLOW researchers.
Dark matter search
Recently, the Low-Frequency ARray (LOFAR) has been used to search for the elusive dark matter in dwarf spheroidal galaxies using radio continuum observations. One of the leading dark matter candidates are the weakly interacting massive particles (WIMPs), which can produce via several annihilation channels cosmic-ray electrons. With the micro-Gauss magnetic field strengths we expect in galaxies, these electrons generate non-thermal synchrotron emission that peaks in the range of hundreds of MHz. With a new 8-hour observation at 150 MHz as part of the LOFAR Two-metre Sky Survey (LoTSS), upper limits for the radio continuum emission in the dwarf spheroidal galaxy Canes Venatici I (CVnI) were obtained. These can be converted into new upper limits for the cross-section of WIMPs. LOFAR has quite good sensitivity, so that for a reasonable choice of model parameters such as magnetic field strength and comic-ray diffusion coefficient, the limits are comparable with those set by the Fermi Large Area Telescope using gamma-ray observations of this particular galaxy. The benchmark limits of this new work exclude already several thermal WIMP realizations in the [2, 20]-GeV mass range. [Published in Vollmann et al., MNRAS, 496, 2663 (2020)]
Figure. (a) 150-MHz map of Canes Venatici I observed with LOFAR, where the large circle shows the extent of the stellar disc and the small circles show the position of background sources; these were subtracted from the map to search for the diffuse emission from annihilating WIMPs. (b) new resulting cross-section of WIMPs annihilating into electron–positron pairs. Several models with various magnetic field strengths and diffusion coefficients are presented.
LOFAR reveals galactic windsLOFAR is the ideal telescope to study the faint radio haloes surrounding star-forming galaxies. These consist of aged cosmic-ray electrons that spiral around magnetic field lines. Radio haloes may be the result of galactic winds, which are driven by stellar feedback. Cosmic rays are thought to play an important role in them, driving them more effectively than the hot thermal gas alone. New observations with LOFAR have looked at the nearby galaxies M108 (NGC 3556) and NGC 5775. The LOFAR map shown here on the left has a size many times larger than the apparent size of the Moon (bottom left corner). The sources that are visible on this map are galaxies and not foreground stars in our own Galaxy. The galaxy, shown in the two insets, is seen in edge-on position. In the optical emission (bottom right), we see the highly inclined stellar disc. Now the 150-MHz radio emission, shown in the top right, extends vertically much further away than the stars with emission above and below the stellar plane; this emission is also shown as contours in the bottom right.
The analysis of the radio data showed that the magnetic plasma must accelerate while expanding into the circum-galactic medium. The wind speed exceeds the escape velocity of the galaxy and its dark matter halo, meaning that the gas will eventually leave the galaxy. A model for such a wind was developed for the analysis of NGC 5775 which takes the form of an expanding flux tube, which can explain the decrease of cosmic ray and magnetic field energy densities (see right picture). It this vertical pressure gradient that can explain the acceleration of the wind. [Published in Miskolczi et al., Astronomy & Astrophysics, 622, A9 (2019) and Heald et al., Montly Notices of the Royal Astronomical Society, 509, 658, 2022]
Left: M108 (NGC 3556) by Miskolczi et al., where a radio halo becomes visible; Right: Model developed for NGC 5775 by Heald et al., which assumes an expanding flux tube.
Radio halos tracing the evolution of merging clusters of galaxiesMassive, merging galaxy clusters often host giant, diffuse radio sources that arise from shocks and turbulence. These sources cover large regions ("halos") in the cluster where synchrotron radiation is emitted by relativistic electrons spiraling around magnetic field lines. In a pilot study using the recently published LOFAR Two-Metre Sky Survey (LoTSS) three galaxy cluster were examined and it appears that they are in pre-merging, merging, and post-merging states, respectively. Systematic studies of this kind over a larger sample of clusters will help constrain the time scales involved in turbulent re-acceleration and the subsequent energy losses of the underlying electrons. [Published in Wilber et al., Astronomy & Astrophysics 622, A25 (2019)]
Why lightning often strikes twiceIt had been know for many years that lightning strikes emit radio waves, but it wasn't until LOFAR started measuring lightning strikes that it was understood how much detail about lightning initiation and propagation could be learned from high-resolution radio signals. LOFAR has a much higher antennas density and faster recording speed than the typical lightning interferometers. Thus, the images that can be reconstructed from LOFAR are of much higher quality and have revealed and keep revealing unknown details. For example, LOFAR data can explain the flickering of a lightning strike through little 'needles' that store charge along the negative leaders and LOFAR data shed light on the size of the initiation region, which may help to finally explain how lightning is initiated. [Published in B. Hare et al., Nature, Volume 568, pages 360–363 (2019) and O. Scholten et al., Phys. Rev. Lett. 124, 105101 (2020)]
Cosmic Magnetic FieldsMagnetic fields pervade the cosmos, and we want to understand how this happened. Cosmological simulations predict that measuring the magnetic field in filaments of the cosmic web, away from clusters of galaxies, can help distinguish between a primordial or astrophysical (i.e. outflows from AGN/galaxies) origin. Although measuring weak magnetic fields in intergalactic space is difficult, LOFAR provides the ability to measure the Faraday rotation effect of these weak fields with unprecedented accuracy. An example is the measurement of the polarised emission from a giant radio galaxy (3.4 Mpc in size) and the associated Faraday rotation of the emission, to constrain the magnetic field properties of cosmic web filaments in the foreground. This demonstrates the unique capability of LOFAR in the study of cosmic magnetic fields. [Published in O'Sullivan et al.: In: Galaxies Vol. 6/4, p.126 (2018)]
Giant radio jets as seen by LOFARLOFAR was from the beginning expected to provide beautiful images of the diffuse and faint radio emission in radio galaxies. The radio emission stems from powerfull outflows (jets) generated in the vicinity of the supermassive black hole located in the center of the galaxies. The new LOFAR 145-MHz map shows that the galaxy 3C 31 has a larger physical size than previously known, reaching 1.1 Mpc (4 million light-years!). This means 3C31 now falls in the class of giant radio galaxies. However, the 145-MHz LOFAR image is not only beautiful, but also very useful for understanding how such huge objects like the jets of 3C31 evolve. The analysis revealed that the plasma flow in the jets must decelerate while expanding into the intergalactic medium. This would suggest an age of the radio galaxy of about 190 Myr, implying supersonic expansion of the tails of plasma. [Published in Heesen et al., In: MNRAS 474, 5049 (2018)]
The first detection of a low-frequency turnover in non-thermal emission from the jet of a young starRadio emission in jets from young stellar objects (YSOs) in the form of non-thermal emission has been seen toward several YSOs. Thought to be synchrotron emission from strong shocks in the jet, it could provide valuable information about the magnetic field in that jet. Using LOFAR, synchrotron emission in two emission knots in the jet of the low-mass YSO DG Tau A at 152 MHz has been detected now, the first time non-thermal emission has been observed in a YSO jet at such low frequencies.
Furthermore, in one of the knots a low-frequency turnover in its spectrum is clearly seen compared to higher frequencies -- the first time such a turnover has been seen in non-thermal emission in a YSO jet. Of the several possible mechanisms, the Razin effect appears to be the most likely explanation for this turnover. From the Razin effect fit, an estimate for the magnetic field strength within the emission knot of ∼ 20 μG can be obtained. If the Razin effect is the correct mechanism, this is the first time the magnetic field strength along a YSO jet has been measured based on a low-frequency turnover in non-thermal emission. [Published in Feeney-Johansson et al., The Astrophysical Journal Letters 885, L7 (2019)]
First detection of frequency-dependent, time-variable dispersion measuresRadio pulsars are rapidly rotating neutron stars that are seen as pulsating sources of radio emission due to the "lighthouse" effect. When the pulses pass through the interstellar medium, they are affected by several frequency-dependent effects that are most pronounced at low frequencies. With LOFAR, we can precisely monitor the dispersion measure (DM), which is equivalent to the amount of electrons between us and the source. For the first time, we were able to detect a frequency dependence of the DM as the radiation takes slightly different paths at different frequencies. This helps us to understand the interstellar medium and its effect on pulsar timing experiments. [Published in Donner et al., Astronomy & Astrophysics, Vol. 624, 2019]
Constraining the intergalactic medium properties with LOFAR upper limit on the 21cm signalDetection of the 21cm line of neutral hydrogen from the high redshift intergalactic medium (IGM) is expected to accurately probe its reionization, shedding light on one of the most elusive epoch in the history of our Universe. The LOFAR Epoch of Reionization (EoR) team has recently published the best upper limits on the 21cm signal power spectrum at z=9.1, based on 141h of data, improving by a factor of about 8 on the previously reported LOFAR upper limit. Using a combination of state-of-the-art N-body simulations, 1D radiative transfer calculations and a Bayesian inference framework to constrain the parameters which describe the physical state of the IGM, the LOFAR EoR team found that the new upper limits exclude some reionization models. This exciting result shows that in the near future, once more data will be processed, observations with LOFAR will be able to constrain the physical properties of the IGM at high redshift and the history of reionization. [Published in Ghara et al., MNRAS 493, 4728 (2020) and Mertens et al., MNRAS 493, 1662 (2020)]
Joint observations with Parker Solar Probe and Solar OrbiterLOFAR observations of the Sun can be complemented ideally by spacecraft like NASA’s Parker Solar Probe (PSP) or ESA’s Solar Orbiter, to study the solar activity processes in the outer corona and the near-Sun interplanetary space.
The Key Science Project (KSP) “Solar Physics and Space Weather with LOFAR” prepares observing time proposals for the Sun with LOFAR. In the period 2018 – 2020 a Long-Term proposal has provided a total of 1064 hours for the first four perihelion passes of PSP. After the end of the long-term period, the Solar KSP submits semester-wise observing cycle proposals. The cycle 14 proposal “Deciphering the state of the inner heliosphere with synergistic observations from LOFAR, PSP, and Solar Orbiter” has been granted the full requested 224 hours. It covers the PSP perihelia on 27 June and 27 September 2020, and furthermore provides for the first time joint observations with Solar Orbiter during its remote-sensing checkout window on 15 - 22 June 2020. LOFAR observes the Sun with a combination of imaging and spectroscopic modes, plus scintillation and Faraday rotation studies in near-Sun interplanetary space. These observations are complemented by remote sensing and in-situ spacecraft data from the inner heliosphere.
The combination of LOFAR with the FIELDS instrument onboard PSP (Figure, middle panel) is able to measure the solar and interplanetary radio radiation in the range 10 kHz -20 MHz and 10 MHz – 240 MHz, respectively (Figure, left panel). Thus, LOFAR and FIELDS deliver comprehensive radio data for studying the evolution of solar activity in the corona and their propagation into the interplanetary space. The analysis of these LOFAR data is a current activity. The right panel in the Figure shows an example of a type III radio burst, tracing energetic electrons from their source in the solar corona through interplanetary space.
LOFAR hunting for supermassive black holes
It is widely agreed that galaxies harbor black holes which weigh as much as hundreds of millions of Suns in their centers. Such supermassive black holes can be strong emitters of radio emission. Actually, the radio sky as seen with LOFAR at very low frequencies is dominated by such galaxies, also called Active Galactic Nuclei (AGN). The radio images of two them are reproduced in a composite in the left panel of the Figure together with two Supernova remnants. These are the four most powerful radio sources in the northern hemisphere. Historically, the brightest radio sources in the sky were named after the constellation in which they were found followed by a letter starting with an "A". They were then grouped in the so-called A-team. The upper two objects in the Figure (Cassiopeia A and Taurus A) are the supernova remnants: leftovers of the explosions of two stars in our own Galaxy. The bright dot centered on Taurus A is the Crab pulsar. In the center the AGN Cygnus A is pictured, an extremely powerful radio galaxy 600 million light years away, whose two lobes are powered by jets of energetic particles formed near a supermassive black hole. At the bottom, the other AGN Virgo A consists of an extended structure (larger than an entire galaxy) that surrounds the famous supermassive black hole at the centre of the galaxy M87, the same black hole recently imaged by the Event Horizon Telescope. [For more details see de Gasperin et al., Astronomy & Astrophysics, Vol. 635, A150, (2020)]
More distant AGN have fainter radio emissions and can be discovered in large numbers by surveying the sky with LOFAR at very low frequencies. Recently the largest and sharpest map of the sky at such ultra-low radio frequencies around 50 MHz had been completed. It reveals more than 25,000 active supermassive black holes in distant galaxies. At a first glance, the map (reproduced in the right panel of the Figure below) looks like an image of a starry night sky. However, stars are almost invisible in the radio band, but instead diffuse matter in the large scale structure of the Universe; fading jets of plasma ejected by supermassive black holes, and exoplanets whose magnetic fields are interacting with their host stars dominate the picture. Albeit among the largest of its kind, the published map only shows two percent of the sky. The survey (LoLSS: LOFAR LBA Sky Survey) will continue for several years until a map of the entire northern sky will be completed. There is a reason why the Universe at very long radio wavelengths is almost uncharted: such observations are very challenging. The ionosphere, a layer of free electrons that surrounds the Earth, acts as a lens continuously moving over the radio telescope. The effect of the ionosphere can be compared to trying to see the world while being submerged in a swimming pool. Looking upwards, the waves on the water bend the light rays and distort the view. To account for ionospheric disturbances, the scientists used supercomputers and new algorithms to reconstruct its effect every four seconds over the course of 256 hours of observation. The map was originally published by de Gasperin et al., Astronomy & Astrophysics, Vol. 648, A104, (2021).
Not So Recent LOFAR Highlights
Some slightly older (but still exciting) science highlights that were featured on this page at some earlier time can be found here.