Science Highlights

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

The Low-Frequency ARray (LOFAR) has been used to search for the elusive dark matter in six 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 microgauss magnetic field strengths we expect in galaxies, these electrons generate non-thermal synchrotron emission that peaks in the range of hundreds of megahertz. With new 50 hours observation at 150 MHz as part of the LOFAR Two-metre Sky Survey (LoTSS), upper limits for the radio continuum emission in six dwarf spheroidal galaxies 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) and in Gajovic et al. 2023, A&A 673, A108]

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 winds
LOFAR 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 three nearby galaxies NGC 3556, NGC 4631, and NGC 5775. These galaxies are seen in edge-on position, where optical emission shows the stellar disc and the X-ray emission the hot gas i the halo. The 150-MHz radio emission extends vertically much further away than the stars with emission above and below the stellar plane and has a morphology that is similar to the hot gas traced by X-ray emission.

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. It this vertical pressure gradient that can explain the acceleration of the wind. [Published in Miskolczi et al., Astronomy & Astrophysics, 622, A9 (2019), Heald et al., Montly Notices of the Royal Astronomical Society, 509, 658, 2022, and Stein et al. 2023, A&A 670, A158]

Left: NGC 4631 with the 150-MHz radio continuum emission coded in orange and the X-ray emission in blue; Right: Vertical cosmic-ray electron advection speed in NGC 5775 as function of distance to the disc. The dashed area shows the expected range of escape velocities.
Radio halos tracing the evolution of merging clusters of galaxies
Massive, 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)]

The galaxy cluster Abell 1314 hosts large-scale radio sources that have been affected by its merger with another cluster. Non-thermal radio emission detected with LOFAR is shown in red and pink, and thermal X-ray emission detected with Chandra is in gray, overlaid on an optical image.
Why lightning often strikes twice
It 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)]

Lightning above LOFAR (montage). Credit: University of Groningen, Olaf Scholten.
Cosmic Magnetic Fields
Magnetic 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)]

Left: A giant radio galaxy (contours) and its Faraday rotation measure distribution (colour). The insets show the Faraday spectra with the red crosses marking the Faraday rotation measure value of the polarised emission from the radio galaxy.Right:Estimated size and location of foreground cosmic web filaments.
Giant radio jets as seen by LOFAR
LOFAR 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 twisting structure in red shows the radio emission traced by LOFAR at 145 MHz of a famous and well-studied radio galaxy known as 3C31. This is superposed to an optical image (white objects which trace regions with stars) of the field. Straightaway, one can see the huge extent of the radio emission compared to the optical size. The galaxy is surrounded also by a hot rarefied gas cloud, which has been discovered in X-ray light (shown in blue).
The first detection of a low-frequency turnover in non-thermal emission from the jet of a young star
Radio 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)]

Contour plot of the DG Tau A LOFAR observation at 152 MHz overlaid on a colour map of a 6 GHz image of DG Tau A from the VLA (Purser et al. 2018). The dashed line indicates the blue approaching lobe and the red receding lobe of the DG Tau jet.
Determining timing properties of 35 pulsars discovered by LOFAR
The LOFAR Tied-Array All-Sky Survey (LOTAAS) detected 73 so far unknown pulsars using the LOFAR superterp, as was published in 2019. Subsequently, these newly detected pulsars were further analyzed and described by different groups of scientists. This undertaking has recently been completed with the publication of timing models for the remaining 35 sources. For this, these 35 pulsars were monitored with several international LOFAR stations (including the six GLOW stations in Germany) and other radio telescopes. [Published in Emma van der Waateren et al., Astronomy & Astrophysics, Vol. 669, 2023]

Pulse profiles of the 35 described pulsars. By averaging the repeating signal that we can observe for each source, reveals a shape that is relatively stable over time and that is characteristic for the particular pulsar. The different colors show the profile for different observing frequencies. Different properties like the spin period of the specific pulsar and a measure for the dispersion of the signal that we receive have to be determined, so that the observational data of the individual pulses can be combined like this. These properties were part of the data that was recently published.
Constraining the intergalactic medium properties with LOFAR upper limit on the 21cm signal
Detection 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)]

Left panel: map of 21cm signal at z=9.1 for one of the reionization models studied in Ghara et al. (2020). Right panel: the curves show the power spectra of the 21cm signal at different scales for the 1495 models studied in Ghara et al. (2020). The red points with error-bars show the upper limits from the LOFAR observations by Mertens et al. (2020), while the blue dashed curve represents the model corresponding to the map in the left panel. All the models above the observational points are excluded by LOFAR observations.
Joint observations with Parker Solar Probe and Solar Orbiter
LOFAR 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.

Left panel: Solar distances of plasma frequencies covered by the frequency ranges of LOFAR (10 - 240 MHz) and the FIELDS instrument on PSP (5 kHz - 16 MHz). LOFAR covers the middle and upper corona, while FIELDS extends far into interplanetary space. Middle panel: NASA’s Parker Solar Probe. Right panel: LOFAR and PSP/FIELDS observations of a type III radio burst. The energetic electrons causing the radio emission can be traced through the corona into interplanetary space up to the spacecraft location.
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).

Left: The A-team as seen with the LOFAR at 60 MHz. In this composite the relative sizes are preserved. The A-team consists of two Supernova remnants located in our own Galaxy (at the top) and two distant radio galaxies harboring supermassive black holes. Right: Mosaic image of the preliminary release of the LOFAR LBA Sky Survey (LoLSS), covering 740 deg2 on the Northern Sky. The image contains >25000 radio sources.