Science Highlights

Recent LOFAR-related highlight publications by GLOW members

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.

  • 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 the nearby galaxy M108 (NGC 3556). The LOFAR map shown here 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. [Published in Miskolczi et al., Astronomy & Astrophysics, 622, A9 (2019)]

    Halos

     

  • 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)]

    Halos
    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)]

    Faraday rotation
    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)]

    3C 31
    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.

     

  • First detection of frequency-dependent, time-variable dispersion measures

    Radio 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]

     
    Variability of dispersion measure at different frequencies over time.

     

  • 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 images cosmic radio monsters

    Pareidolia is a tendency that pushes humans to see shapes in clouds or faces in inanimate objects. The picture shown here is a composition of four cosmic radio sources that can in fact look like a scary monster. To obtain this effect, the sources have been rearranged compared to their original position in the sky but their apparent sizes were preserved. However, in some sense, these sources are real monsters. Their names are: Cassiopeia A (top left), Taurus A (top right), Cygnus A (center), and Virgo A (bottom). 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, like the famous TV series from the 80s.  
    The nature of these four sources is very diverse. The eyes of the monster (Cassiopeia A and Taurus A) are two supernova remnants: the leftovers of the explosions of two stars in our own Galaxy. The evil pupil that stares at you in Taurus A is the Crab pulsar. The nose of the monster, Cygnus A, is 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. The mouth of the monster (Virgo A) is the 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.  
    These four sources are well known to radio astronomers, but this is the first time that they were able to see them in such great detail at the extremely long wavelengths of 5 meters, close to the longest wavelength we can observe with ground instruments. But what does it mean to see a source at “long wavelength”? While we can directly observe the sky with our naked eyes, capturing electromagnetic radiation at visible wavelengths, some cosmic sources emit also (or only) at very different wavelengths, all the way from radio waves to gamma-rays. To make radio images, astronomers need to use radio telescopes, instruments that are similar to optical telescopes but built to capture specifically radio waves. Using this information, astronomers can reconstruct the structure of a radio source as we would see it if we had “radio sensitive” eyes. The images used to make the radio monster were obtained with the Low Frequency Array (LOFAR), a pan-European radio telescope made by 52 stations spread across 8 different countries (The Netherlands, Germany, France, UK, Poland, Sweden, Latvia, Ireland, and soon Italy) and coordinated by a supercomputer in Groningen (NL).  [Published in de Gasperin et al, Astronomy & Astrophysics, Vol. 635, 150 2020]

    The A-team as seen with the LOFAR at 60 MHz

     

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.