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.

  • 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. [Publication: Wilber et al.: In: Astronomy & Astrophysics (2019), in press]

    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.

     

  • 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. [Publication: O'Sullivan et al.: In: Astronomy & Astrophysics (2019), in press]

    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. [Publication: Heesen et al.: LOFAR reveals the giant: a low-frequency radio continuum study of the outflow in the nearby FR I radio galaxy 3C 31, 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).

     

  • Low frequency radio emission from a young star

    The young star T Tau was successfully observed at 149 MHz. These observations show the low-frequency turn-over in its free-free spectrum, allowing for more accurate estimates of the physical parameters of the ionised gas around this newly forming star. LOFAR observations and analysis has been carried out by team of researchers from Dublin and Tautenburg. [Publication: Coughlan et al.: A LOFAR Detection of the Low-mass Young Star T Tau at 149 MHz, In: The Astrophysical Journal, Vol. 834(2), 2017]

    The young star TTau observed with LOFAR and the Giant Metrewave Radio Telescope (GMRT)

     

  • LOFAR millisecond pulsars

    Millisecond pulsars are extremely rapidly rotating neutron stars that are often used to carry out the most sensitive tests of Einstein's theories of gravitation. Until now, it was not known if it would be possible to see these objects with LOFAR because at these low frequencies it seemed likely that interstellar gas clouds would scatter their pulses too much. In this paper we make a first census of millisecond pulsars with LOFAR and we find that an unexpectedly large number of them is indeed detectable, meaning it is possible to now involve LOFAR in a range of high-impact experimental gravity tests, such as the quest for nanohertz-frequency gravitational waves. [Published in Kondratiev et al.: A LOFAR census of millisecond pulsars, In: Astronomy & Astrophysics, Vol. 585, 2016]

     
    Detectability of millisecond pulsars. As we look at ever more distant pulsars, the amount of interstellar gas that scatters away the signal, increases. This was expected to dramatically affect the detectability of these pulsars, particularly at low frequencies where the scattering is strongest. This plot shows all millisecond pulsars that were observed with LOFAR as a function of their DM (which is a rough measure of distance) and the amount of scattering expected for these pulsars. In theory pulsars near the top of the diagram should be fully undetectable (shown in red boxes), but in practice about half of these are still easily observed (green dots).

     

  • Predictions for the 21 cm-galaxy cross-power spectrum

    It has been suggested that using LOFAR observations in combination with observations in different frequency bands would help in studying the 21cm signal from neutral hydrogen, which is expected to provide unique information and constraints on the process known as cosmic reionization. Researchers at the Max PLanck Institute for Astrophysics have investigated this in a series of papers. [Published in Vrbanec et al.: Predictions for the 21 cm-galaxy cross-power spectrum observable with LOFAR and Subaru, In: Oxford University Press on behalf of the Royal Astronomical Society, 2016]

    Predictions for cross-correlation of 21cm lines in LOFAR observations.

     

  • Solar Imaging Pipeline and Data Center

    The solar imaging pipeline and data center is developed and operated at the Leibniz Institut für Astrophysik Potsdam (Germany) in close collaboration with ASTRON. Solar imaging is challanging since the Sun is an extended radio source with a highly spatial and temporal variablity. The pipeline bases on both an external calibrator and self-calibration. The archiving of LOFAR's solar radio data in the LOFAR Solar Data Center is additionally described in the paper.  [Publication: Breitling et al.: The LOFAR Solar Imaging Pipeline and the LOFAR Solar Data Center, In: Astronomy and Computing, Vol. 13, 2015]

    LOFAR Image of a solar type III radio burst at 65 MHz. The bright region shows the location of the radio source. It moves in the direction of the green arrow while it drifts from 60 to 30 MHz, revealing the propagation of an energetic electron beam along magnetic field lines (white lines) in the corona.

     

  • The ultra-low frequencies: LOFAR calibration challenges

    Translating LOFAR data into images is extremely challenging. At LOFAR observing frequencies the sky temperature is high and the systematic effects coming from ionospheric disturbances dominate the error budget. Furthermore, LOFAR stations work as phased arrays, this implies a direction-dependent beam response that needs to be properly accounted for. The effect of the ionosphere on LOFAR observations has been recently analyzed by the ultra-low frequency group in Hamburg (de Gasperin et al. 2018). The same group also developed a unified calibration scheme for LOFAR data that will become the standard calibration strategy for calibrators observed with LOFAR. The direction dependent effect of the ionosphere is currently mitigated by a calibration strategy, called "facet calibration", implemented by Dr. David Rafferty from the University of Hamburg (in a project funded by the BMBF Verbundforschung and presented in van Weeren et al. (2016): LOFAR Facet Calibration ). [Publication: de Gasperin et al.: Systematic effects in LOFAR data: A unified calibration strategy, In: Astronomy & Astrophysics (2019), in press]

    Comparison of a 25′′ image at 150 MHz before facet calibration (left) and the high-resolution (8.0′′ × 6.5′′) full-bandwidth image after facet calibration (right). The red borders mark the regions ("facets"), which are independently calibrated form each other using a bright point source included.

     

Not So Recent LOFAR Highlights

Some slightly older (but still exiting) science highlights that were featured on this page at some earlier time can be found here.