More Science Highlights

Some more LOFAR-related highlight publications by GLOW members

Some more LOFAR Highlights

The LOFAR telescope allows to study the low frequency radio sky with unprecedented resolution and sensitivity. Here we feature some results with significant contribution from GLOW researchers.

  • 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 622, A5 (2019)]

    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.


  • 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)


  • Plasma in galaxy clusters

    On the largest scales, the Universe consists of voids and filaments making up the cosmic web. Galaxy clusters are located at the knots in this web, at the intersection of filaments. Regions of diffuse radio emission are thought to trace relativistic electrons in the intracluster plasma accelerated by low-Mach-number shocks. A long-standing problem is how low-Mach-number shocks can accelerate electrons so efficiently to explain the observed radio relics. Here, we report the discovery of a direct connection between a radio relic and a radio galaxy. This discovery indicates that fossil relativistic electrons from active galactic nuclei are re-accelerated at cluster shocks. It also implies that radio galaxies play an important role in governing the non-thermal component of the intracluster medium in merging clusters. Researchers at the University of Hamburg led the interpretation of the results. [Publication: van Weeren et al.: The case for electron re-acceleration at galaxy cluster shocks, In: Nature Astronomy, Vol. 1, 2017]

    Radio emission in the galaxy cluster Abell 3411


  • 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.


  • 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).


  • 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.