More Science Highlights

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.

• 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]
  
  

   

   

• 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 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)]
  
  

  

   

• 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]
  
  

  

   

• 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]
  
  

  

   

• 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]
  
  

  

   

• 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]
  
  

   

   

• 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]