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 622, A25 (2019)]
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: Galaxies Vol. 6/4, p.126 (2018)]
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)]
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.: First detection of frequency-dependent, time-variable dispersion measures, In: Astronomy & Astrophysics, Vol. 624, 2019]
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)]
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.