REINOUT VAN WEEREN







Research


Galaxy clusters are the largest gravitationally bound objects in the Universe and form through accretion of gas and by mergers with other clusters and galaxy groups. They are unique laboratories to study some of the most fundamental questions in astrophysics, related to the physics of particle acceleration and cosmic rays, the growth of large-scale structure, and the nature of dark matter. Massive clusters, particularly merging clusters, are also powerful cosmic telescopes, capable of magnifying distant galaxies, thus providing a probe of the early Universe.


DIFFUSE CLUSTER RADIO EMISSION

Abell 2256 with LOFARRadio observations of galaxy clusters have revealed the existence of large megaparsec-size diffuse synchrotron emitting sources: so-called radio halos and relics, depending on their location in the cluster center or outskirts. The synchrotron radiation implies the presence of cosmic rays and magnetic fields in the intracluster medium (ICM). With their enormous extent, these sources trace some of the largest particle accelerators in the Universe.

It is thought that halos and relics follow shocks waves and turbulence which are created when two or more galaxy clusters collide and merge. At these shocks, particles are accelerated to relativistic energies and in the presence of a magnetic field these relativistic particles (cosmic rays) emit synchrotron radiation observable with radio telescopes. The physics of the acceleration process and origin of cluster-wide magnetic fields are still poorly understood.

The acceleration mechanism can be studied via radio observations, in particular by mapping out the shape of the radio spectrum via observations at multiple frequencies. The properties of the magnetic field can be determined by obtaining polarization measurements. X-ray observations also play an important role, as they allows us to determine the shock Mach numbers and temperature and density of the ICM.

Over the past few years, our focus has been on utilizing LOFAR for two main purposes: (i) constructing a large sample of clusters to investigate the occurrence rates of halos and relics, and (ii) conducting detailed studies of selected clusters and their surroundings.

LOW-FREQUENCY RADIO OBSERVATIONS AND CALIBRATION

The diffuse radio emission from merging clusters, as well as other radio sources, is typically very faint, making their study difficult. The brightness of this emission generally increases towards lower frequencies, and low-frequency observations are therefore preferred. However, high-quality low-frequency images are difficult to make due to the presence of the ionosphere, which blurs the radio images.

In our group we are working on developing a calibration and imaging schemes that corrects for the blurring effects of the ionosphere, allowing detailed studies cluster and other sources at frequencies below 200 MHz using . LOFAR is  new revolutionary radio telescope operating at the lowest frequencies accessible from the ground. During the last year, we have focused on the development of (i) extraction and re-calibration of sources from the LoTSS survey, (ii) sub-arcsecond widefield imaging with LOFAR's European baselines, and (iii) pushing LOFAR to the lowest possible frequencies that can be observed from the ground, the decameter band.


In our group, we are developing calibration and imaging schemes to correct for the blurring effects of the ionosphere, enabling detailed studies of clusters and other sources at frequencies below 200 MHz using LOFAR. LOFAR is a revolutionary  radio telescope operating at the lowest frequencies accessible from the ground. Over the past year, our focus has been on (i) extracting and recalibrating sources from the LoTSS survey, (ii) achieving sub-arcsecond widefield imaging with LOFAR's European baselines, and (iii) extending LOFAR's observations to the lowest possible frequencies observable from the ground, in the decameter band.