| Abstract: |
Relativistic electrons and magnetic fields permeate the intra-cluster medium (ICM) and manifest themselves as diffuse sources of synchrotron emission observable at radio wavelengths, namely radio halos and radio relics. Although there is broad consensus that the formation of these sources is connected to turbulence and shocks in the ICM, the details of the required particle acceleration are poorly known. Low-frequency cluster surveys have the potential to shed light on the existence of radio halos with very steep radio-spectra, which are a key prediction of turbulent models that should be generated in less energetic merger events and thus be more common in the Universe. I will summarise the results from a series of recent papers, where we investigated all 309 clusters in the second catalog of Planck Sunyaev Zel'dovich that lie within the 5634 deg2 covered by the Second Data Release of the LOFAR Two-meter Sky Survey (LoTSS-DR2). We have studied the low-frequency radio emission and, where available, archival X-ray data of these objects in order to understand the physics that governs the frequency with which radio haloes occur as a function of mass and redshift. For objects with sufficient X-ray data quality, we performed power spectral analysis of the X-ray surface brightness fluctuations to estimate the turbulent velocity dispersion. We find that the injected power for particle acceleration calculated from turbulent dissipation is correlated with the radio halo power, with a best-fit slope close to unity that supports the turbulent (re)acceleration scenario. XRISM observations are needed to further quantify the dependence of radio halo power on the turbulent Mach number, which appears to be very weak based on the current results (but with a large systematic scatter). |
