First evidence of a shock in a radio mini-halo cluster: implications from new LOFAR data

Diffuse non-thermal radio emission in galaxy clusters provides direct information on the relativistic particles and magnetic fields in the ICM. With the advent of the LOFAR array, new properties of the steep diffuse emission in galaxy cluster have been revealed in the low radio frequencies. One of these recent discoveries is the presence of ultra-steep spectrum radio emission on Mpc scale outside the mini-halo. The origin of the mini-halos in cool-core cluster is still debated. New LOFAR observations of the cool-core galaxy cluster RBS797 were requested with the aim to investigate signs of steep large scale emission extending beyond the mini-halo. In order to perform an accurate investigation of this scenario, we carried out a multi-wavelength study to reveal the presence of surface brightness discontinuities in the ICM. The low resolution LOFAR image produced in this thesis work does not show Mpc diffuse emission. Comparing the low resolution 140MHz LOFAR image with the 1.4 GHz VLA image, we found a more extended diffuse emission at the high frequency. Spectral index analysis of this mini-halo was performed and a flattening of the spectral index toward the external region was found. The X-ray analysis was carried out by means of detailed morphological and spectroscopic study of archival Chandra observation. We found an outer discontinuity, which presents thermodynamic properties consistent with those typically found in shock fronts. The presence of a possible “cocoon shock” propagating in the ICM, which compresses and heats the plasma, can re-accelerate relativistic particles to higher energy, and hence, can explain a larger extension of the 1.4 GHz emission with respect to the 140 MHz LOFAR emission, and the flattening of the spectral index increasing with the distance from the center. Therefore, cocoon shock in this cool-core cluster can provide new evidence of the link between the AGN feedback and the mini-halo origin.