The discovery of radio halos in the Frontier Fields clusters Abell S1063 and Abell 370

Abstract: Massive merging galaxy clusters often host diffuse Mpc-scale radio synchrotron emission. This emission originates from relativistic electrons in the ionized intracluster medium (ICM). An important question is how these synchrotron emitting relativistic electrons are accelerated. Our aim is to search for diffuse emission in the Frontier Fields clusters Abell S1063 and Abell 370 and characterize its properties. While these clusters are very massive and well studied at some other wavelengths, no diffuse emission has been reported for these clusters so far. We obtained 325 MHz Giant Metrewave Radio Telescope (GMRT) and 1--4 GHz Jansky Very Large Array (VLA) observations of Abell S1063 and Abell 370. We complement these data with Chandra and XMM-Newton X-ray observations. In our sensitive images, we discover radio halos in both clusters. In Abell S1063, a giant radio halo is found with a size of $\sim 1.2$ Mpc. The integrated spectral index between 325 MHz and 1.5 GHz is $-0.94\pm0.08$ and it steepens to $-1.77 \pm 0.20$ between 1.5 and 3.0 GHz. This spectral steepening provides support for the turbulent re-acceleration model for radio halo formation. Abell 370 hosts a faint radio halo mostly centred on the southern part of this binary merging cluster, with a size of $\sim 500-700$ kpc. The spectral index between 325 MHz and 1.5 GHz is $-1.10\pm0.09$. Both radio halos follow the known scaling relation between the cluster mass proxy $Y_{500}$ and radio power, consistent with the idea that they are related to ongoing cluster merger events.