High significance detection at 4.8 GHz of the radio halo in the Coma galaxy cluster with the Sardinia Radio Telescope

ABSTRACT We present the results of observations of the radio halo in the Coma galaxy cluster at 4.8 GHz performed with the Sardinia Radio Telescope. The radio halo in this cluster is detected for the first time at this frequency with a statistical significance higher than $3\sigma$. After the removal of the radio frequency interference and of the discrete sources contribution, and after the correction for the Sunyaev–Zel’dovich effect, we estimate a flux density of $61\pm 11$ mJy, higher than the value previously reported in literature at this frequency. By using the value we obtained, it is possible to estimate an integrated spectral index between 4.8 and 6.6 GHz of $\alpha \sim 1.17$, where $F(\nu)\propto \nu ^{-\alpha }$, indicating a possible higher frequency slowdown of the spectral steepening observed between 1.4 and 4.8 GHz. Such a spectral behaviour is compatible with turbulent reacceleration if the seed electrons have a spectrum extending up to high energies, as in the case of continuous injection by hadronic interactions or dark matter annihilation. We also report the detection at 4.8 GHz of a polarized spot inside the halo, without an evident counterpart, already detected at 6.6 GHz.