Constraints on dark matter annihilation and turbulent reacceleration set by high-frequency observations of the radio halo in the Coma cluster

ABSTRACT We study the impact of the recent observation of the radio halo in the Coma galaxy cluster at 6.6 GHz with the Sardinia Radio Telescope on models based on turbulent reacceleration of electrons produced in dark matter annihilation processes. Observing at that frequency it is possible to obtain information on the electrons spectrum at energies where the effect of turbulent reacceleration becomes sub-dominant with respect to energy losses, and therefore to obtain information on the properties of seed electrons. Under the assumption that dark matter particles are neutralino-like particles annihilating at a rate close to the maximum allowed by Fermi-Large Area Telescope (LAT) upper limits in dwarf galaxies, we obtain some constraints on the intensity of the reacceleration and on the value of the neutralino mass. In particular, models with mass of the order of 10 GeV are generally disfavoured, because they produce a high-frequency radio spectrum that can not reproduce the possible flattening observed between 5 and 6.6 GHz; on the other hand, models with mass of the order of 500 GeV, in order to reproduce the observed spectrum at frequencies below 100 MHz, require a reacceleration phase longer than $10^9$ yr, which would require more than one event responsible of the generation of turbulence in the cluster. The resulting optimal mass values are in the range 100–200 GeV, with a preference for the quark annihilation channel.