HARD X-RAYS FROM COMA CLUSTER REGION

The region containing the Coma cluster of galaxies was studied on 1987 July 15 in the energy range (15-310 keV) with a balloon-borne experiment, comprising scintillation and multiwire proportional counter detectors. During the observation, a high-energy non thermal component was detected in the energy band 18-130 keV, and the emitted photon spectrum is well described by a single power law of photon index -1.77 with an intensity of 8.9×10-5 photons cm-2 s-1 keV-1 at 50 keV. This emission is not consistent with the thermal one detected by HEAO I A-2/A-4 (1978/1979) and supports the presence of a hard tail, as already suggested by early measurements from OSO 8 and the HXR/Frascati balloon experiment. The high-energy emission could be due to the inverse Compton mechanism (IC) of relativistic electrons permeating the cluster off the 2.7 K background radiation, or to the presence in the field of an active galactic nucleus, possibly X Comae, as suggested by the EXOSAT/CMA image. In the last case, the high-energy component varies more than a factor of 4 on a time scale of years, while assuming the hard X-rays as due to IC emission a lower limit of 0.37×10-7 G is derived for the intracluster magnetic field.