Confirmation of the Presence of Nonthermal Hard X-Ray Excess in the Cluster A2256 from Two Epoch Observations

Abstract

After confirmation of the presence of a nonthermal hard X-ray excess with respect to the thermal emission in the Coma Cluster from two independent observations, obtained using the Phoswich Detection System on board BeppoSAX, we also present in this Letter the results of two observations of A2256 performed with a time interval of about 2.5 yr. In both spectra a nonthermal excess is present at a confidence level of ~3.3 σ and ~3.7 σ, respectively. The combined spectrum obtained by adding up the two spectra allows us to measure an excess at the level of ~4.8 σ in the 20-80 keV energy range. The nonthermal X-ray flux is in agreement with the published value of the first observation (Fusco-Femiano et al.) and with that measured by a Rossi X-Ray Timing Explorer observation (Rephaeli & Gruber).