The Diffuse Soft Excess Emission in the Coma Cluster from the Rosat All-sky Survey

ROSAT All-Sky Survey data near the North Galactic Pole was analyzed in order to study the largescale distribution of soft X-ray emission from the Coma cluster. These ROSAT data constitute the only available X-ray observations of Coma that feature an in situ – temporally and spatially contiguous – background, with unlimited and continuous radial coverage. These unique characteristics of the ROSAT All-Sky Survey data are used to deliver a final assessment on whether the soft excess previously detected in the Coma cluster is due to background subtraction errors, or not. This paper confirms the presence of soft X-ray excess associated with Coma, and reports the detection of 1/4 keV band excess out to 5 Mpc from the cluster center, the largest soft excess halo discovered to date. We propose that the emission is related to filaments that converge towards Coma, and generated either by non-thermal radiation caused by accretion shocks, or by thermal emission from the filaments themselves. Subject headings: galaxies: clusters: individual (Coma); cosmology: large-scale structure of universe