Characterization of the warm--hot intergalactic medium near the Coma
  cluster through high--resolution spectroscopy of X Comae
  We have analyzed all available archival XMM-Newton observations of X Comae, a
bright X-ray quasar behind the Coma cluster, to study the properties of the
warm-hot intergalactic medium in the vicinity of the nearest massive galaxy
cluster. The RGS observations confirm the possible presence of a Ne IX K-alpha
absorption line at the redshift of Coma, although with a limited statistical
significance. This analysis is therefore in line with the earlier analysis by
Takei et al. (2007) based on a sub-set of these data. Its large column density
and optical depth, however, point to implausible conditions for the absorbing
medium, thereby casting serious doubts to its reality. Chandra has never
observed X Comae and therefore cannot provide additional information on this
source. We combine upper limits to the presence of other X-ray absorption lines
(notably from O VII and O VIII) at the redshift of Coma with positive
measurements of the soft excess emission from Coma measured by ROSAT (Bonamente
et al. 2003). The combination of emission from warm-hot gas at kT~1/4 keV and
upper limits from absorption lines provide useful constraints on the density
and the sightline length of the putative WHIM towards Coma. We conclude that
the putative warm-hot medium towards Coma is consistent with expected
properties, with a baryon overdensity >=10 and a sightline extent of order of
tens of Mpc.
