The X-ray view of the quasar MR 2251-178 and its host cluster: Variability, absorption, and intracluster gas emission

MR 2251-178 was the first quasar initially discovered in X-rays, and the fi rst one found to host a warm absorber. The quasar turned out to be an outst anding object in many respects. It has a high ratio of X-ray/optical lumino sity, is surrounded by the largest quasar emission-line nebula known, and i s located in the outskirts of a cluster of galaxies. Here, we present resul ts from an analysis of the X-ray spectral, temporal, and spatial properties of this source and its environment based on deep ROSAT observations. Remar kably, we do riot detect any excess X-ray cold absorption expected to origi nate from the giant gas: nebula surrounding R IR 2251-178. This excludes th e presence of a huge HI envelope around the quasar. The X-ray spectrum of M r 2251-178 is best fit by a warm-absorbed powerlaw with an ionization param eter log U = 0.5 and a column density log N-w = 22.6 which, however, cannot be the same material as the giant optical emission line nebula. The mean ( 0.1-2.4) keV X-ray luminosity amounts to 10(45) erg/s. A spatial analysis s hows that the bulk of the X-ray emission from the quasar is consistent with a point source, as expected in view of the powerlaw-shaped X-ray spectrum and the rapid X-ray variability we detect. In addition, extended emission a ppears at weak emission levels, including a bridge between the quasar and t he cD galaxy of the cluster. The X-ray emission from the intra-cluster medi um is weak or absent. The derive an upper limit on the X-ray luminosity of L-x less than or equal to 1.6 10(42) erg/s, weaker than other clusters: of comparable richness. None of the other member galaxies of the cluster to wh ich MR 2251-178 belongs, are detected in X-rays. However, east of the quasa r there is a significant excess of X-ray sources, several of them without o ptical counterparts on the UK Schmidt plates.