The ROSAT view of the quasar MR2251-178: a number of surprises

Stefanie Komossa

MPE Garching


The quasar MR 2251-178 is one of the most luminous soft X-ray
emitters in the local universe ($L_{x} \sim 10^{45}$ erg/s).
It was the first quasar in which an ionized absorber was
discovered. MR 2251-178 is surrounded
by the largest quasar emission-line nebula known, and
is located in the outskirts of a cluster of galaxies.

We present results from ROSAT observations of the quasar
MR 2251-178 and its environment, which revealed some unexpected results.
We find that (i) there is no evidence for excess X-ray {\em cold}
absorption. Such absorption was predicted from some formation
models of the giant gas nebula surrounding MR 2251-178.
There are (ii) indications for an X-ray flaring event with
variability by a factor $\sim$2 during the {\sl ROSAT} all-sky survey;
rarely observed in such a luminous source.
(iii) None of the other optically bright member galaxies of the cluster
to which MR\,2251-178 belongs, are detected in X-rays.
However, (iv) east of the quasar there is a significant excess of
X-ray sources several of which do not have optical counterparts
on the UK Schmidt plates.