Tempestuous life beyond R_500: X-ray view on the Coma cluster with
  SRG/eROSITA. I. X-ray morphology, recent merger, and radio halo connection
  This is the first paper in a series of studies of the Coma cluster using the
SRG/eROSITA X-ray data obtained in course of the Calibration and Performance
Verification observations. The data cover $\sim3^\circ\times 3^\circ$ area
around the cluster with a typical exposure time of more than 20 ks. The
stability of the instrumental background and operation of the SRG Observatory
in the scanning mode provided us with an excellent data set for studies of the
diffuse emission up to a distance of $\sim 1.5R_{200}$ from the Coma center. In
this study, we discuss the rich morphology revealed by the X-ray observations
(also in combination with the SZ data) and argue that the most salient features
can be naturally explained by a recent (on-going) merger with the NGC 4839
group. In particular, we identify a faint X-ray bridge connecting the group
with the cluster, which is convincing proof that NGC 4839 has already crossed
the main cluster. The gas in the Coma core went through two shocks, first
through the shock driven by NGC 4839 during its first passage through the
cluster some Gyr ago, and, more recently, through the "mini-accretion shock"
associated with the gas settling back to quasi-hydrostatic equilibrium in the
core. After passing through the primary shock, the gas should spend much of the
time in a rarefaction region, where radiative losses of electrons are small,
until the gas is compressed again by the mini-accretion shock. Unlike "runway"
merger shocks, the mini-accretion shock does not feature a rarefaction region
downstream and, therefore, the radio emission can survive longer. Such a
two-stage process might explain the formation of the radio halo in the Coma
cluster.
