X-Ray Study of the Outer Region of Abell 2142 with Suzaku
  We observed outer regions of a bright cluster of galaxies A2142 with Suzaku.
Temperature and brightness structures were measured out to the virial radius
($r_{200}$) with good sensitivity. We confirmed the temperature drop from 9 keV
around the cluster center to about 3.5 keV at $r_{200}$, with the density
profile well approximated by the $\beta$ model with $\beta = 0.85$. Within
$0.4\r_{200}$, the entropy profile agrees with $r^{1.1}$, as predicted by the
accretion shock model. The entropy slope becomes flatter in the outer region
and negative around $r_{200}$. These features suggest that the intracluster
medium in the outer region is out of thermal equilibrium. Since the relaxation
timescale of electron-ion Coulomb collision is expected to be longer than the
elapsed time after shock heating at $r_{200}$, one plausible reason of the low
entropy is the low electron temperature compared to that of ions. Other
possible explanations would be gas clumpiness, turbulence and bulk motions of
ICM\@. We also searched for a warm-hot intergalactic medium around $r_{200}$
and set an upper limit on the oxygen line intensity. Assuming a line-of-sight
depth of 2 Mpc and oxygen abundance of 0.1 solar, the upper limit of an
overdensity is calculated to be 280 or 380, depending on the foreground
assumption.
