X-Ray Study of the Outer Region of Abell 2142 with Suzaku

We observed outer regions of a bright cluster of galaxies, A 2142, with Suzaku. Temperature and brightness structures were measured out to the virial radius (r(200)) with good sensitivity. We confirmed a 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 0.4r(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 collisions is expected to be longer than the elapsed time after shock heating at r(200), one plausible reason for the low entropy is a low electron temperature compared to that of ions. Other possible explanations would be gas dumpiness, turbulence and bulk motions of the 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.