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

Abstract

Abstract 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 0.4 $r_{200}$ , 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 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 clumpiness, 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.