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 ( r₂₀₀ ) with good sensitivity. We confirmed a temperature drop from 9 keV around the cluster center to about 3.5 keV at r₂₀₀ , with the density profile well approximated by the β -model with β=$ 0.85. Within 0.4 $r₂₀₀$ , the entropy profile agrees with $r1.1$ , as predicted by the accretion shock model. The entropy slope becomes flatter in the outer region and negative around $r₂₀₀$ . 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 r₂₀₀ , 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 r₂₀₀ , 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.