The outer regions of galaxy clusters: Chandra constraints on the X-ray   surface brightness
  (Abridged version) We study the properties of the X-ray surface brightness profiles in a sample of galaxy clusters that were observed with Chandra and have emission detectable with a signal-to-noise ratio higher than 2 at a radius beyond R500 ~ 0.7 R200. Our study aims to measure the slopes of the X-ray surface brightness and of the gas density profiles in the outskirts of massive clusters. These constraints are compared to similar results obtained from observations and numerical simulations of the temperature and dark matter density profiles with the intention of presenting a consistent picture of the outer regions of galaxy clusters. We extract the surface brightness profiles Sb(r) of 52 X-ray luminous galaxy clusters at z>0.3 from X-ray exposures obtained with Chandra. We estimate R200 using both a beta-model that reproduces Sb(r) and scaling relations from the literature. The two methods converge to comparable values. We determine the radius, R_S2N, at which the signal-to-noise ratio is larger than 2 and select the objects in the sample that satisfy the criterion R_S2N/R200 > 0.7. For the eleven selected objects, we model with a power-law the behaviour of Sb(r). We measure a consistent steepening of the Sb(r) profile moving outward from 0.4 R200, where an average slope of -3.6 (sigma=0.8) is estimated. At R200, we evaluate a slope of -4.3 (sigma=0.9) that implies a slope in the gas density profile of -2.6 and a predicted mean value of the surface brightness in the 0.5-2 band of 2e-12 erg/s/cm2/deg2. Combined with estimates of the outer slope of the gas temperature profile and expectations about the dark matter distribution, these measurements allow us to describe properly how X-ray luminous clusters behave out to the virial radius. 
S. Ettori, I. Balestra