The outer regions of galaxy clusters: Chandra constraints on the X-ray surface brightness

Context. 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 per radial bin at a radius beyond R(500) approximate to 0.7 x R(200). Aims. Our study aims to measure the slopes in both the X-ray surface brightness and gas density profiles in the outskirts of massive clusters. These constraints are compared with 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. Methods. We extract the surface brightness profiles S(b)(r) of 52 X-ray luminous galaxy clusters at z > 0.3 from X-ray exposures obtained with Chandra. These objects, which are of both high X-ray surface brightness and high redshift, allow us to use Chandra either in ACIS-I or even ACIS-S configuration to survey the cluster outskirts. We estimate R(200) using both a beta-model that reproduces the surface brightness profiles 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 higher than 2, and select the objects in the sample that satisfy the criterion R(S2N)/R(200) > 0.7. For the eleven selected objects, we model by a power-law function the behaviour of S(b)(r) to estimate the slope at several characteristic radii expressed as a fraction of R(200). Results. We measure a consistent steepening of the S(b)(r) profile moving outward from 0.4R(200), where an average slope of -3.6 (sigma = 0.8) is estimated. At R(200), we evaluate a slope of -4.3 (sigma = 0.9) that implies a slope in the gas density profile of approximate to-2.6 and a predicted mean value of the surface brightness in the 0.5-2 keV band of 2 x 10(-12) erg s(-1) cm(-2) deg(-2). Conclusions. Combined with estimates of the outer slope of the gas temperature profile and expectations about the dark matter distribution, these measurements lie well within the physically allowed regions, allowing us to describe properly how X-ray luminous clusters behave out to the virial radius.