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 } \approx 0.7 \times 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 _ { S 2 N } , at which the signal-to-noise ratio is higher than 2 , and select the objects in the sample that satisfy the criterion R _ { S 2 N } / 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 0.4 R _ { 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 \approx - 2.6 and a predicted mean value of the surface brightness in the 0.5 - 2 keV band of 2 \times 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 .