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We present the analysis of a local (z = 0.04 - 0.2) sample of 31 galaxyclusters with the aim of measuring the density of the X-ray emitting gas incluster outskirts. We compare our results with numerical simulations to setconstraints on the azimuthal symmetry and gas clumping in the outer regions ofgalaxy clusters. We exploit the large field-of-view and low instrumentalbackground of ROSAT/PSPC to trace the density of the intracluster gas out tothe virial radius. We perform a stacking of the density profiles to detect asignal beyond r200 and measure the typical density and scatter in clusteroutskirts. We also compute the azimuthal scatter of the profiles with respectto the mean value to look for deviations from spherical symmetry. Finally, wecompare our average density and scatter profiles with the results of numericalsimulations. As opposed to some recent Suzaku results, and confirming previousevidence from ROSAT and Chandra, we observe a steepening of the densityprofiles beyond \sim r500. Comparing our density profiles with simulations, wefind that non-radiative runs predict too steep density profiles, whereas runsincluding additional physics and/or treating gas clumping are in betteragreement with the observed gas distribution. We report for the first time thehigh-confidence detection of a systematic difference between cool-core andnon-cool core clusters beyond \sim 0.3r200, which we explain by a differentdistribution of the gas in the two classes. Beyond \sim r500, galaxy clustersdeviate significantly from spherical symmetry, with only little differencesbetween relaxed and disturbed systems. We find good agreement between theobserved and predicted scatter profiles, but only when the 1% densest clumpsare filtered out in the simulations. [Abridged]

The gas distribution in the outer regions of galaxy clusters