Outer regions of the cluster gaseous atmospheres

We present a systematic study of the hot gas distribution in the outer regi ons of regular clusters using ROSAT PSPC data. Outside the cooling how regi on, the beta-model describes the observed surface brightness closely, but n ot precisely. Between 0.3 and 1 virial radii, the profiles are characterize d by a power law with slope, expressed in terms of the beta-parameter, in t he range beta = 0.65-0.85. The values of beta in this range of radii are ty pically larger by approximate to 0.05 than those derived from the global fi t. There is a mild trend for the slope to increase with temperature, from [ beta] approximate to 0.68 for 3 keV clusters to approximate to 0.8 for 10 k eV clusters; however, even at high temperatures there are clusters with fla t gas profiles, beta < 0.7. Our values of beta at large radius are systemat ically higher, and the trend of beta with temperature is weaker than was pr eviously found; the most likely explanation is that earlier studies were af fected by an incomplete exclusion of the central cooling how regions. For o ur regular clusters, the gas distribution at large radii is quite close to spherically symmetric, and this is shown not to be an artifact of the sampl e selection. The gas density profiles are very similar when compared in uni ts of the cluster virial radius. The radius of fixed mean gas overdensity 1 000 (corresponding to the dark matter overdensity 200 for Omega = 0.2) show s a tight correlation with temperature, R similar to T-0.5, as expected fro m the virial theorem for clusters with the universal gas fraction. At a giv en temperature, the rms scatter of the gas overdensity radius is only appro ximate to 7%, which translates into a 20% scatter of the gas mass fraction, including statistical scatter due to measurement uncertainties.