Outer regions of the cluster gaseous atmospheres

We present a systematic study of the hot gas distribution in the outer regions of regular clusters using ROSAT PSPC data. Outside the cooling how region, the beta-model describes the observed surface brightness closely, but not precisely. Between 0.3 and 1 virial radii, the profiles are characterized by a power law with slope, expressed in terms of the beta-parameter, in the range beta = 0.65-0.85. The values of beta in this range of radii are typically larger by approximate to 0.05 than those derived from the global fit. 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 keV clusters; however, even at high temperatures there are clusters with flat gas profiles, beta < 0.7. Our values of beta at large radius are systematically higher, and the trend of beta with temperature is weaker than was previously found; the most likely explanation is that earlier studies were affected by an incomplete exclusion of the central cooling how regions. For our 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 sample selection. The gas density profiles are very similar when compared in units of the cluster virial radius. The radius of fixed mean gas overdensity 1000 (corresponding to the dark matter overdensity 200 for Omega = 0.2) shows a tight correlation with temperature, R similar to T-0.5, as expected from the virial theorem for clusters with the universal gas fraction. At a given temperature, the rms scatter of the gas overdensity radius is only approximate to 7%, which translates into a 20% scatter of the gas mass fraction, including statistical scatter due to measurement uncertainties.