Tracing the X-ray emitting intra-cluster medium of clusters of galaxies   beyond r_200
  (Abridged) We present in this paper a sample of 14 nearby clusters of galaxies observed with the ROSAT/PSPC. We only select clusters with low galactic nH in order to trace the X-ray emitting intra-cluster medium (ICM) out to large radii. We convert the X-ray surface brightness profiles of the clusters into emission measure profiles scaled to the classical scaling relations based on the spherical collapse model. We sort the clusters into different temperature bins and stack the scaled emission measure (ScEM) profiles of clusters together. The stacked profiles allow us to observe out to radii r>r_200. In the center we find that the ScEM profiles deviate from predicted scaling laws. This result is in very good agreement with current studies on the L_X-T relation and the entropy - temperature relation (S \propto T^0.65) found recently. At radii r>0.4r_200 we find that the ScEM profiles agree well within the error bars, suggesting self-similarity. Fitting beta-models to the overall ScEM profiles we find for the different sub samples r_c=0.15-0.18 r_200 and beta=0.8, which is higher than beta=2/3 often found. The beta-model is generally a better representation for hotter than for cooler clusters. We see indications for continuous steepening of the profiles with increasing radius: at radii r>0.8r_200 the profiles are systematically below the beta-model curve with beta=0.8. We discuss our results with respect to the observed X-ray luminosity L_X-T relation, the gas mass M_gas-T relation and the total mass M-T relation. We also address implications on the origin of the observed S-T relation. Furthermore we discuss the observed steepness of the X-ray profiles, which falls off more rapidly than predicted from the NFW-profile for cold dark matter halos. 
D.M. Neumann (AIM -CEA/Saclay DSM/DAPNIA/Service d'Astrophysique,   France)