XMM-Newton observations of two X-ray-bright galaxy groups -: pushing out to r500

We report on the results of XMM-Newton observations of the z = 0.18 galaxy group WARPJ0943.7+1644 and the nearby poor cluster WARPJ0943.5+1640 at z = 0.256. Tracing the X-ray gas out to similar to425 and similar to500 h(-1) kpc in the two systems, corresponding to roughly 70 per cent of the estimated virial radii, we find the surface-brightness profile of both groups to be well described by standard beta-models across the entire radial range, but with a significantly lower value of beta in the cooler WARPJ0943.7+1644. A Navarro-Frenk-White (NFW) model for the gas density can also provide a good fit to the surface-brightness data, but a large scale-radius is required, and models in which gas traces mass are strongly ruled out. Both systems fall close to the observed L-X-T relation for clusters. Gas mass fractions increase with radius out to the radii probed, reaching extrapolated values at the virial radius of 0.08 and 0.09 h(-3/2), respectively. The latter is concordant with results obtained for more massive systems. Gas entropy profiles show evidence for excess entropy out to at least r(500), and are inconsistent with predictions of simple pre-heating models. This study emphasizes the need for observing galaxy groups out to large radii, to establish their global properties securely.