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 ~425 and ~500 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 β-models across the entire radial range, but with a significantly lower value of β 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 r₅₀₀, 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.