Tracing the Gas to the Virial Radius (R 100) in a Fossil Group

We present a Chandra, Suzaku, and ROSAT study of the hot intragroup medium (IGrM) of the relaxed fossil group/poor cluster RX J1159+5531. This group exhibits an advantageous combination of flat surface brightness profile, high luminosity, and optimal distance, allowing the gas to be detected out to the virial radius (R vir≡ R 108 = 1100 kpc) in a single Suzaku pointing, while the complementary Chandra data reveal a round morphology and relaxed IGrM image down to kpc scales. We measure the IGrM entropy profile over ~3 orders of magnitude in radius, including three data bins beyond ~0.5R 200 that have good azimuthal coverage (>30%). We find no evidence that the profile flattens at large scales (>R 500), and when corrected for the enclosed gas fraction, the entropy profile is very close to the predictions from self-similar structure formation simulations, as seen in massive clusters. Within R vir, we measure a baryon fraction of 0.17 ± 0.02, consistent with the cosmological value. These results are in sharp contrast to the gas behavior at large scales recently reported in the Virgo and Perseus clusters and indicate that substantial gas clumping cannot be ubiquitous near R vir, at least in highly evolved (fossil) groups.