2010ApJ...708.1376V

We report the discovery of seven new fossil systems in the 400d cluster survey. Our search targets nearby, z ≤ 0.2, and X-ray bright, L_X≥ 10⁴³ erg/s, clusters of galaxies. Where available, we measure the optical luminosities from Sloan Digital Sky Survey images, thereby obtaining uniform sets of both X-ray and optical data. Our selection criteria identify 12 fossil systems, out of which five are known from previous studies. While in general agreement with earlier results, our larger sample size allows us to put tighter constraints on the number density of fossil clusters. It has been previously reported that fossil groups are more X-ray bright than other X-ray groups of galaxies for the same optical luminosity. We find, however, that the X-ray brightness of massive fossil systems is consistent with that of the general population of galaxy clusters and follows the same L_X-L_optscaling relation.