Serendipitous XMM-Newton Discovery of a Cluster of Galaxies at z = 0.28

We report the discovery of a galaxy cluster serendipitously detected as an extended X-ray source in an offset observation of the group NGC 5044. The cluster redshift, z=0.281, determined from the optical spectrum of the brightest cluster galaxy, agrees with that inferred from the X-ray spectrum using the Fe Kα complex of the hot ICM (z=0.27+/-0.01). Based on the 50 ks XMM-Newton observation, we find that within a radius of 383 kpc the cluster has an unabsorbed X-ray flux f_X(0.5-2 keV)=(3.34^+0.08_-0.13)×10^-13 ergs cm^-2 s^-1, a bolometric X-ray luminosity L_X=(2.21^+0.34_-0.19)×10⁴⁴ ergs s^-1, kT=3.57+/-0.12 keV, and metallicity 0.60+/-0.09 Z_solar. The cluster obeys the scaling relations for L_X and T observed at intermediate redshift. The mass derived from an isothermal NFW model fit is M_vir=3.89+/-0.35×10¹⁴ M_solar, with a concentration parameter, c=6.7+/-0.4, consistent with the range of values expected in the concordance cosmological model for relaxed clusters. The optical properties suggest this could be a ``fossil cluster.''