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 ¼ 0:281, determined from the optical spectrum of the brightest cluster galaxy, agrees with that inferred from the X-ray spectrum using the Fe K complex of the hot ICM (z ¼ 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 fX(0:5Y2 keV ) ¼ (3:34þ0:08 0:13) ; 1013 ergs cm2 s1 , a bolometric X-ray luminosity LX ¼ (2:21þ0:34 0:19) ; 1044 ergs s1 , kT ¼ 3:57 0:12 keV, and metallicity 0:60 0:09 Z. The cluster obeys the scaling relations for LX and T observed at intermediate redshift. The mass derived from an isothermal NFW model fit is Mvir ¼ 3:89 0:35 ; 1014 M, with a concentration parameter, c ¼ 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.’’