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 K alpha 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: 13)(+0.18)); 10(-13) ergs cm(-2) s(-1), a bolometric X-ray luminosity L-X (2.21(-0.19)(+0. 34) 1044 ergs s(-1), kT 3: 57 +/- 0: 12 keV, and metallicity 0: 60 +/- 0: 09 Z(circle dot). The cluster obeys the scaling relations for LX and T observed at intermediate redshift. The mass derived from an isothermal NFW model fit is M-vir 3.89 +/- 0.35; 10(14) M-circle dot, 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.''