SerendipitousXMM‐NewtonDiscovery of a Cluster of Galaxies atz= 0.28

We report the discovery of a galaxy cluster serendipitously detected as anextended X-ray source in an offset observation of the group NGC 5044. Thecluster redshift, z=0.281, determined from the optical spectrum of thebrightest cluster galaxy, agrees with that inferred from the X-ray spectrumusing the Fe K alpha complex of the hot ICM (z=0.27 +/- 0.01). Based on the 50ks XMM observation, we find that within a radius of 383 kpc the cluster has anunabsorbed X-ray flux, f_X (0.5-2 keV) = 3.34 (+0.08, -0.13) x 10^{-13}erg/cm^2/s, a bolometric X-ray luminosity, L_X = 2.21 (+0.34, -0.19) x 10^{44}erg/s, kT = 3.57 +/- 0.12 keV, and metallicity, 0.60 +/- 0.09 solar. Thecluster obeys the scaling relations for L_X and T observed at intermediateredshift. The mass derived from an isothermal NFW model fit is, M_vir = 3.89+/- 0.35 x 10^{14} solar masses, with a concentration parameter, c = 6.7 +/-0.4, consistent with the range of values expected in the concordancecosmological model for relaxed clusters. The optical properties suggest thiscould be a ``fossil cluster''.Comment: 5 pages, 4 colour figures, accepted for publication in Ap