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 Ka 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 fX(0.5-2 keV) = (3.34 -0.13+0.08) × 10-13 ergs cm-2 s-1, a bolometric X-ray luminosity LX = (2.21 0.19+0.34) × 1044 ergs s-1, kT = 3.57±0.12 ke V, 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 = 3.89 ± 0.35 × 1014 M⊙, 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."
- Galaxies: clusters: general
- X-rays: general