A cluster of galaxies hiding behind M31:: XMM-Newton observations of RX J0046.4+4204

We report on our serendipitous discovery with the XMM-Newton Observatory of a luminous X-ray-emitting cluster of galaxies that is located behind the Andromeda galaxy (M31). X-ray emission from the cluster was detected previously by ROSAT and cataloged as RX J0046.4+4204, but it was not recognized as a galaxy cluster. The much greater sensitivity of our XMM-Newton observations revealed diffuse X-ray emission that extends at least 50 and has a surface brightness profile that is well fit by the alpha-beta model with beta = 0.70 +/- 0.08, a core radius r(c) = 56 +/- 16, and alpha = 1.54 +/- 0.25. A joint global spectral fit of the EPIC MOS1, MOS2, and pn observations with the Mewe-Kaastra-Liedahl plasma emission model gives a cluster temperature of 5.5 +/- 0.5 keV. The observed spectra also show high significance iron emission lines that yield a measured cluster redshift of z = 0: 290 with 2% accuracy. For a cosmological model with H-0 = 71 km s(-1) Mpc(-1), Omega(M) = 0.3, and Omega(Lambda) = 0.7, we derive a bolometric luminosity of L-X = (8.4 +/- 0.5); 10(44) ergs s(-1). This discovery of a cluster behind M31 demonstrates the utility of X-ray surveys for finding rich clusters of galaxies, even in directions of heavy optical extinction.