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 5' and has a surface brightness profile that is well fit by the α-β model with β=0.70+/-0.08, a core radius r_c=56^''+/-16^'', and α=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 H₀=71 km s^-1 Mpc^-1, Ω_M=0.3, and Ω_Λ=0.7, we derive a bolometric luminosity of L_X=(8.4+/-0.5)×10⁴⁴ 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.