Discovery of an absorbed cluster of galaxies (XMMU J183225.4-103645) close to the Galactic plane with XMM-Newton

During an XMM-Newton observation of the galactic supernova remnant G21.5-09 a bright, previously uncatalogued, source (XMMU J183225.4-103645) was detected 18' from G21.5-09. The European Photon Imaging Camera data inside 1' (180 h(50)(-1) kpc) radius are consistent with a source at a redshift of 0.1242 +/- (0.0003)(0.0022) with an optically thin thermal spectrum of temperature 5.8 +/-0.6 keV and a metal abundance of 0.60 +/-0.10 solar. This model gives a 2-10 keV luminosity of 3.5(-0.4)(+0.8) h(50)(-2) x 10(44) erg s(-1.) These characteristics, as well as the source extent of 2.'0 (350 h(50)(-1) kpc), and the surface brightness profile are consistent with emission from the central region of a moderately rich cluster containing a cooling ow with mass ow rate of similar to 400-600 M-circle dot yr(-1). The absorption is (7.9 +/-0.5) x 10(22) atom cm(-2), 5 times that inferred from low-resolution HI data but consistent with higher spatial resolution infrared dust extinction estimates. XMMU J183225.4-103645 is not visible in earlier ROSAT observations due to high amount of absorption. This discovery demonstrates the capability of XMM-Newton to map the cluster distribution close to the Galactic plane, where few such systems are known. The ability of XMM-Newton to determine cluster redshifts to 1% precision at z = 0.1 is especially important in optically crowded and absorbed fields such as close to the Galactic plane, where the optical redshift measurements of galaxies are difficult.