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 0 from G21.5-09. The European Photon Imaging Camera data inside 1 0 (180 h 1 50 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 +0:8 0:4 h 2 50 10 44 erg s 1 . These characteristics, as well as the source extent of 2: 0 (350 h 1 50 kpc), and the surface brightness prole are consistent with emission from the central region of a moderately rich cluster containing a cooling flow with mass flow rate of 400{600 M yr 1 . The absorption is (7:9 0:5) 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 elds such as close to the Galactic plane, where the optical redshift measurements of galaxies are dicult.