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 h50-1 kpc) radius are consistent with a source at a redshift of $0.1242 \pm ^{0.0003}_{0.0022}$ with an optically thin thermal spectrum of temperature $5.8 \pm 0.6$ keV and a metal abundance of $0.60 \pm 0.10$ solar. This model gives a 2-10 keV luminosity of $3.5^{+0.8}_{-0.4} h_{50}^{-2} \times 10^{44}$ erg s-1. These characteristics, as well as the source extent of 2$\farcm$0 (350 h50-1 kpc), and the surface brightness profile are consistent with emission from the central region of a moderately rich cluster containing a cooling flow with mass flow rate of ${\sim \! 400}$- ${600} M_{\odot}$ yr-1. The absorption is $(7.9 \pm 0.5) \times 10^{22}$ 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.