2001A&A...374...66N

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' (180h₅₀^–1kpc) 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±0.6keV and a metal abundance of 0.60±0.10 solar. This model gives a 2-10keV luminosity of 3.5^+0.8_–0.4h₅₀^–2x10⁴⁴erg/s. These characteristics, as well as the source extent of 2.0' (350h₅₀^–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 ∼400-600M_☉/yr. The absorption is (7.9±0.5)x10²²atom/cm², 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.