  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
arcmin from G21.5-09. The European Photon Imaging Camera data inside 1 arcmin
(180/h_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_50^2 10^44 erg/s. These characteristics,
as well as the source extent of 2.0 arcmin (350/h_50 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
400-600 M_Sun/yr. 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 fields such as close
to the Galactic plane, where the optical redshift measurements of galaxies are
difficult.
