An ASCA observation of M51 (NGC 5194): Iron K emission from an obscured active galactic nucleus

We present an ASCA observation of the nearby spiral galaxy M51 (NGC 5194). We detected hard X-ray emission with a photon index of similar to 1.4 and a luminosity of L-x similar to 1.1 x 10(40) ergs s(-1) in the 2-10 keV band (assuming a distance of 9.6 Mpc). A strong fluorescent iron K line (equivalent width similar to 900 eV) was detected at 6.4 keV in the X-ray spectra. Such an intense iron line is characteristic of Seyfert 2 galaxies and strongly suggests the presence of a heavily obscured active galactic nucleus (AGN). However, the X-ray image is extended even at energies above 2 keV. From the strong iron line and the extended hard X-ray image we speculate that the AGN is obscured by matter with a hydrogen column density more than several times 10(23) cm(-2) and that the observed 2-10 keV X-ray flux is not dominated by emission from the AGN but rather by other components, such as low-mass X-ray binaries, which typically dominate the X-ray emission of normal spiral galaxies. Emission lines from O K, Ne K, Fe L, Mg K and Si K were detected in the soft energy spectra, which indicate the presence of hot gas. The soft component is well represented by a Raymond-Smith thermal plasma model (with kT similar to 0.4 keV), which suggests a lower iron abundance (<0.1 solar) than other elements (similar to 0.1-0.4 solar), or by a two temperature (kT similar to 0.3 keV and kT similar to 0.8 keV) model with similar to 0.1 solar abundance, which is reminiscent of the X-ray-emitting gas in starburst galaxies.