An<i>ASCA</i>Observation of M51 (NGC 5194): Iron K Emission from an Obscured Active Galactic Nucleus

Abstract

We present an ASCA observation of the nearby spiral galaxy M51 (NGC 5194). We detected hard X-ray emission with a photon index of D1.4 and a luminosity of ergs s~1 in the 210 L X D 1.1 ] 1040 keV band (assuming a distance of 9.6 Mpc). A strong uorescent iron K line (equivalent width D900 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 1023 cm~2 and that the observed 210 keV X-ray ux 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 D 0.4 keV), which suggests a lower iron abundance (\0.1 solar) than other elements (D0.10.4 solar), or by a two temperature (kT D 0.3 keV and kT D 0.8 keV) model with D0.1 solar abundance, which is reminiscent of the X-rayemitting gas in starburst galaxies.