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

Details An ASCA Observation of M51 (NGC 5194): Iron K Emission from an Obscured Active Galactic Nucleus An ASCA Observation of M51 (NGC 5194): Iron K Emission from an Obscured Active Galactic Nucleus

Mar 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998apj...496..210t&link_type=abstract

Astrophysical Journal v.496, p.210

Astronomy and Astrophysics

Astronomy

28

Galaxies: Individual Messier Number: M51, Galaxies: Nuclei, X-Rays: Galaxies

Scientific paper

We present an ASCA observation of the nearby spiral galaxy M51 (NGC 5194). We detected hard X-ray emission with a photon index of ~1.4 and a luminosity of LX ~ 1.1 x 1040 ergs s-1 in the 2-10 keV band (assuming a distance of 9.6 Mpc). A strong fluorescent iron K line (equivalent width ~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 1023 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 ~ 0.4 keV), which suggests a lower iron abundance (<0.1 solar) than other elements (~0.1-0.4 solar), or by a two temperature (kT ~ 0.3 keV and kT ~ 0.8 keV) model with ~0.1 solar abundance, which is reminiscent of the X-ray-emitting gas in starburst galaxies.

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