EDP Sciences
Free Access
Volume 374, Number 1, July IV 2001
Page(s) 73 - 82
Section Extragalactic astronomy
DOI https://doi.org/10.1051/0004-6361:20010708

A&A 374, 73-82 (2001)
DOI: 10.1051/0004-6361:20010708

Excess hard X-ray emission from the obscured low luminosity AGN in the nearby galaxy M 51 (NGC 5194)

Y. Fukazawa1, 2, N. Iyomoto3, 2, A. Kubota2, Y. Matsumoto2 and K. Makishima2

1  Department of Physical Sciences, Graduate School of Science, Hiroshima University, 1-3-1 Kagamiyama, Higashi-Hiroshima, Hiroshima 739-8526, Japan
2  Department of Physics, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
3  Institute of Space and Astronautical Science, 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510, Japan

(Received 29 January 2001 / Accepted 27 April 2001 )

We observed the nearby galaxy M 51 (NGC 5194) with BeppoSAX. The X-ray properties of the nucleus below 10 keV are almost the same as the ASCA results regarding the hard component and the neutral Fe K$\alpha$ line, but the intensity is about half of the ASCA 1993 data. Beyond this, in the BeppoSAX PDS data, we detected a bright hard X-ray emission component which dominates above 10 keV. The 10-100 keV flux and luminosity of this component are respectively $2\times10^{-11}$ erg s-1 cm-2 and $2\times10^{41}$ erg s-1. These are about 10 times higher than the extrapolation from the soft X-ray band, and similar to the flux observed with Ginga, which found a bright power law component in 2-20 keV band. Considering other wavelength properties and the X-ray luminosity, together with strong neutral Fe K line, the hard X-ray emission most likely arises from a low luminosity active nucleus, which is obscured with a column density of ~1024 cm-2. This suggests that hidden low luminosity AGNs may well be present in other nearby galaxies. We interpret the discrepancy between Ginga and other X-ray satellites to be due to a large variability of absorption column density toward the line of sight over several years, suggesting that the Compton thick absorption material may be present on a spatial scale of a parsec. Apart from the nucleus, several ultra-luminous off-nuclear X-ray sources detected in M 51 exhibit long-term time variability, suggesting the state transition similar to that observed in Galactic black hole candidates.

Key words: galaxies: active -- galaxies: nuclei -- galaxies: individual: M 51

Offprint request: Y. Fukazawa, fukazawa@hirax6.hepl.hiroshima-u.ac.jp

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