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

(Received 29 January 2001 / Accepted 27 April 2001 )

Abstract
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 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 erg s^-1 cm^-2 and 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.