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

Citation
Y. Fukazawa et al., Excess hard X-ray emission from the obscured low luminosity AGN in the nearby galaxy M 51 (NGC 5194), ASTRON ASTR, 374(1), 2001, pp. 73-82
Citations number
67
Categorie Soggetti
Space Sciences
Journal title
ASTRONOMY & ASTROPHYSICS
ISSN journal
14320746 → ACNP
Volume
374
Issue
1
Year of publication
2001
Pages
73 - 82
Database
ISI
SICI code
1432-0746(200107)374:1<73:EHXEFT>2.0.ZU;2-O
Abstract
We observed the nearby galaxy M 51 (NGC 5194) with BeppoSAX. The X-ray prop erties 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 inten sity 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 ab ove 10 keV. The 10-100 keV flux and luminosity of this component are respec tively 2x10(-11) erg s(-1) cm(-2) and 2x10(41) erg s(-1). These are about 1 0 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 lum inosity, together with strong neutral Fe K line, the hard X-ray emission mo st likely arises from a low luminosity active nucleus, which is obscured wi th a column density of similar to 10(24) cm(-2). This suggests that hidden low luminosity AGNs may well be present in other nearby galaxies. We interp ret 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 ov er 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-ter m time variability, suggesting the state transition similar to that observe d in Galactic black hole candidates.