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
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.