1 Introduction

M 51 is a face-on spiral galaxy, also known as the Whirlpool galaxy; it is located at a distance of 9.6 Mpc (Sandage & Tammann 1975). Optical studies of emission lines classified it as a LINER (Low Ionization Nuclear Emission Region) or a Seyfert 2 galaxy (Stauffer 1982; Filippenko & Sargent 1985; Ho et al. 1997b). Furthermore, Ho et al. (1997c) suggested the presence of a broad H$\alpha$line. Kohno et al. (1996) found a nuclear molecular disk, and constrained the dynamical mass within 70 pc of the nucleus to be (4 - 7) $\times10^6~M_{\odot}$. This indicates that M 51 hosts as massive a black hole as many bright AGNs.

Figure 1: BeppoSAX MECS images of M 51. a) MECS image (contour) in the total band overlaid on the optical image (gray scale). b) MECS image in the energy band of 5-7 keV. The center position is (13h 30m 00s, 47d 13m 00s). The axis unit is arcmin.

Observations in other wavelengths also strongly suggest the presence of an AGN in M 51, although previous X-ray observations of M 51 gave puzzling results. Einstein (Palumbo et al. 1985) and ROSAT (Marston et al. 1995; Ehle et al. 1995) observations constrained the soft X-ray luminosity of the M 51 nucleus to be $L_{\rm X}<5\times10^{39}$ ergs^-1. Ginga scanning observations in 1988 detected bright hard X-ray emission with $L_{\rm X}=1.2\times10^{41}$ erg/s (2-20 keV) with a photon index of $1.43\pm0.08$ (Makishima et al. 1990) and an intrinsic absorption of < $7\times10^{21}$ cm^-2. Such hard nuclear X-ray emission is often considered to be evidence of Low Luminosity AGN (LLAGN), but it should have been detected with Einstein and ROSAT if the flux did not change significantly. After the Ginga observations, ASCA observed M 51 in the hard X-ray band (2-10 keV) in 1993, and did not detect such a bright hard component (Terashima et al. 1998a). Instead, a faint hard X-ray continuum with a neutral Fe K line was detected, whose flux was an order of magnitude lower than that measured by Ginga. One possibility is that the difference between these observations is due to time variability with a large amplitude. Such a strong time variability is not often observed for low luminosity AGNs, and therefore is quite interesting if it is correct. The other possibility is a strong change of the intrinsic absorption. The key phenomenon that can help in resolving this mystery is the presence of a neutral Fe K line emission detected by ASCA (Terashima et al. 1998a), which indicates that during the ASCA observations, the nucleus of M 51 was heavily obscured by Compton thick material below 10 keV, as found in some Seyfert 2 galaxies (e.g. Risaliti et al. 1999). This idea can be confirmed by X-ray observations at energies above 10 keV.

Detection of an obscured hard X-ray emission from M 51 would suggest that many galaxies may harbor previously unknown hidden LLAGNs emitting below 10 keV. Moreover, such a detection can be valuable for a study of the physical structure of Compton thick absorbing material around AGNs. To further investigate such a possibility, we performed an X-ray observation of M 51 with BeppoSAX, sensitive to hard X-ray emission (Boella et al. 1997b).