Astron. Astrophys. 319, 397-400 (1997)

1. Introduction

The extragalactic sky at MeV energies, observed by CGRO COMPTEL and EGRET, has been found to be dominated by the radio loud class of blazars, not radio quiet objects such as Seyfert galaxies, as had been expected before the launch of CGRO (e.g. v. Montigny et al. 1995). In a study of the X-ray brightest Seyferts at energies 0.75-3 MeV, Maisack et al. (1995, hereafter Paper I) found no evidence for emission from Seyfert galaxies, deriving upper limits more than an order of magnitude below previously reported fluxes from balloon experiments (Perotti et al. 1981a, 1981b). No Seyfert galaxy has been detected by EGRET, either (Lin et al. 1993).

At the same time, OSSE observations ranging up to several 100 keV detected spectra significantly steeper than those found in the standard X-ray bands (Johnson et al. 1994), suggesting a thermal nature of the high-energy emission. Zdziarski et al. (1995, hereafter Z95) combined (non-simultaneous) Ginga and OSSE data of 9 Seyfert galaxies to derive a mean spectrum which is characterised by a continuum which falls off exponentially with an e-folding energy of several 100 keV, plus a Compton reflected component which contributes mainly between 10 and 50 keV.

Therefore, it is plausible that the primary hard X-rays are thermal X-rays from a hot corona (Haardt & Maraschi 1993) or other active regions above an accretion disk (Stern et al. 1995) which acts as the cold reflector. A Compton reflected component has been observed in many Seyfert galaxies (e.g., Nandra and Pounds 1994). However, a nonthermal origin of the primary emission is still not ruled out, since the nonthermal spectrum could turn over at 100 keV as well or, if it extends to -ray energies, it could be anisotropic. Therefore, this radiation could go unobserved in most sources, and be mainly visible as reprocessed radiation. A likely candidate for such anisotropic emission are primary X-rays from a misdirected X-ray jet. Klein-Nishina driven nonthermal emission could also be highly anisotropic irradiating the disk (Ghisellini et al. 1991). Analogously, photo-pair or photo-pion driven hadronic cascades arising in a turbulent, low Mach number jet also produce much more flux in the direction of the disk than toward the observer (Mannheim 1995a). The cascade spectrum reaches up to a few MeV, and a diffuse high-energy neutrino background with an energy flux comparable to the CXB is expected from such hadronic models (Stecker et al. 1991). We probe the presence of a radiation component peaking at high energies by direct COMPTEL measurements employing the method of Paper I to the 3-30 MeV high energy range.

The remainder of the paper is organised as follows: in Sects. 2 and 3, we describe the instrument and data analysis, and give the results. We then discuss the upper limits in connection to the recent COMPTEL results of the CXB.

© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998
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