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Astron. Astrophys. 326, 51-58 (1997)

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1. Introduction

Up to the end of 1994 more than 50 extragalactic radio sources were detected as emitters of high-energy [FORMULA] -rays by EGRET. The majority of the sources are flat-spectrum radio quasars (FSRQ) and about 20% are classified as BL Lacertae (BL Lac) objects. Especially in their flare state FSRQs often emit the bulk of their luminosity in the form of [FORMULA] -rays (von Montigny et al. 1995). For a number of objects COMPTEL data at a few MeV (Schönfelder et al., 1996), OSSE data at a few hundred keV (McNaron-Brown et al. 1995) or the extrapolation of X-ray data imply a spectral break at a few MeV photon energy. This break may be attributed to incomplete cooling of radiating electrons (Dermer and Schlickeiser 1993, Sikora et al. 1994) or bremsstrahlung and annihilation emission of cooling pair plasma (Böttcher and Schlickeiser 1995).

Individual [FORMULA] -ray spectra in the EGRET range, i.e. above 30 MeV, can generally be well described by power-laws. For the BL Lac Mrk421 the power-law behaviour extends up to TeV energies (Punch et al. 1992). Though a number of papers deal with the multifrequency spectrum of [FORMULA] -ray AGN, to our knowledge no attempt has yet been made to investigate whether there are systematic deviations from power-law behaviour in the [FORMULA] -ray spectra of AGN. Due to the different environmental conditions one might expect a different impact of opacity effects, for example, on FSRQs on one side and on BL Lacs on the other side.

In this paper we analyse the class-averaged spectra of AGN by summing the observed intensity and the statistic of power-law fits to the observed emission. We also derive the spectrum of the average [FORMULA] -ray AGN which is to be compared to the spectrum of the diffuse extragalactic background. In the next section (Sect. 2) we consider the time-averaged behaviour of sources by using the summed EGRET data of Phases 1,2 and 3 corresponding to observation times between May 1991 and October 1994. Since many of the extragalactic sources are variable at [FORMULA] -rays we discuss the behaviour at flare state in Sect. 3. A discussion of the results is in Sect. 4.

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© European Southern Observatory (ESO) 1997

Online publication: April 20, 1998