Astron. Astrophys. 358, 841-844 (2000)

1. Introduction

In the third catalog of high-energy -ray sources, Hartman et al. (1999) listed 66 high-confidence identification blazars (i.e. flat-spectrum radio quasars (FSRQs) and BL Lac objects) which emit most of their bolometric luminosity in the -rays. Many of the -ray emitters also show superluminal components (Vermeulen & Cohen 1994, see also Fan et al. 1996) and very rapid -ray variability (von Montigny et al. 1995; Mattox et al. 1997; Mukherjee et al. 1997; Wehrle et al. 1998; Hartman et al. 1999). These facts strongly suggest that the -ray emission is from the jet of a blazar, and Doppler factors are derived for -ray loud blazars in the papers (Dondi & Ghisellini 1995; Cheng et al. 1999; Fan et al. 1999).

Models for -ray emission from AGNs are of two kinds: leptonic and hadronic. In the leptonic model, high energy -rays are produced by the inverse Compton scattering of high energy electrons in a soft photon field. The soft photons may be emitted from the nearby accretion disk (Dermer et al. 1992) or they may arise from disk radiation reprocessed in some region of AGNs (e.g. a broad emission line region; Sikora et al. 1994; Blandford & Levinson 1995; Xie et al. 1997, 1998); or they may come from the synchrotron emission in the jet (synchrotron self-Compton or SSC; Maraschi et al. 1992; Zdziarski & Krolik 1993; Bloom & Marscher 1996; Marscher & Travis 1996), or from a differential rotating flux tube near the inner edge of the accretion disk (Cheng et al. 1993). In the hadronic model, high energy -rays are produced by the synchrotron emission from ultrarelativistic electrons and positrons created in a proton-induced cascade (PIC; Mannheim & Biermann 1992; Mannheim 1993; Cheng & Ding 1994). There is no consensus yet on the dominant emission process. It is well known that the emission mechanisms might imply different relations between wave bands that can be used to choose between emission mechanisms. Such correlations have been discussed in many papers (Dondi & Ghisellini 1995; Mücke et al. 1997; Fan 1997a; Fan et al. 1998; Xie et al. 1997, 1998; Cheng et al. 2000). Fan (1997a) has investigated the correlation between the -ray band and lower energy bands by means of a multiple regression method, and proposed that the correlation between the -ray and the radio bands is probably due to the fact that both the -ray and the radio emissions are beamed. In this paper, we will discuss the relation between the -rays and the emission lines.

H = 75 km s^-1 Mpc^-1 and q = 0.5 are adopted.

© European Southern Observatory (ESO) 2000

Online publication: June 20, 2000
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