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Astron. Astrophys. 325, 19-26 (1997)

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1. Introduction

One of the most important discoveries by the Compton Gamma Ray Observatory is the detection of about 50 high energy gamma ray blazars by the EGRET instrument on board (Fichtel et al. 1994; Thompson et al. 1995). These sources have been identified as BL Lacs and Flat Spectrum Radio Loud Quasars (FSRQ). Some of them show optically violent variability and high polarization. The apparent gamma ray luminosity (assuming isotropic emission) of these sources are extremely high (up to [FORMULA] [FORMULA]) and may exceed the Eddington limit on the accretion power. Also rapid time variability (days) has been seen in a number of blazars and this indicates the smallness of the gamma ray emission region. Therefore, relativistic jets are generally considered to be the emitting medium to boost the apparent luminosity and to fast the time variability (Blandford & Königl 1979; Marscher 1980; Königl 1981; Reynolds 1982).

A number of jet models have been proposed for the gamma ray emission from blazars. The popular ones include, synchrotron self-Compton (Ginzburg & Syrovatskii 1965; Rees 1967; Jones, O'Dell & Stein 1974; Maraschi, Ghisellini & Celotti 1992; Zdziarski & Krolik 1993; Bloom & Marscher 1996); and inverse Compton scattering on photons from the accretion disk (Dermer, Schlickeiser & Mastichiadis, 1992; Melia & Königl 1989); inverse Compton scattering on clouds-reprocessed photons (Sikora, Begelman & Rees 1994; Blandford & Levinson 1995); and synchrotron radiation by ultra-high energy electrons/positrons created in the ultra-high energy proton interactions (Mannheim 1993; Ghisellini 1993). Kinematically, these models can be divided into two classes: 1) relativistic electron scattering on internal photons or magnetic field, and 2) on external photons, in the jet comoving frame. Accordingly, the relativistic boosting effects are significantly different for these two cases (Dermer 1995).

One of the important characteristics of the gamma ray blazars is the presence of a strong core-dominated radio emission. Previous studies show that there is a moderate correlation between the gamma ray flux above 100 MeV and radio flux at 5 GHz, but no correlation between gamma ray flux and optical or X-ray flux (Dondi & Ghisellini 1995; Erlykin & Wolfendale 1995). The existence of the correlation indicates that the gamma ray emission is connected with the radio emission in blazars, either physically or kinematically. This can be understood in a way that the radio emission also emerges from the same jet and is thus Doppler-beamed to some extent. In this work, we use the radio flux as a reference to differentiate the beaming effect of gamma rays and to discriminate the emission models. In Sect. 2, we derive a beaming statistics for the jet models and show the theoretical difference in the the two classes of gamma ray emission mechanisms. In Sect. 3, we exemplify the test method with a limited sample of data. A correlation study is presented to show the link between the radio and gamma ray emission. Then we perform the test with the limited sample of data. In Sect. 4, we verify the test method by examining various factors involved in the beaming statistics with Monte-Carlo simulations. The effects of the random spreads in the relevant parameters on the test result are estimated. Finally, in Sect. 5, we discuss the need for simultaneous observations of VLBI with high energy gamma ray telescopes to produce a larger, better quality sample of data.

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© European Southern Observatory (ESO) 1997

Online publication: May 5, 1998