Astron. Astrophys. 336, 123-129 (1998)

1. Introduction

About 50 blazars have been detected by the Compton Gamma Ray Observatory in the MeV - GeV energy range (Fichtel et al. 1994, von Montigny et al. 1995, Thompson et al. 1995, Mukherjee et al. 1997), and 3 blazars, of the BL Lac type, are discovered in the TeV -rays by the Whipple Observatory (Punch et al. 1992, Quinn et al. 1996, Catanese et al. 1997). These blazars can reach very high -ray luminosities which are variable on time scales as short as a part of a day, in the case of optically violent variable quasars, or even several minutes, in the case of BL Lacs. These observations strongly suggest that -ray emission from blazars is collimated towards the observer within a small angle as a result of relativistic motion of plasma in the jet or directional acceleration of particles.

High energy processes occurring in blazars are popularly explained in terms of the inverse Compton scattering (ICS) model in which -rays are produced in ICS of soft photons by electrons in a blob moving relativistically along the jet. Different modifications of this general model mainly concern the origin of soft photons, i.e. whether they come internally from the blob in the jet (synchrotron self-Compton (SSC) model, e.g. Maraschi et al. 1992, Bloom & Marscher 1993), directly from the disk (e.g. Dermer et al. 1992, Bednarek et al. 1996a,b), are produced in the disk but reprocessed by the matter surrounding the disk (external comptonization (EC) model, e.g. Sikora et al. 1994, Blandford & Levinson 1995), or produced in the jet but reprocessed by the matter surrounding the jet (the so-called mirror model, Ghisellini & Madau 1996, henceforth GM). In this last paper it is mentioned that SSC model and external comptonization of photons produced by the broad line region clouds (BLR) illuminated by the disk (EC model) may also contribute to the -ray emission producing a first -ray pre-flare. For the SSC model the amplitude of the -ray variation is expected to be proportional to the square of the variation observed in IR-optical-UV energy range. For the EC model the -ray emission should vary linearly with the low energy synchrotron emission. Such behaviour is not observed in the case of the 1996 flare from 3C 279 in which the -ray variation is more than the square of the synchrotron variation. Moreover, in the -ray light curve of this flare (see Fig. 1 in Wehrle et al. 1997), there is no clear evidence for a double peak structure which could eventually correspond to the first -ray flare produced in terms of SSC or EC models and the second -ray flare produced in terms of the mirror model. Therefore, although the SSC model cannot be completely rule out, Wehrle et al. (1997) concludes that the mirror model is favourite by the multiwavelength observations of a strong flare in February 1996 from 3C 279 since it predicts -ray flare with observed features.

In this paper we test the mirror model by comparing predictions of the kinematic analysis with the observational results. The possible contributions from SSC and EC models to the -ray production during this flare are neglected since, as we mentioned above, there is no observational support for their importance. Simultaneous analysis of all these models will require an introduction of additional free parameters (density of electrons in the blob, the perpendicular extent of the blob, definition of the disk radiation) which are not all well constrained by the observations.

© European Southern Observatory (ESO) 1998

Online publication: July 7, 1998
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