3. Confrontation with the -ray light curve of 1996 flare from 3C 279
The -ray blazar 3C 279 is one of the best studied up to now. It was the first blazar detected by the Compton GRO in June 1991, showing the bright flare with the -ray light curve which increased for a few days and finished on a much shorter time scale (Kniffen et al. 1993, Hartman et al. 1996). Its -ray emission in December 1992 - January 1993 was about an order of magnitude lower (Maraschi et al. 1994). However on February 1996, 3C 279 again shows strong flare with the light curve very similar to this one observed in 1991 (Wehrle et al. 1997, Collmar et al. 1997). Significant variability of the -ray flux during this flare has been measured on a time scale of hr. The -ray flux increased continuously for about an order of magnitude during days and later dropped sharply during day (see Fig. 1 in Wehrle et al. 1997). The rapid -ray variability, simultaneous variability of X- and 10 GeV -rays, and the condition that the -ray emission region should be transparent, requires for the Lorentz factor of relativistic blob (Wehrle te al. 1997). According to Wehrle et al., the multiwavelength observations of this flare (the lack of evident simultaneous variations of the optical - UV flux and the -ray flux) favour the mirror model for the -ray production in this source.
As we already mentioned in Sect. 2.1, the -ray flare of the type observed in 3C 279 cannot be produced by a single blob with negligible dimension moving with the Lorentz factor , provided that the mirror is located from the base of the jet at a characteristic distance of the BLR clouds cm. The rise time scale of the flare has to be connected with the longitudinal extent of the blob in the jet. The observed time scale of the flare requires that the extent of the blob should be of the order of cm, if the mirror is at the distance cm and . However the requirements on the time of flight of photons and the blob show that only soft photons re-emitted by the parts of the mirror within radius , centered on the jet axis (see Eq. (19)), can contribute to the observed -ray flux. Therefore the scattering mirror has to be located within the jet cone, provided that the jet typical opening angle is of the order of . This conclusion is inconsistent with the assumptions made by Ghisellini & Madau (GM) in their computations of the density of reprocessed photons seen by relativistic electrons in the blob frame.
The shape of the -ray light curve can be explained in terms of the mirror model if the density of relativistic electrons increases exponentially towards the end of the blob. Good consistency with the observed rise time scale of the -ray flare in February 1996 from 3C 279 is obtained for the density distribution of electrons in the blob of the type (see long-dashed curve in Fig. 2a and b a) where r is measured from the front of the blob. However such distribution of electrons along the blob is difficult to motivate in terms of the standard relativistic shock model moving along the jet. The electron distributions with the maximum on the front of the blob and the trail streaming away from the shock on its downstream side seems to be more likely (Mastichiadis & Kirk 1996, Kirk et al. 1998). However such electron distribution gives rapidly rising and exponentially decaying light curve (see dot-dashed curve in Fig. 2a and b a), which is in contrary to the observations of 3C 279. Therefore, the -ray light curve of 3C 279 suggests that for the mirror model the single large scale shock front is not likely mechanism of injection of relativistic electrons along the blob. The sequence of smaller scale shocks or another mechanism of acceleration of particles (magnetic reconnection in the jet?) might give more appropriate explanation.
© European Southern Observatory (ESO) 1998
Online publication: July 7, 1998