The homogeneous blob can be understood as a perturbation or shock traveling down the jet. For the high energy variability of blazars such a scenario was also tested e.g. by Celotti et al. To estimate the synchrotron intensity in the jet at this position we take into account only the local contribution of the jet. To complete the picture, we will also test how the existence of relatively weak additional external radiation field can modify the inverse-Compton spectrum. We assume that the external radiation is isotropic and defined by a simple Planck function at temperature .
External photons are blue-shifted in the blob frame.
This can attenuate the high energy tail of the gamma ray spectra in blazars. The absorption coefficient was calculated for two values of the IR background which may be considered as a lower and upper limits. The absorption parameter is approximated by a 3rd order polynomial and is valid for redshift less than 0.3. This results from two effects. The main one is the undetermined contribution of the host galaxy, which depends on instrumental techniques, aperture or integration area used for photometric measurements, and seeing condition.