On the Viewing Angle Dependence of Blazar Variability
  Internal shocks propagating through an ambient radiation field, are subject to a radiative drag that, under certain conditions, can significantly affect their dynamics and, consequently, the evolution of the beaming cone of emission produced behind the shocks. The resultant change of the Doppler factor combined with opacity effects leads to a strong dependence of the variability pattern produced by such systems, specifically, the shape of the light curves and the characteristics of correlated emission, on viewing angle. One implication is that objects oriented at relatively large viewing angles to the observer should exhibit a higher level of activity at high synchrotron frequencies (above the self-absorption frequency) and at gamma-ray energies below the threshold energy to pair production, than at lower (radio/millimeter) frequencies. 
Avigdor Eldar, Amir Levinson (TAU)