Gravitational microlensing of gamma-ray blazars

Abstract

We present a detailed study of the effects of gravitational microlensing on compact and distant γ -ray blazars. These objects have γ -ray-emitting regions that are small enough to be affected by microlensing effects produced by stars lying in intermediate galaxies. We compute the gravitational magnification taking into account effects of the lensing and show that, whereas the innermost γ -ray spheres can be significantly magnified, there is little magnification either for very high γ -ray energies or for lower (radio) frequencies (because these wavelengths are emitted from larger regions). We analyse the temporal evolution of the gamma-ray magnification for sources moving in a caustic pattern field, where the combined effects of thousands of stars are taken into account using a numerical technique. We propose that some of the unidentified γ -ray sources (particularly some of those lying at high galactic latitude with gamma-ray statistical properties that are very similar to detected γ -ray blazars) are indeed the result of gravitational lensing magnification of background undetected active galactic nuclei (AGN). This is partly supported from a statistical point of view: we show herein as well, using the latest information from the third EGRET catalogue, that high-latitude γ -ray sources have similar averaged properties to already detected γ -ray AGN. Some differences between both samples, regarding the mean flux level, could also be understood within the lensing model. With an adequate selection of lensing parameters, it is possible to explain a variety of γ -ray light curves with different time-scales, including non-variable sources. The absence of strong radio counterparts could be naturally explained by differential magnification in the extended source formalism.