2005A&A...436..763I

We report the very first evidence of the possible detection of γ-ray resonant absorption along the line of sight towards γ-ray bright quasars (QSOs), like 3C279, 3C273, PKS0528+0134, and BL Lacertae. These detections resulted from the analysis of COMPTEL and EGRET data that were collected either during monitoring campaigns of the Virgo and galactic anticenter regions by the Compton Gamma Ray Observatory (CGRO) or during ToO observations of QSOs flares. We discuss three resonant absorption mechanisms that affect the γ-ray spectrum of point-like sources when crossing the surrounding warm and cold absorbers, as well as the potential of this γ-ray photon absorption method to constrain the abundance of the absorber. We detected two absorbers along the line of sight towards γ-ray bright QSOs, one at the QSO rest frame redshift and another at approximately zero redshift. We tentatively identify the latter with an absorber in the Galactic halo, while the former is undoubtedly caused by photon absorption in the host galaxy of the QSO. We discuss the advantages and drawbacks of this method in studies of absorbers in different astrophysical environments and compare this new method to absorber studies at X-ray or other wavelengths. We applied this γ-ray absorption method to identify a few of the EGRET unidentified (EUID) sources as QSOs, and determine their redshifts.