Propagation of high energy gamma rays in the blazar jets and resulting effects on their spectral energy distribution

Abstract: Active galactic nuclei (AGN) emit electromagnetic (EM) radiation brightly across the entire EM spectrum. Although their physical structure has been studied for decades by analysing their EM emission, we now know that they have an anisotropic struc-ture, but they are still not fully understood. The subclass of AGN known as blazars are the most luminous and variable of AGN, and we study a peculiar feature that appears in the spectrum of the blazar 3C 279, which also appears in the spectra of several other blazars. This feature present in the spectral energy distribution (SED) of 3C 279 is a distinct dip in the gamma-ray band (in the ∼10 – 100 GeV range). We perform a spectral analysis of 3C 279 and find that its SED dip is caused by gamma rays in the blazar jet interacting with the nearby broad-line region (BLR) to produce electron-positron pairs (e±). The magnetic fields present in the jet accelerate these e± to produce synchrotron radiation. Secondary gamma rays emitted through syn-chrotron self-Compton (SSC) can partially compensate for the attenuated gamma rays. We find that the primary gamma rays in the ∼ 30 – 200 GeV range are pro-duced towards the outer edge of the BLR and travel a distance of 6 × 1014 cm within the BLR region. Furthermore, we conclude that gamma-gamma absorption in the BLR is responsible for the SED dips if the magnetic field in the jet is ≥ 0.1 G, and SSC emission from secondary e± can compensate for lost primary gamma-ray flux. If the magnetic field is weaker, then the observed dips would be caused by a differ-ent mechanism, and the gamma rays would be produced much further away from the BLR.M.Sc. (Physics