Fermi Observations of Blazars: Implications for Gamma-ray Production

Abstract: The brightest blazars detected by the Fermi Gamma-ray Space Telescope Large Area Telescope (Fermi/LAT) show significant breaks in their spectra at a few GeV. The sharpness and the position of the breaks can be well reproduced by absorption of $\gamma$-rays via photon--photon pair production on He ii and H i Lyman recombination continua (LyC) produced in the broad-line region (BLR). Using 138 weeks of LAT observations of the brightest GeV blazar 3C 454.3 we find a power-law dependence of the peak energy on flux and discover anti-correlation between flux and the column density of the He ii LyC which is responsible for absorption of the >2.5 GeV photons in this object. The strength and the variability of the absorption implies the location of the gamma-ray emitting zone close to the boundary of the high-ionization part of the BLR and moving away from the black hole when the flux increases. A combination of the GeV breaks with the detection of a few powerful blazars in the TeV range puts strong constraints on the BLR size. Additional spectral breaks at ~100 and ~400 GeV due to absorption by the Balmer and Paschen lines could be detected by the Cherenkov Telescope Array.