Fermi Observations of Blazars: Implications for Gamma-ray Production

TL;DR

This paper analyzes Fermi observations of blazars, revealing spectral breaks caused by photon absorption in the broad-line region, and discusses implications for the location of gamma-ray emission zones and BLR size constraints.

Contribution

It demonstrates that spectral breaks in blazar gamma-ray spectra are due to photon-photon absorption in the BLR, providing new insights into the emission region and BLR structure.

Findings

Spectral breaks are caused by photon absorption in the BLR.

The gamma-ray emission zone is near the BLR boundary and moves outward with increased flux.

Constraints on BLR size are derived from combined GeV and TeV observations.

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.