GeV breaks in blazars as a result of gamma-ray absorption within the broad-line region

TL;DR

This paper explains the GeV spectral breaks in blazars as gamma-ray absorption within the broad-line region, revealing the location of the emission zone and providing a new method to study the BLR photon field.

Contribution

It demonstrates that gamma-ray absorption by photon--photon pair production explains spectral breaks, indicating the blazar zone is within the high-ionization region of the BLR.

Findings

Spectral breaks are well modeled by photon-photon absorption.

The blazar emission region is within a light-year of the black hole.

Spectral features probe the extreme-UV/soft X-ray photon field.

Abstract

Spectra of the brightest blazars detected by the Fermi Gamma-ray Space Telescope Large Area Telescope cannot be described by a simple power law model. A much better description is obtained with a broken power law, with the break energies of a few GeV. We show here that the sharpness and the position of the breaks can be well reproduced by absorption of gamma-rays via photon--photon pair production on HeII Lyman recombination continuum and lines. This implies that the blazar zone lies inside the region of the highest ionization of the broad-line region (BLR) within a light-year from a super-massive black hole. The observations of gamma-ray spectral breaks open a way of studying the BLR photon field in the extreme-UV/soft X-rays, which are otherwise hidden from our view.