The mystery of spectral breaks: Lyman continuum absorption by photon-photon pair production in the Fermi GeV spectra of bright blazars

TL;DR

This study reanalyzes Fermi gamma-ray spectra of bright blazars, revealing spectral breaks at specific energies caused by photon-photon pair production, which constrains the location and structure of the gamma-ray emission region within the broad-line region.

Contribution

It provides new evidence linking spectral breaks to ultraviolet emission features of the quasar broad-line region, refining understanding of gamma-ray emission sites in blazars.

Findings

Detection of spectral breaks at 5 GeV and 20 GeV in bright blazars.

Spectral breaks are associated with hydrogen and helium Lyman continuum absorption.

Gamma-ray emission likely originates within or near the broad-line region.

Abstract

We reanalyze Fermi/LAT gamma-ray spectra of bright blazars with a higher photon statistics than in previous works and with new Pass 7 data representation. In the spectra of the brightest blazar 3C 454.3 and possibly of 4C +21.35 we detect breaks at 5 GeV (in the rest frame) associated with the photon-photon pair production absorption by He II Lyman continuum (LyC). We also detect confident breaks at 20 GeV associated with hydrogen LyC both in the individual spectra and in the stacked redshift-corrected spectrum of several bright blazars. The detected breaks in the stacked spectra univocally prove that they are associated with atomic ultraviolet emission features of the quasar broad-line region (BLR). The dominance of the absorption by hydrogen Ly complex over He II, rather small detected optical depth, and the break energy consistent with the head-on collisions with LyC photons imply that the gamma-ray emission site is located within the BLR, but most of the BLR emission comes from a flat disk-like structure producing little opacity. Alternatively, the LyC emission region size might be larger than the BLR size measured from reverberation mapping, and/or the gamma-ray emitting region is extended. These solutions would resolve a long-standing issue how the multi-hundred GeV photons can escape from the emission zone without being absorbed by softer photons.