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

Abstract

<p> <span style="font-size: 10pt; font-family: Times;">We re-analyze </span><span style="font-size: 10pt; font-family: Times; font-style: italic;">Fermi</span><span style="font-size: 10pt; font-family: MTMI;">/</span><span style="font-size: 10pt; font-family: Times;">LAT </span><span style="font-size: 10pt; font-family: MTMI;">&gamma; </span><span style="font-size: 10pt; font-family: Times;">-ray spectra of bright blazars using the new Pass 7 version of the detector response files and detect breaks at </span><span style="font-size: 10pt; font-family: MTSY;">&sim;</span><span style="font-size: 10pt; font-family: Times;">5 GeV in the rest-frame spectra of 3C 454.3 and possibly also 4C +21.35, associated with the photon&ndash;photon pair production absorption by the He </span><span style="font-size: 10pt; font-family: Times;">ii </span><span style="font-size: 10pt; font-family: Times;">Lyman continuum (LyC). We also detect significant breaks at </span><span style="font-size: 10pt; font-family: MTSY;">&sim;</span><span style="font-size: 10pt; font-family: Times;">20 GeV associated with hydrogen LyC in both the individual spectra and 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 the hydrogen Ly complex over He </span><span style="font-size: 10pt; font-family: Times;">ii</span><span style="font-size: 10pt; font-family: Times;">, a small detected optical depth, and break energy consistent with head-on collisions with LyC photons imply that the </span><span style="font-size: 10pt; font-family: MTMI;">&gamma; </span><span style="font-size: 10pt; font-family: Times;">-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</span><span style="font-size: 10pt; font-family: MTMI;">/</span><span style="font-size: 10pt; font-family: Times;">or the </span><span style="font-size: 10pt; font-family: MTMI;">&gamma; </span><span style="font-size: 10pt; font-family: Times;">-ray emitting region is extended. These solutions would resolve the long-standing issue of how the multi-hundred GeV photons can escape from the emission zone without being absorbed by softer photons.&nbsp;</span></p>