paper

On the origin of gamma rays in Fermi blazars: beyond the broad line region

arXiv:1804.02408 · doi:10.1093/mnras/sty887

Abstract

The gamma-ray emission in broad-line blazars is generally explained as inverse Compton (IC) radiation of relativistic electrons in the jet scattering optical-UV photons from the Broad Line Region (BLR), the so-called BLR External Compton scenario. We test this scenario on the Fermi gamma-ray spectra of 106 broad-line blazars detected with the highest significance or largest BLR, by looking for cut-off signatures at high energies compatible with gamma-gamma interactions with BLR photons. We do not find evidence for the expected BLR absorption. For 2/3 of the sources, we can exclude any significant absorption (), while for the remaining 1/3 the possible absorption is constrained to be 1.5-2 orders of magnitude lower than expected. This result holds also dividing the spectra in high and low-flux states, and for powerful blazars with large BLR. Only 1 object out of 10 seems compatible with substantial attenuation (). We conclude that for 9 out of 10 objects, the jet does not interact with BLR photons. Gamma-rays seem either produced outside the BLR most of the time, or the BLR is ~100x larger than given by reverberation mapping. This means that i) External Compton on BLR photons is disfavoured as the main gamma-ray mechanism, vs IC on IR photons from the torus or synchrotron self-Compton; ii) the Fermi gamma-ray spectrum is mostly intrinsic, determined by the interaction of the particle distribution with the seed-photons spectrum; iii) without suppression by the BLR, broad-line blazars can become copious emitters above 100 GeV, as demonstrated by 3C454.3. We expect the CTA sky to be much richer of broad-line blazars than previously thought.

arXiv comment28 pages (including 11 pages of online material), 8 figures, 4 tables. Accepted for publication in MNRAS

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