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

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 γ-γ 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 (τ max < 1), 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 (τ max > 5). 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 ∼ 100× 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 3C 454.3. We expect the CTA sky to be much richer of broad-line blazars than previously thought.

NuSTAR Observations and Multiwavelength Modeling of the High-redshift BL Lacertae Object 4FGL J2146.5-1344

“…Hence, the main photon energy density contributing to the EC is the torus one. This is in agreement with the recent results of Costamante et al 2018 andMeyer et al 2019, who found the emission region location outside the BLR while studying the emission of Fermi-LAT broad-line blazars. However, if this source is an FSRQ, it would need to support a very high radiative power, even larger than the kinetic one.…”

Proton Synchrotron Gamma-Rays and the Energy Crisis in Blazars

“…Given that the median radius of the broad line region (BLR) for the sources of our sample is 0.15 pc, our results suggest that the γ-ray production site should be well within the BLR. This conclusion is, however, in tension with the lack of strong absorption features in the GeV γ-ray spectrum of luminous quasars (e.g., Costamante et al 2018). The sub-pc location of the emission region is also inconsistent with the radius inferred by the average observed variability, i.e., r ′ = cDt v /(1 + z).…”

Investigating the connection between γ-ray activity and the relativistic jet in 3C 273 during 2015−2019

“…The γ-ray/radio correlations (e.g., Pushkarev et al 2010;León-Tavares et al 2012;Ramakrishnan et al 2015;Lico et al 2017) particularly played an important role to link the γ-ray production site to the VLBI radio core which is generally identified as the brightest, compact, synchrotron self-absorbed feature in the Very Large Baseline Interferometry (VLBI) images of blazar jets (Kovalev et al 2009;Kim, J. et al 2018; but see also Lee et al 2016b, for optically thin spectra of blazars at mm-wavelengths in dominance of the core). In addition, the absence of γ-ray absorption by broad-line region (BLR) photons further supports a location of the γ-ray dissipation zone downstream the relativistic jet (i.e., > 10 4 R s , with R s being the Schwarzschild radius), where the parsec scale radio core appears (e.g., Harris et al 2012;Costamante et al 2018;Meyer et al 2019; see also Jorstad et al 2013;Kravchenko Article number, page 1 of 14 arXiv:2003.05659v1 [astro-ph.HE] 12 Mar 2020 Kim, D. et al 2018, for VLBI studies consistent with the idea). However, the BLR region, which is closer to the central black hole, is also a well-known γ-ray production site as revealed by observations of several objects (e.g., Rani et al 2013b;Berton et al 2018; but see also Hodgson et al 2018;Rani et al 2018, for discussion of multiple γ-ray sites).…”

“…Tavares et al 2013). Recent studies found indication that the majority of γ-ray bright FSRQs radiate γ-rays beyond the BLR region, thus supporting the parsec-scale scenario (Costamante et al 2018;Meyer et al 2019). Dotson et al (2012) suggested energy-dependent cooling times for γ-ray emission produced via infrared (IR) seed photons from the dusty torus.…”

“…DW_273_fin indices of γ-ray bright FSRQs, which is around −2.4, however, 3C 273 shows a softer photon index (see Harris et al 2012;Linford et al 2012). Although 3C 273 was less γ-ray bright than those extreme FSRQs (e.g., 3C 279 and 3C 454.3), it seems that the difference in photon index values is a sign of different source physics, like a spectral break or additional emitting components (Harris et al 2012;H.E.S.S Collaboration et al 2018; see also Costamante et al 2018, for the exception of 3C 273 at constraining high/low states). We also note that our parsec-scale scenario supports what Harris et al (2012) reported: γ-ray absorption by BLR photons is disfavored in γ-ray bright FSRQs.…”