The γ-ray emitting region in low synchrotron peak blazars

Testing self-synchrotron Compton and external Compton scenarios

¹ Science Data Center della Agencia Spaziale Italiana, SSDC - ASI, Rome, Italy ;
e-mail: bruno.arsioli@ssdc.asi.it; yuling.chang@ssdc.asi.it
² Instituto de Física Gleb Wataghin, UNICAMP, Rua Sérgio B. de Holanda 777, 13083-859 Campinas, Brazil
e-mail: arsioli@ifi.unicamp.br
³ ICRANet-Rio, CBPF, Rua Dr. Xavier Sigaud 150, 22290-180 Rio de Janeiro, Brazil
⁴ Sapienza Università di Roma, Dipartimento di Fisica, Piazzale Aldo Moro 5, 00185 Roma, Italy
⁵ ICRANet, P.zza della Repubblica 10, 65122 Pescara, Italy

Received: 12 March 2018
Accepted: 12 April 2018

Abstract

Aims. From the early days in γ-ray astronomy, locating the origin of GeV emission within the core of an active galactic nucleus (AGN) persisted as an open question; the problem is to discern between near- and far-site scenarios with respect to the distance from the super massive central engine. We investigate this question under the light of a complete sample of low synchrotron peak (LSP) blazars which is fully characterized along many decades in the electromagnetic spectrum, from radio up to tens of GeV. We consider the high-energy emission from bright radio blazars and test for synchrotron self-Compton (SSC) and external Compton (EC) scenarios in the framework of localizing the γ-ray emission sites. Given that the inverse Compton (IC) process under the EC regime is driven by the abundance of external seed photons, these photons could be mainly ultraviolet (UV) to X-rays coming from the accretion disk region and the broad-line region (BLR), therefore close to the jet launch base; or infrared (IR) seed photons from the dust torus and molecular cloud spine-sheath, therefore far from jet launch base. We investigate both scenarios, and try to reveal the physics behind the production of γ-ray radiation in AGNs which is crucial in order to locate the production site.

Methods. Based on a complete sample of 104 radio-selected LSP blazars, with 37 GHz flux density higher than 1 Jy, we study broadband population properties associated with the nonthermal jet emission process, and test the capability of SSC and EC scenarios to explain the overall spectral energy distribution (SED) features. We use SEDs well characterized from radio to γ rays, considering all currently available data. The enhanced available information from recent works allows us to refine the study of Syn to IC peak correlations, which points to a particular γ-ray emission site.

Results. We show that SSC alone is not enough to account for the observed SEDs. Our analysis favors an EC scenario under the Thomson scattering regime, with a dominant IR external photon field. Therefore, the far-site (i.e., far from the jet launch) is probably the most reasonable scenario to account for the population properties of bright LSP blazars in cases modeled with a pure leptonic component. We calculate the photon energy density associated with the external field at the jet comoving frame to be U′_ext = 1.69 × 10⁻² erg cm⁻³, finding good agreement to other correlated works.