Location of gamma-ray flaring region in quasar 4C +21.35

Abstract

Abstract 4C +21.35 is a flat-spectrum-radio-quasar-type blazar, in which the rapid variability of very high energy (VHE, Eγ ≳ 100 GeV) emission as short as ∼ 10 min was observed by MAGIC Cherenkov telescopes, and the VHE spectrum extends up to at least 400 GeV. In this paper, by using a flat broad-line region (BLR) structure, we study the location and properties of the γ-ray-emitting region of 4C +21.35 under the constraints of multiwavelength data. We fit three quasi-simultaneous spectral energy distributions using a homogeneous one-zone leptonic model, in which a flat BLR with aperture angle α = 25° and a spherically symmetric hot dusty torus with a temperature of Tsub = 1200 K are assumed. The results show that the jet structure of 4C +21.35 is nearly conical with a half-opening angle of θj ≃ 0 $_{.}^{\circ}$29–0 $_{.}^{\circ}$6. Furthermore, the emitting region is located within the BLR clouds and approaches the outer radius of the BLR during the flaring states, while it is well beyond the dusty torus in quiescent state. The quiescent high-energy emission is dominated by the synchrotron self-Compton process, the high-energy emission during the flaring periods is dominated by Compton scattering of BLR and dusty torus photons with a value of the Compton-dominance parameter of about 30. Moreover, the fit to optical/ultraviolet data provides further support that the central black hole mass of 4C +21.35 is 6 × 108 M⊙.