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

Abstract

4C +21.35 is a flat-spectrum-radio-quasar-type blazar, in which the rapid variability of very high energy (VHE, $E_{\gamma}\gtrsim 100$\,GeV) emission as short as $\sim$ 10 minutes 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 $\gamma$-ray emitting region of 4C +21.35 under the constraints of multiwavelength data. We fit three quasi-simultaneous spectral energy distributions (SEDs) using homogeneous one-zone leptonic model, in which the flat BLR with the aperture angle of $\alpha=25^{\circ}$ and a spherically symmetric hot dusty torus with the temperature of $T_{\rm sub}=1200$\,K, are assumed. The results show that the jet structure of 4C +21.35 is nearly conical with a half-opening angle of $\theta_{\rm j}\simeq 0.29^{\circ}-0.6^{\circ}$. Furthermore, the emitting region is located within the BLR clouds and approaches to 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 synchrotron self-Compton (SSC) process, the high-energy emission during the flaring periods is dominated by Compton scattering of BLR and dusty torus photons with the value of Compton-dominance parameter is about 30. Moreover, the fit to optical/ultraviolet data provides a further support that the central black hole (BH) mass of 4C +21.35 is $6\times 10^{8}$\,$M_{\odot}$.