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We propose a two-zone synchrotron self-Compton (SSC) model, including an inner gamma-ray emitting region with spherical shape and a conical radio emitting region located at the extended jet, to alleviate the long-standing "bulk Lorentz factor crisis" in blazars. In this model, the spectral energy distributions (SEDs) of blazars are produced by considering the gamma-ray emitting region inverse Compton scattering of both the synchrotron photons itself and the ambient photons from the radio emitting region. Applying the model to Mrk 501, we obtain that the radio emitting region has a comoving length of ~0.15 pc and is located at sub-parsec scale from the central engine by modeling the radio data; the flux of the Compton scattering of the ambient photons is so low that it can be neglected safely. The characteristic hard gamma-ray spectrum can be explained by the superposition of two SSC processes, and the model can approximately explain the very high energy (VHE) data. The insights into the spectral shape and the inter-band correlations under the flaring state will provide us with a diagnostic for the bulk Lorentz factor of radio emitting region, where the low and upper limits of 8 and 15 are preferred, and for the two-zone SSC model itself. In addition, our two-zone SSC model shows that the gamma-ray emitting region creates flare on the timescale of merely a few hours, and the long time outbursts more likely originate from the extended radio emitting region. [ABSTRACT FROM AUTHOR]

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 onezone 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 ≃0.29°-0.6°. 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⊙. [ABSTRACT FROM AUTHOR]

The broad-line region (BLR) of flat-spectrum radio quasars (FSRQs) could have a "flat" geometrical structure to allow GeV gamma-ray photons to escape, to produce the observed gamma-ray flares with short timescales. In this paper, we collect the quasisimultaneous spectral energy distributions (SEDs) of the FSRQ 3C 454.3 obtained by the multi-wavelength campaigns spanning from 2007 July to 2011 January, and use a model with a "flat" structure BLR, an accretion disc and a dust torus to explain the SEDs of gamma-ray outbursts. We obtain the following results: (i) the jet is almost in equipartition between magnetic and particle energy densities during the outbursts; (ii) when the emitting region is located inside the cavity of the BLR, the covering factor fBLR of the BLR is very small-as the emitting region goes into the BLR structure, fBLR increases; (iii) the aperture angle α describing the BLR structure is about 45°; (iv) the central black hole mass is about 5×108 M⊙ rather than 4.4×109 M⊙. [ABSTRACT FROM AUTHOR]