| 其他摘要 | Blazars is an extreme class of active galactic nuclei (AGNs), which is divided into BL Lacertae objects (BL Lacs) and flat-spectrum radio quasars (FSRQs), according to the intensity of observed broad-emission lines. Most extragalactic objects identified in $\gamma$-rays belong to this category. Owing to their relativistic jet pointing aligned with the line of sight, the non-thermal radiations produced will dramatically be Doppler-boosted. It has been the window to explore the matter accretion, acceleration mechanism, jet content and the interactions between particles and particles or particles and photons, together with jet and circumnuclear matter. Therefore, the investigations on blazars has been an active area in modern astrophysics. Under the framework of the leptonic models, the spectrum from GeV to TeV band originates from Compton scattering of local photons by relativistic electrons for BL Lacs. Whereas for FSRQs, the observed GeV photons come from Compton scattering of external soft photons, those photons could come from broad-line region (BLR) or dust torus, such framework can satisfactorily explain the spectral energy distributions (SEDs) and correlation between X- and $\gamma$-rays. But we have to solve the problem: how to reduce the absorption to GeV photons when the emission region is located within the cavity of the BLR? Under the context of spherical BLR structure, the $\gamma$-rays photons with energies above tens of GeV will suffer from $\gamma+\gamma \to e^{\pm}$ processes, and cannot avoid the fate of absorption. Therefore, we propose a BLR model with ``flat" structure measured by aperture angle $\alpha$. Under such structure, the energy density of the BLR is the function of the radial distance. So that, almost all of the photons originating from the emission region can escape the diffuse photon field of the BLR, even if the emission region located within the cavity of the BLR. Moreover, we collect the quasi-simultaneous SEDs of 3C 454.3 obtained by the multi-wavelength campaigns in 2007 July to 2011 January, and use a model with the ``flat" structure BLR, accretion disc and dust torus to explain the SEDs of gamma-ray outbursts. In the model, the low-energy component would come from the synchrotron radiation of relativistic electrons and the high-energy component consists of two humps would be produced by the synchrotron self-Compton (SSC) and external Compton (EC) processes, in which the first hump peaking at X-rays would be produced by the SSC emission, the second one peaking at GeV $\gamma$-rays would come from the Compton scattering of dust torus and BLR radiations. We obtain the following results: (i) The aperture angle $\alpha$ describing the BLR structure is about $45^{\circ}$; (ii) The jet is almost dominated by the matter during the outbursts; (iii) The central black hole (BH) mass is about $5\times 10^{8}$\,$M_{\odot}$ rather than $4.4\times 10^{9}$\,$M_{\odot}$. Mrk 501 is a typical BL Lac. In 2009, Fermi and other telescopes carried out a multifrequency campaign, obtained a complete SED. However, those SED data cannot fit well by one-zone homogeneous SSC model. The analysis related to the spectrum and flux suggested that the properties within the emission region is more complex. Under such arguments, we propose another explanation related to two zones. Such that we can satisfactorily reproduce SED . Meanwhile, it can impose some constraints on acceleration mechanism. The result show that the diffusive shock acceleration mechanism could play an important role in baseline state, while the flaring state, may be a result of multiple pre-acceleration mechanism. |
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