The Origin Of High-energy Gamma-ray From Blazars

Blazars are a special subclass of active galaxy nucleis (AGNs). Radiationfrom blazars is thought to originate in a relativistic jet oriented at a small anglewith respect to the line of sight. They usually show rapid variability, high polar-ization and strong non-thermal emission. Spectral energy distributions (SEDs)of blazars are characterized by two broad components: a low-energy componentfrom radio to X-ray frequency, and a high-energy component from X-ray to γ-ray.It is generally accepted that the low-energy component of blazars SEDs is pro-duced by synchrotron radiation from relativistic electrons in the jet. However,the origin of high-energy component is still a matter of debate. There are twoclasses of models that are used to explain the high-energy emission of blazars: theleptonic model and the hadronic model. In the leptonic model, the high-energyemission is produced by inverse Compton scattering of electrons on backgroundphotons. In the hadronic model, the high-energy emission originates from protonsynchrotron or photon-hadronic interactions. We study the origin of high-energyemission from blazars based on leptonic model and the hadronic model. Themain results are as following:1. We study the location of the GeV emission region of21fat spectrumradio quasars (FSRQs) with quasi-simultaneous spectral energy distributions. Wepropose a method to constrain the location of the GeV emission region based onthe spectral shapes. If the γ-ray emission region is located inside the BLR, the ICscattering could occur at the Klein-Nishina (KN) regime and the γ-ray spectrumshould be steeper than the optical-infrared spectrum. If the γ-ray emission regionis located far beyond the the broad-line region (BLR), the IC scattering couldtake place at the Thomson regime and the γ-ray spectrum should have the samespectral index as the optical-infrared spectrum. In order to test our scenario, Wereproduced the simultaneous SEDs of21FSRQs using one-zone leptonic modelwith the synchrotron-self Compton (SSC) and external Compton (EC) processes.We suggest that the X-ray emission is produced by SSC process, the GeV emission comes from the external Compton (EC) process, in which the EC emission mayoriginate from the inverse Compton (IC) scattering of photons from BLR andaccretion disc or dust torus by the same electron population. We infer from thespectral shapes and SED modeling that the location of the GeV emission regionis inside the BLR for5FSRQs and beyond the BLR for16FSRQs. Our resultsshow that the ratio of the magnetic feld and electron energy density is close toequipartition condition for21FSRQs.2. We investigate the X-ray and γ-ray fares of Mrk421on2008June6-15using the SSC model with electron acceleration, in which an evident correlationbetween the X-ray and γ-ray bands appears, while no signifcant correlation be-tween the optical and X-ray band is observed. We argue that the emission fromMrk421may originate from two diferent components. One is the steady compo-nent from the outer region that is mainly attributed to the optical radiation, inwhich the electrons are accelerated by frst-order Fermi acceleration mechanism.We use a steady electron spectrum to produce the SSC emission from the steadycomponent. The other is the variable component from the inner region, in whichthe electrons are accelerated by the stochastic acceleration process. We use thetime-dependent SSC model to produce the emission from the variable componen-t. We suggest that the fares are due to the hardening of the electron spectrumunder the process of the stochastic acceleration, which leads to the hardeningof the observed spectrum in the X-ray and γ-ray bands. Furthermore, we fndthat the energy densities of electrons and magnetic felds are near equipartitionin both jet regions.3. The very hard γ-ray spectrum from distant blazars challenges the tra-ditional SSC model, which may indicate that there is the contribution of anadditional high-energy component beyond the SSC emission. We study the pos-sible origin of the hard γ-ray spectrum from distant blazars1ES1101-232. TheH.E.S.S. observation indicated that the very high energy (VHE) γ-ray fux showsno evidence for variability on any time. It is possible that hard γ-ray spectrumhas a hadronic origin. We suggest that the hard γ-ray spectrum from blazars1ES1101-232may originate from interactions of high-energy protons. We develop amodel to explain the hard γ-ray spectrum from distant blazar1ES1101-232. In the model, the optical and X-ray radiation comes from the synchrotron radiationof primary electrons and secondary pairs and that the GeV emission is producedby the SSC process, however, the hard γ-ray spectrum would originate from thedecay of neutral pion produced through proton-photon interactions with the syn-chrotron radiation photons within the jet. Our model can explain the observedSED of1ES1101-232well, especially the very hard γ-ray spectrum. However,our model requires extreme proton power to efciently produce the γ-ray throughproton-photon interactions.