| Active galactic nuclei (AGNs) are the centers of galaxies with violent activities.With great improvements in both the space resolution and detectionsensitivity, AGN jets and their sub-structures, including knots, hot spots, andlobes were observed in the radio, infrared, optical, and X-ray bands for manyAGNs. Emission in the MeV-GeV-TeV band has also been detected for someAGNs with space and ground-based telescopes, placing strong constraints on theradiation mechanisms of the AGNs. Research on the high energy emission ofAGNs and their extended regions is one of the most active fields in astrophysics.This thesis first presents a detail review on the observational and theoreticalprogress for AGNs and their sub-structures, and then reports my relevant worksin this field.So far, almost all identified TeV AGNs are blazars, the only exceptionsbeing two radio galaxies (Cen A and M87), and most of them are HBLs. WComae was the first IBL, which was confirmed to be a TeV source. The SEDsquasi-simultaneously obtained during the TeV flare and during the optical/X-rayoutburst are well fitted by a single-zone synchrotron + synchrotron-self-Compton(Syn+SSC) model. We find that the 7-ray emission can be describedSSC leptonic jet model and the TeV flare and the X-ray/optical outburst maybe not simultaneous.It is believed that hot spots are the terminals of jets. They may be due to theinteraction of a jet with surrounding medium, and are the sites for reaccelerationof relativistic electrons. These electrons may be able to produce high energyγ-ray photons through inverse Compton (IC) scattering. We study the radiationmechanism of the X-rays from the western hot spot of Pictor A, a nearby FR IIradio galaxy, and investigate the possible GeV-TeV emission from this hot spot.We find that the single-zone Syn+SSC model can not describe the ultravioletdata well and the multi-zone model presents a much better fit, consistent withthe VLBI observation by Tingay et al. (2008). Both the single-zone and multi- zone models predict Pictor A as a GeV-TeV source if deviating the equipartitioncondition, which could be marginally detectable with Fermi/LAT and HESS. Bycomparing the GeV-TeV flux from the nucleus and from the hot spot predictedby the model, we show that the GeV-TeV flux is dominated by the hot spot.Based on our results of the hot spot in Pictor A, we make a systematicalanalysis for a sample of 21 hot spots and 46 knots in order to investigate theradiation mechanisms of the X-rays and the physical differences between thetwo kinds of extended structures. Their radiation mechanisms can be well constrainedwith the observed SEDs that cover the radio, optical, and X-ray bands.The single-zone model fits to the observed SEDs show that the mechanisms forthe X-ray emission from the hot spots and the knots could be diverse. The X-rayemission for a small fraction of the hot spots and knots can be explained withthe synchrotron radiation, but an IC scattering component is required to modelthe X-rays for most the hot spots and knots. Without considering the beamingeffect, the X-rays of some hot spots can roughly be explained by the SSC modelwith a magnetic field strength no more than an order of magnitude smaller thanthe equipartition magnetic field, but the derived BSSCδ=1 significantly deviates fromBeqδ=1 for knots. Measuring this deviation with a ratio RB=Beqδ=1/BSSCδ=1, we findthat RB is strongly correlated with RL for both the knots and hot spots, especiallyfor the knots, where RL = L1keV/L5GHz. Considering the beaming effect,an IC/CMB model well explains the X-ray emission under the equipartition condition,and the derived B+eq in this scenario are in the same order for both theknots and hot spots. Theδand RL are correlated. Our results indicate thatthe differences on the X-ray observations for the knots and hot spots may bedue to the beaming effect if the magnetic filed for the knots and hot spots agreewith the equipartition condition. Both the SSC and IC/CMB dominated modelspredict a prominent GeV-TeV component in the SEDs for some sources, whichare detectable by HESS and Fermi/LAT.
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