The Study On The Physical Properties Of Blazar Jets

Active galactic nucleis (AGNs) are special class of galaxies, which have violent active phenomena and intense physical processes in the nuclei. Blazar is a subclass of AGNs, which has relativistic jet with a small jet viewing angle. Therefore, boosting effect is very normally important and almost all the observed radiation is dominated by the jet. The jet physics is very unclear, such as jet formation, jet collimation, jet matter content etc. The multi-waveband radiation of blazar is dominated by jet, which provides an ideal laboratory for studying the jet physics.The first chapter of this thesis introduces the recent progress of blazars. We further introduce the jet model as commonly used in blazars in the second chapter. In the third chapter, we fit (quasi-) simultaneous multi-waveband spectral energy distributions (SED) for a sample of low-synchrotron-peaked (LSP) blazars with the jet model and%-minimization procedure, which takes into account different soft seed photon fields (broad line region or a molecular torus). We find that the SED fitting with external seed photon from IR torus is systematically better than that from BLR based on a χ2test, which suggests that γ-ray emitting region most possibly stay outside the BLR.The minimum electron Lorentz factor,γmin, is constrained from the modeling of these LSP blazars with good soft X-ray data (ranging from5to160with a median value of55), which plays a key role in jet power estimation. Assuming one-to-one ratio of proton and electron, we find that the jet power for LSP blazars is systematically higher than that of FR II radio galaxies. A possible reason for this is that there are some e±pairs in the jets of these blazars. If this is the case, the jet power will be reduced. Therefore, we propose a mixed composition of e±-p in the jets of these LSP blazars. If we assume that the jet power of LSP blazars is the same as that of FR Ⅱs, the number density of e±pairs should be several times higher than that of e--p pairs, but the jet power is still dominated by proton.For HSP BL Lac PKS1424+240, the SED fitting with the SSC model gave unreasonable fitting parameters (e.g., a very large Doppler factorδ～60). In this work, we take into account the possible external seed photon field and then fit the multi-waveband SEDs of blazar PKS1424+240with one-zone leptonic jet models in both states. We find the SSC+EC model can give a better fitting results if EC process is included. However, the needed energy density of external seed photon field (Uext)is much lower than the typical value as used in high power FSRQs. This result is consistent with the results of some other BL Lacs, where BLR or torus is very week or disappearing. There is evolution of the energy density of external seed photon field with the decrease of the luminosity of blazar (FSRQs-BL Lac:LBL-IBL-HBL).We futher explore the possible evolution of the external seed photon field of blazars based on EC process. We employ one-zone homogeneous leptonic jet model and χ2-minimization procedure to fit the simultaneous or quasi-simultaneous multi-waveband SEDs for a sample of blazars with wide distribution luminosities. In our model, we set Uext as a free parameters. We find1) the Uextof the high luminosity blazar (FSRQs and LBLs) keep rougly as a constant, which is, however, smaller than that constrained from BLR observations. Assuming IR as the source of seed photons, the Uext is roughly consistent with the torus observational result. This further supports the result that the external soft photon field may originate from torus, and the γ-ray emitting region of these LSP blazars locate outside the BLR. For IBLs or HBLs, the EC process may be still needed, but the photon energy density is less than the typical values of the photon energy density of BLR (or dust torus) as in FSRQs. Where the Uext is decreases with the decreasing of the luminosity. This evolution is consistent with the BLR or torus as directly constranined fom radio-quiet AGN. The final part summarizes the study on the subject and makes some suggestions for further research.