Investigation Of The Absorption In Active Galactic Nuclei Using VLBI

In this dissertation, we mainly discuss the observationl effects caused bythe synchrotron self-absorption (SSA) and free-free absorption (FFA). We studyPKS 0528+134 by using VLBA observations at five frequencies at 2001.64. Wealso study 3C 66A by using VLBA observations at 2.3, 8.4, and 22.2 GHz at sixepochs.The radio radiation from AGN is likely to be absorbed more or less by theabsorbing medium in AGN and the medium between AGN and the observer.The absorption depresses the radiation intensity we receive, but it also gives usa new access to study the environment in and outside of AGN. For the radiocontinuum, the main absorption mechanisms are SSA and FFA. Synchrotronphotons may be absorbed through the interaction with low energy non-thermalelectrons; this process is called SSA. The energy of thermal electrons may increaseas a result of radio photons being absorbed when radio photons interact withthermal electrons, this process is called FFA. Some observing effects caused bySSA and FFA are quite similar. Both SSA and FFA can induce the inverse ofthe radio continuum at low frequencies. For a uniform synchrotron source, thespectral index at low frequencies is 2.5 for SSA, but for FFA, it is steeper than 2.5.The position of VLBI core is defined as the position where the optical depth is1. The higher the observing frequency, the deeper region will be detected, so thepositions of the VLBI core at high frequencies will be closer to the jet apex thanat low frequencies, i.e., there is a positional offset of the VLBI core at differentfrequencies. This positional offset has been observed by many VLBI observations,but we cannot discriminate which is the main mechanism responsible for such anoffset, SSA or FFA. Another similar observational effect caused by SSA and FFAis the polarization angle variation of the radio wave when at different frequencies.The polarization angle difference at high frequencies and low frequencies due toSSA is～90°. The plasma screen for the FFA can cause the change of thepolarization angle at different frequencies too; this change is proportional to the square of the observing wavelength, which is known as the Faraday rotation.Due to these similar effects caused by SSA and FFA, people are searching themethods to discriminate them.We performed VLBA observations of PKS 0528+134 at 2.3, 5.0, 8.4, 15.4,and 22.2 GHz, and find that PKS 0528+134 has a structure extending north-eastand the jet has two bendings at about 0.3 and 1.4 mas from the core. We finda new component n2, which may be related to the radio burst at 1999.5. Byextrapolating the proper motion of component a, we estimate its ejection time of1991.94, but we cannot ascertain which radio burst is related to its ejection. Wealso find a phenomenon of position-angle regression; i.e., the position angle ofthe jet component gradually decreases with its increasing distance from the coreuntil 25°. The quasi-simultaneous VLBA observations at five frequencies enableus to study the spectral distribution of the jet components without the effectof flux-density variation. We identify k as the core because of its flat spectrum(α=0.30±0.08) and the spectral inverse at low frequencies. We try to use thehelical model to describe the bending of the jet; the kinetic parameters obtainedfrom the helical model are consistent with ones obtained from our data. Thishelical model can also explain the position-angle regression.We proposed to observe PKS 0528+134 with VLBA at eight frequencies forstudying the absorption mechanism in the core region and the spectral distrib-ution of the jet components. The proposal has been approved. Combining theprevious VLBI observations, we can also study the kinematics of the jet compo-nents in PKS 0528+134.We present results of VLBA observations of 3C 66A at three frequenciesduring six epochs. We find from our VLBA maps that there are two bendingsat 1.2 and 4 mas from the core. We identify the brightest k component as thecore from our simultaneous spectral data which show an inverse. Combining theexisting VLBI data at 5 GHz, we use the SSA model to fit the core spectrum atepoch 2001.48 for the first time, and obtain the best-fit optically-thin spectralindex of -0.08 with a maximum flux density 0.66 Jy at the turnover frequencyof 6.52 GHz. We do not detect any obvious proper motions in jet components during our shortly-spanned observing epochs. We give a possible identificationof our jet components with the jet components in previous VLBI observations.The positional offset of component d relative to the core at 2.3 and 8.4 GHz isconsistently seen at all six epochs. We also study the light curves of 3C 66Aat different frequencies, and find that the simultaneous radio burst at differentfrequencies may be due to the rotation of the jet axis, then the change of ourviewing angle to the jet emission, and thus the change of flux density induced bythe change of Doppler factor.