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We present the results of six-epoch Very Long Baseline Array (VLBA) observations of 3C 66A. The high-resolution Very Long Baseline Interferometer (VLBI) maps obtained at multi-frequency (2.3, 8.4, and 22.2 GHz) simultaneously enabled us to identify the brightest compact component with the core. We find that the spectrum of the core can be reasonably fitted by the synchrotron self-absorption model. Our VLBA maps show that the jet of 3C 66A has two bendings at about 1.2 and 4 mas from the core. We also give possible identifications of our jet components with the components in previous VLBA observations by analysing their proper motions. We find consistent differences of the position from the core in one component between different frequencies at six epochs.

We present results of Very Long Baseline Array (VLBA) observations of PKS 0528+134 at five frequencies (2.3, 5.0, 8.4, 15.4, and 22.2 GHz). These quasi-simultaneous data enable us to study the spectral distribution of Very Long Baseline Interferometer (VLBI) components for the first time in this highly variable source, from which the central compact core is identified. Our observations indicate that there are two bendings for the jet motion at parsec scale. We provide an approximate spatial fit to the curved jet trajectory using the Steffen et al. (1995) helical model. We further investigate the proper motions of three jet components, which all show superluminal motion. At high frequencies (15.4 and 22.2 GHz) we detected a new component, which is estimated to be related to a radio burst peaking at about 2000.

We report on VLBA observations of a γ-ray bright blazar NRAO 530 at multiple frequencies (5, 8, 15, 22, 39, 43, and 45 GHz) in 1997 and 1999. These multi-epoch multi-frequency high-resolution VLBI images exhibit a consistent core-dominated morphology with a bending jet to the north of the core. The quasi-simultaneous data observed at five frequencies (5, 8, 15, 22, and 43 GHz) in February 1997 enabled us to estimate the spectra of compact VLBI components in this highly variable source. Flat spectra are seen in the two central components (A and B), and the most compact component A that has the flattest spectral index at the south end is identified as the core. Based on the synchrotron cooling timescale argument, it is suggested that the observed inverted spectrum of component C is caused by the free-free absorption (FFA), although the synchrotron self-absorption (SSA) model cannot be ruled out definitely. While the SSA probably exists in component B, it is likely that the same FFA would produce the spectral turnover toward component B since the fitted FFA coefficients in both B and C components are almost the same. If so, the projected size of such an absorbing medium is at least 25 pc.
By adding our new measurements to previous data, we obtain apparent velocities of two components, B of 10.2 c and E of 14.5 c. These are consistent with the emergence of VLBI component associated with the flux density outburst; i.e. components B and E are related to strong γ-ray flares in 1994.2–1994.6 and 1995.4–1995.5, respectively. We investigate the spectral variability further by making use of the single-dish measurements covering a complete outburst profile from mid-1994 to mid-1998. It shows a continuous increase in the turnover frequency during the rising phase, and a gradual decrease after passing the peak of the flare. Finally, we discuss the equipartition Doppler-factor ($\delta_{\rm eq}$) based on analysis of the magnetic field and obtain $\delta_{\rm eq}$s of 3.7, 7.2, and 0.8 for components A, B, and C, respectively, which are all consistent with a larger flux density in component B, the non-detection of proper motion in component C, and a bent jet.