Multi - Frequency VLBI Observations of NRAO 530

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 coredominated 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 enable us to estimate the spectra of compact VLBI components in this highly variable source. Flat spectra are seen in central two components (A and B), and the most compact component A with 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), though the synchrotron self-absorption (SSA) model cannot not be definitely ruled out. 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 about 25 pc. By adding our new measurements to previous data, we obtain apparent velocities of two components (B and E) of 10.2 c and 14.5 c, respectively. These are consistent with that the emergence of VLBI component is 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 further investigate the spectral variability by making use of the single-dish measurements covering a complete outburst profile from mid-1994 to mid-1998. It shows a continuous increasing in the turnover frequency during the rising phase, and a gradual decreasing after passing the peak of the flare. Finally, we discuss the equipartition Doppler-factor (δeq) based on analysis of magnetic field and obtain δeqs of 3.7, 7.2 and 0.8 for components A, B and C, respectively, which are consistent with bf a larger flux density in component B, the non-detection of proper motion in component C and a bent jet.