A Procedure for Making High Dynamic-range Radio Images: Deep Imaging of the Kiloparsec-scale Radio Structures of a Distant Blazar, NRAO 530, with JVLA Data

Using JVLA data obtained from A-, B-, and C-array observations of Sgr A* and NRAO 530 (as a calibrator) at 5.5, 9, and 33 GHz during the period between 2012 and 2015, we developed a procedure for the reduction of wideband data at high angular resolution. We have demonstrated that, correcting for residual interferometer errors, such as antenna-based errors caused by residual delays as well as baseline-based closure errors, radio astronomers can now achieve high-fidelity radio images with a dynamic range (peak:rms) exceeding 1,000,000:1. We outline the procedure in detail, noting that it can have broad application in the analysis of broadband continuum observations. Here, we apply this procedure to observations of a distant blazar, NRAO 530, revealing the radio structures surrounding the core in unprecedented detail. Our wideband JVLA image of this source at 5.5 GHz (C-band) shows that the kiloparsec radio structure of NRAO 530 is prominently characterized by a moderately curved western jet terminating at a hot spot where the radiation is stretched farther out into a diffuse radio lobe or plume. Close to the radio core, an abrupt bending of the jet is revealed in the high-resolution (<100 mas) images at 33 GHz (Ka band), showing the evolution of the position angle of the jet from the north at the VLBI scale (50 mas, or -400 pc projected) and increasing toward the west at larger VLA scales (1 ", or similar to 10 kpc). The continuation of the jet axis drift forms a curved western jet extending out to 200 kpc. In contrast to the main radio structure, a faint and broad counter jet is present on the eastern side with a curvature antisymmetric to that of the western jet. The eastern jet terminates at a bright hot spot, forming an edge-brightened diffuse lobe. A newly recognized compact component, located 0/16 east of the radio core, is detected in the A-array images at 9 GHz (X-band), suggesting that a more recent ejection has taken place toward the east. In addition, a lower brightness emission extended N-S from the core is detected at a level of 1-10 K in brightness temperature in our 5.5 GHz C-array data. The observed contrast in surface brightness between the western and eastern jet components suggests that the jets on the VLA scale are mildly relativistic. The radiation from the western jet is boosted while the radiation from the receding eastern jet is plausibly suppressed owing to the relativistic Doppler effect.