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

Abstract

Using JVLA data of Sgr A* and NRAO 530 (as a calibrator) at 5.5, 9 and 33 GHz, we developed a procedure for the reduction of wideband data. We have demonstrated that, correcting for residual interferometer errors such as residual delays, astronomers can now achieve high-fidelity radio images with a dynamic range exceeding 1,000,000:1. We outline the detailed procedure, noting that it can have broad application to the analysis of broadband continuum observations. We apply this procedure to observations of a distant blazar, NRAO 530, revealing its structures in unprecedented detail. Our 5.5-GHz image shows that the structure of NRAO 530 is prominently characterized by a moderately curved western jet terminating at a hot spot. Close to the radio core, an abrupt bending of the jet is revealed in the high-resolution (<100 mas) images at 33 GHz, showing an evolution of the position angle of the jet from the north at the VLBI scale (50 mas, or ~400 pc projected), increasing toward the west at the larger VLA scales (1 arcsec, or ~10 kpc). The continuation of the jet axis drift forms the curved western jet extending out to 20 arcsec. In contrast, a faint and broad counter-jet is present on the eastern side with a curvature antisymmetric to the western jet. The eastern jet terminates at a bright hotspot, forming an edge-brightened diffuse lobe. The observed contrast in brightness between the western and eastern jets 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.