Abstract
Recently published results from VLBI observations at 3 and 7 millimeters of the radio galaxy Cygnus A are reviewed in this article, and discussed within the model of a prominently stratified jet outflow. At the source redshift ( z = 0.056), mm-VLBI allows a spatial resolution down to 200 Schwarzschild radii to be achieved, providing an extremely detailed view of the two-sided jet base. Through a study of the kinematic properties of the flow and of its transverse structure, it is shown that the radio emission is produced by an accelerating, mildly relativistic, parabolically expanding disk-wind. The observed transverse stratification, both of the flux density and of the bulk speed, supports the presence of an invisible faster spine close to the jet axis, powered either by the inner regions of the accretion disk or by the spinning black hole.
Highlights
Since the early days of radio astronomy, the radio galaxy Cygnus A has been one of the favorite targets for studies aimed at understanding the physics of relativistic jets in AGN
We summarize results from a detailed mm-Very Long Baseline Interferometry (VLBI) study of Cygnus A, first presented in Boccardi et al 2016a, 2016b [14,15], and we interpret them in a model of a jet outflow with significant transverse stratification
Our VLBI study of Cygnus A at millimeter wavelengths provides a coherent description of the mechanisms taking place in the nuclear regions of this jet
Summary
Since the early days of radio astronomy, the radio galaxy Cygnus A has been one of the favorite targets for studies aimed at understanding the physics of relativistic jets in AGN. By measuring the transverse width in the vicinity of the jet apex, one can constrain the radial distance at which the flow is initially anchored If it is larger than the inner radius of the accretion disk, defined by the location of the innermost stable circular orbit (ISCO), this indicates that the disk is, at least partially, driving the flow, as described in the work of Blandford and Payne [11]. Thanks to the much reduced synchrotron opacity at high radio frequencies and to an observing beam as small as 200 RS (for MBH = 2.5 × 109 M [16]), we could resolve transversely the base of the two-sided flow and obtain a comprehensive picture of the jet parameters in the acceleration and collimation region. For a more exhaustive description of the data analysis and scientific discussion, the reader is referred to the original articles [14,15]
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