The Subparsec-scale Structure and Evolution of Centaurus A. III. A Multi-Epoch Spectral And Polarimetric VLBA Study

The Centaurus A radio galaxy, due to its proximity, presents itself as one of the few systems that allow the study of relativistic jet outflows at sub-parsec distances from the central supermassive black holes, with high signal to noise. We present the results from the first multi-epoch spectropolarimetric observations of Centaurus A at milliarcsecond resolution, with a continuous frequency coverage of $4.59-7.78$\,GHz. Using a Bayesian framework, we perform a comprehensive study of the jet kinematics, and discuss aspects of the jet geometry including the jet inclination angle, jet opening angle, and the jet expansion profile. We calculate an upper limit on the jet's inclination to the line of sight to be $<25^{\circ}$, implying the lower limit on the intrinsic jet speed to be $0.2$\,c. On the observed VLBA scales we detect new jet components launched by the central engine since our previous study. Using the observed frequency-dependent core shift in Centaurus A, we find the jet to have reached constant bulk speed and conical outflow at the regions probed by the base of the jet at $7.78- 4.59$\,GHz, and we also estimate the location of the central black hole further upstream. Through polarimetric analysis (by applying RM synthesis for the first time on VLBI data), we find evidence to suggest the possible onset of acceleration towards the leading edge of Centaurus A's subparsec-scale jet studied here.