The Core of a Blazar Jet

The nature of the bright upstream end of jets as seen in VLBI images, known as the core, is difficult to specify because it is only weakly resolved on the images. Here the author discusses evidence indicating that the core is either where the optical depth is of order unity or a standing shock somewhat farther downstream. The jet at this point is turbulent, causing the polarization to be low and variable. At short millimeter wavelengths where the overall continuum spectrum turns over, the core should be at or just downstream of the zone where the flow is accelerated and collimated.