SIMBAD references

We describe a new tool for studying the structure and physical characteristics of ultracompact active galactic nucleus (AGN) jets and their surroundings with µas precision. This tool is based on the frequency dependence of the light curves observed for intra-day variable radio sources, where the variability is caused by interstellar scintillation. We apply this method to PKS 1257-326 to resolve the core-shift as a function of frequency on scales well below ∼12 µas. We find that the frequency dependence of the position of the scintillating component is r∝ν^–0.1±0.24 (99% confidence interval) and the frequency dependence of the size of the scintillating component is d∝ν^{–0.64±0.006}. Together, these results imply that the jet opening angle increases with distance along the jet: d ∝rⁿd_^ with n_d> 1.8. We show that the flaring of the jet, and flat frequency dependence of the core position is broadly consistent with a model in which the jet is hydrostatically confined and traversing a steep pressure gradient in the confining medium with p ∝r^–np_^ and n_p ≳ 7. Such steep pressure gradients have previously been suggested based on very long baseline interferometry studies of the frequency dependent core shifts in AGNs.