The high-frequency compact radio structure of the peculiar quasar 4C 39.25

We present new high angular resolution images of the compact non-thermal radio source 4C 39.25 obtained from VLBI observations at lambda 1.3 cm, lambda 7 mm, and lambda 3 mm wavelengths. These maps and Gaussian modelfits show that the milliarcsecond to sub-milliarcsecond structure of 4C 39.25 consists of a complex bent core-jet structure with embedded moving and stationary VLBI components. Facilitated by the small observing beams and high angular resolutions obtained at mm-wavelengths, we measured the relative positions of the jet components with an accuracy of a few hundred micro-arcseconds. This allows the detailed followup of the ongoing merging process of a westward superluminally moving component ((b) under bar) with a stationary component (a) under bar, located at similar to 2.9 mas east of the putative core (d) under bar. In contrast to the other components of the structure with steeper spectra, the westernmost component (d) under bar exhibits an inverted spectrum peaking between lambda 7 mm and lambda 3 mm, thus further supporting its identification as the VLBI core, self-absorbed at longer wavelengths. From two VLBI maps obtained nearly simultaneously at lambda 7 mm and lambda 1.3 cm, we made the first spectral index map of 4C 39.25 in this wavelength regime. The main characteristics of the spectral index distribution of the jet are pronounced changes of the spectral index between orientations parallel and transverse to the jet axis. Near the merging components (a) under bar and (b) under bar the spec tral index steepens with increasing separation from (d) under bar. However, in the bridge of emission (c) under bar, which connects (d) under bar with (a) under bar and (b) under bar, the spectral index gradient has a direction transverse to the jet axis, suggesting a frequency dependent jet curvature and edge-brightening. A brief discussion of this behaviour within current jet models is presented.