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

Citation:

A. Alberdi, T. P. Krichbaum, D. A. Graham, A. Greve, M. Grewing, J. M. Marcaide, A. Witzel, R. S. Booth, L. B. Baath, F. Colomer, S. Doeleman, A. P. Marscher, A. E. E. Rogers, C. J. Schalinski, and K. Standke. 1997. “The high-frequency compact radio structure of the peculiar quasar 4C 39.25.” Astronomy and Astrophysics, 327, Pp. 513-521.

Abstract:

We present new high angular resolution images of the compact non-thermalradio source 4C 39.25 obtained from VLBI observations at λ1.3cm,λ7mm, and λ3mm wavelengths. These maps and Gaussianmodel-fits show that the milli-arcsecond to sub-milliarcsecond structureof 4C 39.25 consists of a complex bent core-jet structure with embeddedmoving and stationary VLBI components. Facilitated by the smallobserving beams and high angular resolutions obtained at mm-wavelengths,we measured the relative positions of the jet components with anaccuracy of a few hundred micro-arcseconds. This allows the detailedfollowup of the ongoing merging process of a westward superluminallymoving component (b_) with a stationary component a_, located at ~2.9mas east of the putative core d_. In contrast to the other components ofthe structure with steeper spectra, the westernmost component d_exhibits an inverted spectrum peaking between λ7mm andλ3mm, thus further supporting its identification as the VLBIcore, self-absorbed at longer wavelengths. From two VLBI maps obtainednearly simultaneously at λ7mm and λ1.3cm, we made thefirst spectral index map of 4C 39.25 in this wavelength regime. The maincharacteristics of the spectral index distribution of the jet arepronounced changes of the spectral index between orientations paralleland transverse to the jet axis. Near the merging components a_ and b_the spectral index steepens with increasing separation from d_. However,in the bridge of emission c_, which connects d_ with a_ and b_, thespectral index gradient has a direction transverse to the jet axis,suggesting a frequency dependent jet curvature and edge-brightening. Abrief discussion of this behaviour within current jet models ispresented.