VLBI observations of compact radio sources at 609 megahertz

Abstract

This paper reports VLBI observations of seven powerful radio sources, obtained using a four-telescope network which, at 609 MHz, gives a maximum resolution of about 5 milliarcsec. Measurements of the closure phase as well as the fringe amplitude have made it possible to make hybrid maps of five of the sources. In many of the sources the similarity between the position angle of the compact structure and that on a much larger scale (at least about 0.5 arcsec) indicates that there is a close relationship between them. This is particularly evident in 3C 84 and 3C 273, where the compact emission region is asymmetrical and has extensions toward structure on the 10-arcsec scale. The data are, in general, explicable in terms of incoherent electron synchrotron radiation with rough energy equipartition between the relativistic particles and the magnetic field; but in CTA 102 and 3C 454.3 the energetics are probably particle dominated. At the epoch of these observations (March 1975) none of the sources had a peak brightness temperature in excess of 1 million K. The lack of frequency dependence in the structure of individual source components implies that they are relatively homogeneous, although more detailed observations at other frequencies are needed to confirm this.