1999ApJ...515..558S

The gigahertz peaked spectrum (GPS) radio source CTD 93 has long been considered the prototypical compact double radio source described by a simple two-component structure. We have used the NRAO VLBA to observe CTD 93 at 2, 3.6, 6, 13, 18, and 50 cm in order to determine the structure of the two predominant components and to search for a possible central component. We find that the observed structure is characteristic of the core-jet structure commonly seen in other quasars and active galactic nuclei rather than a symmetric double. The northern core component has a complex structure dominated by a wide-angle extension pointing toward the southwest. The jet reappears about 25 mas away, where it appears remarkably well collimated. At a distance of 50 mas from the northern feature the jet reaches a hot spot where it abruptly turns toward the southeast. Comparison with earlier observations of CTD 93 indicates no changes in the separation of the two brightest features greater than 0.025 mas.yr^–1 (0.5 h^–1 c). We suggest that it may be premature to consider GPS radio sources to be necessarily associated with symmetric radio structure. The observed sharp low-frequency cutoff is most likely due to synchrotron self-absorption, but we cannot rule out the possibility that there is also absorption from an intervening ionized medium or from ionized material that is mixed with the synchrotron source. The absence of observed superluminal motion over a time span of 20 years, the lack of flux density variability, and the sharply peaked spectrum suggest that the direction of the jet in CTD 93 lies close to the plane of the sky and/or that any motion is nonrelativistic.