The center of activity in the compact steep-spectrum superluminal source 3C 138

We present the results from the first quasi-simultaneous multifrequency (2.3, 5.0, 8.4, and 15 GHz) very long baseline interferometry (VLBI) observations of the compact steep-spectrum (CSS) superluminal source 3C 138. For the first time, the spectral distribution of the components within its central 10 milliarcsecond (mas) region was obtained. This enables us to identify the component at the western end as the location of the nuclear activity, assuming that the central engine is associated with one of the detected components. The possibility that none of these visible components is the true core is also discussed. The new measurements further clarify the superluminal motions of its inner jet components. The multifrequency data reveal a convex spectrum in one jet component, implying the existence of free-free absorption by the ambient dense plasma.