Detection of a VLBI counterjet in NGC 1275: A possible probe of the parsec-scale accretion region

Abstract

A high dynamic range Very Long Baseline Array (VLBA) image at 8.4 GHz of the radio source in NGC 1275 reveals a previously undetected radio component on parsec scales to the north of the compact core. As reported in a companion Letter by Vermeulen, Readhead, & Backer, this component has also been detected independently at 22 GHz The new component is much stronger and somewhat more extended at 22 GHz than at 8.4 GHz, and it was not detected at 1.7 GHz in a high dynamic range image by Biretta, Bartel, & Deng (1991). Many properties of the source can be explained if it consists of two oppositely directed, symmetric, and mildly relativistic jets at a modest angle to the line of sight, with the counterjet radiation obscured at lower frequencies by free-free absorption in material around the galactic nucleus. If this model is valid, VLBI observations of the counterjet constitute a probe that can provide constraints on physical conditions in the accretion disk or infall region about 1 pc from this active galactic nucleus. The image reported here was the first continuum image made with data from the new VLBA correlator. A dynamic range of over 4000:1 (peak to noise) was obtained on a complex source using data from only eight antennas. The fact that the very first continuum image made with the full set of VLBA electronics is of such high quality, despite the fact that much fine tuning remains to be done, bodes well for the future performance of the VLBA.