Constraints on the Accretion Region in NGC 1275 from VLBA Observations of the Counterjet

Abstract

A northern feature, that is most likely related to a counterjet, was found on parsec scales in 3C 84 (which is in NGC 1275) in “First Science” observations on the VLBA¹ at 8.4 GHz (Walker et al., 1994) and in Global VLBI observations at 22 GHz (Vermeulen et al., 1994). The jet/counterjet length ratio, brightness ratio at 22 GHz, and speeds, as measured over many years with VLBI, fit a simple beaming model with symmetric jets oriented at 30–50 degrees to the line-of-sight and traveling at a speed of 0.3–0.5 times the speed of light. These ranges allow for Hubble constants of between 50 and 100 km s⁻¹ Mpc⁻¹. The brightness of the counterjet at 8.4 GHz was very much lower than expected with this model, assuming that the spectral indices in the near and far side jets are similar. This was interpreted as the result of free-free absorption in an ionized medium that lies in front of the counterjet but not in front of the near-side jet. An accretion disk or torus has an appropriate geometry to show this effect (for an analysis, see Levinson et al., 1995).