What Happened to the NGC 6251 Counterjet?

We have used the VLBA to produce a high dynamic range image of the nucleus ofNGC 6251 at 1.6 GHz and snapshot images at 5.0, 8.4, and 15.3 GHz to search foremission from a parsec-scale counterjet. Previous VLBI images at 1.6 GHz haveset a lower limit for the jet/counterjet brightness ratio near the core atabout 80:1, which is larger than expected given the evidence that the radioaxis is fairly close to the plane of the sky. A possible explanation is thatthe inner few pc of the counterjet is hidden by free-free absorption by ionizedgas associated with an accretion disk or torus. This would be consistent withthe nearly edge-on appearance of the arcsecond-scale dust disk seen in thecenter of NGC 6251 by HST. We detect counterjet emission close to the core at1.6 GHz, but not at the higher frequencies. Given that the optical depth offree-free absorption falls off more rapidly with increasing frequency than theoptically thin synchrotron emission from a typical radio jet, this resultimplies that the absence of a detectable parsec-scale counterjet at highfrequencies is not due to free-free absorption unless the density of ionizedgas is extremely high and we have misidentified the core at 1.6 GHz. The mostlikely alternative is a large jet/counterjet brightness ratio caused byrelativistic beaming, which in turn requires the inner radio axis to be closerto our line of sight than the orientation of the HST dust disk would suggest.Comment: Accepted for publication in ApJ. Includes 10 figure