Morphological changes in the NGC 6251 jet

The giant radio galaxy NGC 6251 is a particularly good object for observational tests of relativistic jet models. Due to its high declination and approximately 0.5 Jy radio nucleus, high-quality Very Long Baseline Interferometry (VLBI) images of the central regions of the source can be made with northern hemisphere arrays. In addition, the large-scale radio morphology strongly suggests that the radio axis lies close to the plane of the sky, so Doppler boosting should be less extreme than in the core-dominated superluminal sources. Earlier 18 cm VLBI observations of NGC 6251 revealed an unexpectedly large jet/counterjet brightness ratio and small transverse motion of a feature in the parsec-scale jet. These early results are difficult to reconcile with the simplest symmetric relativistic jet models. In this paper we present a third-epoch 18 cm VLBI image of the parsec-scale radio jet in NGC 6251, and compare jet morphology over a 5 year time span. The jet shows a minor brightness peak at nearly the same distance from the core as the '25 mas knot' seen in the first- and second-epoch VLBI images. This feature is much less pronounced in the third epoch, and a relatively bright, new knot has appeared approximately 12 mas from the core. If this new component had a constant brightness during the 5 years separating the first and third observing epochs, then it must have moved away from the core with an apparent speed of at least 1.2c (compared with an upper limit of 0.23c for motion of the 25 mas knot). However, we cannot yet rule out a local brightening of the inner jet in favor of a new moving component. We determine a lower limit for the jet/couterjet brightness ratio of 100:1 within 6 mas of the core. We also present a new Very Large Array (VLA) image of the kpc-scale jet with 3 sec resolution, made from data obtained during the VLBI observations. The rate of decrease in jet surface brightness from parsec to kiloparsec scales is similar to jets in known superluminal radio sources.