A Three-Decade VLBI Study of the Nucleus in the Lobe-Dominated Quasar 3C207
We present results from very-long-baseline-interferometry (VLBI) observations of the nucleus in the lobe-dominated quasar 3C207. These observations were completed at 8.4 or 10.7 GHz (X-band) from 1981 to 2010, spanning 29 years. The nucleus of 3C207 is the strongest and most variable in the 3CR complete sample of LDQs, which is under study to test relativistic jet models over a wide range of jet orientation angles. Images have typical resolutions of ~0.5-1.0 milliarcseconds (mas) and sensitivities of ~0.1-0.2 mJy beam^{-1}. The VLBI core region has flux density outbursts at mean intervals of ~7 yr; two of these are multiple outbursts from a stationary "true: core that feeds a "swinging component" ~0.5 mas to the east. The position angle (PA) of the swinging component shows a long-term increase of ~40°, with a short-term reversal of ~10°. A one-sided, curved VLBI jet extends ~25 mas eastward, with components spanning a PA range of ~25°. The jet components have average apparent transverse velocities ~10c. One component shows apparent acceleration from 7c to 14c at 2-3 mas from the true core, where the flow is redirected toward PA ~90°. Another component shows marginal evidence for apparent deceleration. Individual jet components expand until reaching the recollimation zone. Our results are consistent with a physical model in which 3C207 has quasi-periodic outbursts, jet precession by ballistic components on a conical surface with a small opening angle, and a recollimation process that modifies component motions and narrows the conical geometry on a scale of ~100 pc.