Sub‐Milliarcsecond Imaging of Quasars and Active Galactic Nuclei. III. Kinematics of Parsec‐scale Radio Jets

Abstract

We report the results of a 15 GHz (2 cm) multi-epoch VLBA program, begun in 1994 to study the outflow in radio jets ejected from quasars and active galaxies. The observed flow of 208 distinct features measured in 110 quasars, active galaxies, and BL Lac objects shows highly collimated relativistic motion with apparent transverse velocities typically between zero and about 15c, with a tail extending up to about 34c. Within individual jets, different features appear to move with a similar characteristic velocity which may represent an underlying continuous jet flow, but we also see some stationary and even apparently inward moving features which co-exist with the main features. Comparison of our observations with published data at other wavelengths suggests that there is a systematic decrease in apparent velocity with increasing wavelength, probably because the observations at different wavelengths sample different parts of the jet structure.The observed distribution of linear velocities is not consistent with any simple ballistic model. Either there is a rather broad range of Lorentz factors, a significant difference between the velocity of the bulk relativistic flow and the pattern speed of underlying shocks, or a combination of these options. Assuming a ballistic flow, comparison of observed apparent velocities and Doppler factors computed from the time scale of flux density variations is consistent with a steep power law distribution of intrinsic Lorentz factors, an isotropic distribution of orientations of the parent population, and intrinsic brightness temperatures about an order of magnitude below the canonical inverse Compton limit. It appears that the parent population of radio jets is not dominated by highly relativistic flows, and contrary to the assumption of simple unified models, not all sources have intrinsic speeds close to c.Usually, the observed jet flow is in the general direction of an established jet. However, many jets show significant bends and twists, where the observed motions are non-radial, but are alingned with the local jet direction suggesting that the jet flow occurs along pre-existing bent channels. In a few cases we have observed a clear change in the direction of a feature as it flows along the jet. Radio jets which are also strong gamma-ray sources detected by EGRET appear to have significantly faster speeds than the non EGRET sources, consistent with the idea that gamma ray sources have larger Doppler factors than non gamma-ray sources. Sources at high redshift have systematically lower angular speeds than low redshift jets, consistent with standard cosmologies.

Non-radial motion in the TeV blazar S5 0716+714

“…On kpc-scales, the jet is more East-West oriented with some evidence for halo-emission (e.g., Antonucci et al 1986). A rather large range in proper motions (0.05 mas/yr-1.11 mas/yr) has been reported by different authors based on the investigation of an increasing core separation with time modeled for jet components in this source (e.g., Eckart et al 1987;Witzel et al 1988;Schalinski et al 1992;Gabuzda et al 1998;Jorstad et al 2001;Pérez-Torres et al 2004;Kellermann et al 2004;Bach et al 2005;Rastorgueva et al 2008). Based on these results it is not clear, whether S5 0716+714 reveals slow apparent velocities typical for BL Lac objects (e.g., Britzen et al 2007) or faster apparent velocities more typical for quasars (e.g., Kellermann et al 2004).…”

The kinematics in the pc-scale jets of AGN

“…At times when the pc-scale jet appears to be strongly bent, the jet components show apparent superluminal motion. Britzen et al (2007) and Kellermann et al (2004) find stationary components in their 6 and 2 cm AGN surveys. We also note the case of the quasar 3C 395 (Simon et al 1988;Lara et al 1994Lara et al , 1999, whose jet exhibits a possible stationary component due to possible bending of the jet.…”

“…However, as more detailed VLBI-monitoring reveals, this model can not easily explain all the observed motion phenomena in a growing number of AGN. In many sources stationary features or even motion towards the core have been observed (e.g., Alberdi et al 2000;Kellermann et al 2004;Britzen et al 2007).…”

The MASIV Survey – IV. Relationship between intra-day scintillation and intrinsic variability of radio AGNs

“…These sources are mainly drawn from the Radio Fundamental Catalogue (RFC) comprising all sources observed with VLBI for astrometry and geodesy programmes. They also include data from major VLBI surveys such as the VLBA Calibrator Survey (e.g., Beasley et al 2002;Petrov et al 2005;Kovalev et al 2007), the VLBI Imaging and Polarimetry Survey (Helmboldt et al 2007;Petrov & Taylor 2011), and the VLBA 2-cm Survey Kellermann et al (1998Kellermann et al ( , 2004, among others. For their sample of sources, they fit two circular Gaussian components to the self-calibrated visibilities for each source, then select the most compact component as the core component for which they determine the FWHM angular sizes.…”