Abstract
Very long baseline interferometry offers the best means of investigating the complex dynamics of relativistic jets powered by active galactic nuclei, via multi-epoch, sub-milliarcsecond, full-polarization imaging at radio wavelengths. Although targeted studies have yielded important information on the structures of individual AGN jets, the strong selection effects associated with relativistically beaming imply that general aspects of the flows can only be determined via large statistical studies. In this review I discuss major results from the Monitoring of Jets in Active Galactic Nuclei With VLBA Experiments (MOJAVE) program, which has gathered multi-epoch Very Long Baseline Array (VLBA) data at 15 GHz on over 400 AGN jets over the course of two decades. The sample is large enough to encompass a range of AGN optical class, radio luminosity and synchrotron peak frequency, and has been used to show that within a particular jet, individual bright features have a spread of apparent speed and velocity vector position angle about a characteristic value. We have found that in some cases there is a secular evolution of launch angle direction over time, indicative of evolving narrow energized channels within a wider outflow. The majority of the jet features are superluminal and accelerating, with changes in speed more common than changes in direction. Within approximately 100 pc of the AGN, the flows are generally accelerating, while beyond this distance the flows begin to decelerate or remain nearly constant in speed. We also find evidence for a maximum bulk flow Lorentz factor of 50 in the pc-scale radio regime, and a trend of higher jet speeds in lower-synchrotron peaked and gamma-ray-loud blazars.
Highlights
The techniques of radio-wavelength Very Long Baseline Interferometry (VLBI) make it possible to study the the time evolution of AGN jets on parsec-scales, despite their enormous cosmological distances
We argued that in order to have such a good correlation, there must be a narrow range of spectral energy distributions (SED) shape, implying that the only major distinction between bright high synchrotron peaked (HSP) and LSP BL Lacs is the peak energy of their relativistic electron population
To date we have studied the kinematics of five NLSY1 AGN in MOJAVE, and find three to have significant superluminal jet motion [19]
Summary
The techniques of radio-wavelength Very Long Baseline Interferometry (VLBI) make it possible to study the the time evolution of AGN jets on parsec-scales, despite their enormous cosmological distances. The advent of the first dedicated VLBI array: the Very Long Baseline Array (VLBA) in 1994 represented a major advance for jet kinematics studies, by making it possible to observe relatively large samples of jets at regular intervals. AGN jet VLBI studies, the 2 cm Survey [1], and its successor, the MOJAVE MOJAVE is distinct in terms of its large sample size (several hundred AGN, distributed across the northern 2/3 of the celestial sphere), its long continuous time baselines, and its full polarization observations. Other past and current AGN jet monitoring programs are complementary to MOJAVE with regards to their sky coverage (southern hemisphere: TANAMI [3]), observing frequency (43 GHz: Boston U. program [4]; 8 GHz: USNO RFID [5]; 5 GHz: CJ-F survey [6]) and monitoring of faint TeV blazar jets [7]
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