We present multi-epoch, parsec-scale core brightness temperature observations of 447 AGN jets from the MOJAVE and 2cm Survey programs at 15 GHz from 1994 to 2019. The brightness temperature of each jet over time is characterized by its median value a
nd variability. We find that the range of median brightness temperatures for AGN jets in our sample is much larger than the variations within individual jets, consistent with Doppler boosting being the primary difference between the brightness temperatures of jets in their median state. We combine the observed median brightness temperatures with apparent jet speed measurements to find the typical intrinsic Gaussian brightness temperature of (4.1 +- 0.6)*10^10 K, suggesting that jet cores are at or below equipartition between particle and magnetic field energy in their median state. We use this value to derive estimates for the Doppler factor for every source in our sample. For the 309 jets with both apparent speed and brightness temperature data, we estimate their Lorentz factors and viewing angles to the line of sight. Within the BL Lac optical class, we find that high-synchrotron-peaked (HSP) BL Lacs have smaller Doppler factors, lower Lorentz factors, and larger angles to the line of sight than intermediate and low-synchrotron-peaked (LSP) BL Lacs. We confirm that AGN jets with larger Doppler factors measured in their parsec-scale radio cores are more likely to be detected in gamma rays, and we find a strong correlation between gamma-ray luminosity and Doppler factor for the detected sources.
We have analyzed the parsec-scale jet kinematics of 447 bright radio-loud AGN, based on 15 GHz VLBA data obtained between 1994 August 31 and 2019 August 4. We present new total intensity and linear polarization maps obtained between 2017 January 1 to
2019 August 4 for 143 of these AGN. We tracked 1923 bright features for five or more epochs in 419 jets. A majority (60%) of the well-sampled jet features show either accelerated or non-radial motion. In 47 jets there is at least one non-accelerating feature with an unusually slow apparent speed. Most of the jets show variations of 10 to 50 deg in their inner jet position angle (PA) over time, although the overall distribution has a continuous tail out to 200 deg. AGN with SEDs peaked at lower frequencies tend to have more variable PAs, with BL Lacs being less variable than quasars. The Fermi LAT gamma-ray associated AGN also tend to have more variable PAs than the non-LAT AGN in our sample. We attribute these trends to smaller viewing angles for the lower spectral peaked and LAT-associated jets. We identified 13 AGN where multiple features emerge over decade-long periods at systematically increasing or decreasing PAs. Since the ejected features do not fill the entire jet cross-section, this behavior is indicative of a precessing flow instability near the jet base. Although some jets show indications of oscillatory PA evolution, we claim no bona fide cases of periodicity since the fitted periods are comparable to the total VLBA time coverage.
We have carried out a Chandra X-ray and multi-frequency radio VLBA study of the AGN TXS 0128+554, which is associated with the Fermi gamma-ray source 4FGL J0131.2+5547. The AGN is unresolved in a target 19.3 ks Chandra image, and its spectrum is well
fit by a simple absorbed power law model, with no distinguishable spectral features. Its relatively soft X-ray spectrum compared to other CSOs may be indicative of a thermal emission component, for which we were able to obtain an upper temperature limit of kT = 0.08 keV. The compact radio morphology and measured advance speed of 0.32c +- 0.07c indicate a kinematic age of only 82 y +- 17 y, placing TXS 0128+554 among the youngest members of the compact symmetric object (CSO) class. The lack of compact, inverted spectrum hotspots and an emission gap between the bright inner jet and outer radio lobe structure indicate that the jets have undergone episodic activity, and were re-launched a decade ago. The predicted gamma-ray emission from the lobes, based on an inverse Compton-emitting cocoon model, is three orders of magnitude below the observed Fermi LAT flux. A comparison to other Fermi-detected and non-Fermi detected CSOs with redshift z<0.1 indicates that the gamma-ray emission likely originates in the inner jet/core region, and that nearby, recently launched AGN jets are primary candidates for detection by the Fermi LAT instrument.
We present results from a parsec-scale jet kinematics study of 409 bright radio-loud AGNs based on 15 GHz VLBA data obtained between 1994 August 31 and 2016 December 26 as part of the 2cm VLBA survey and MOJAVE programs. We tracked 1744 individual br
ight features in 382 jets over at least five epochs. A majority (59%) of the best-sampled jet features showed evidence of accelerated motion at the >3sigma level. Although most features within a jet typically have speeds within ~40% of a characteristic median value, we identified 55 features in 42 jets that had unusually slow pattern speeds, nearly all of which lie within 4 pc (100 pc de-projected) of the core feature. Our results combined with other speeds from the literature indicate a strong correlation between apparent jet speed and synchrotron peak frequency, with the highest jet speeds being found only in low-peaked AGNs. Using Monte Carlo simulations, we find best fit parent population parameters for a complete sample of 174 quasars above 1.5 Jy at 15 GHz. Acceptable fits are found with a jet population that has a simple unbeamed power law luminosity function incorporating pure luminosity evolution, and a power law Lorentz factor distribution ranging from 1.25 to 50 with slope -1.4 +- 0.2. The parent jets of the brightest radio quasars have a space density of 261 +- 19 Gpc$^{-3}$ and unbeamed 15 GHz luminosities above ~$10^{24.5}$ W/Hz, consistent with FR II class radio galaxies.
We report on the acceleration properties of 329 features in 95 blazar jets from the MOJAVE VLBA program. Nearly half the features and three-quarters of the jets show significant changes in speed and/or direction. In general, apparent speed changes ar
e distinctly larger than changes in direction, indicating that changes in the Lorentz factors of jet features dominate the observed speed changes rather than bends along the line of sight. Observed accelerations tend to increase the speed of features near the jet base, $lesssim 10-20$ parsecs projected, and decrease their speed at longer distances. The range of apparent speeds at fixed distance in an individual jet can span a factor of a few, indicating that shock properties and geometry may influence the apparent motions; however, we suggest that the broad trend of jet features increasing their speed near the origin is due to an overall acceleration of the jet flow out to de-projected distances of order $10^2$ parsecs, beyond which the flow begins to decelerate or remains nearly constant in speed. We estimate intrinsic rates of change of the Lorentz factors in the galaxy frame of order $dot{Gamma}/Gamma simeq 10^{-3}$ to $10^{-2}$ per year which can lead to total Lorentz factor changes of a factor of a few on the length scales observed here. Finally, we also find evidence for jet collimation at projected distances of $lesssim 10$ parsecs in the form of the non-radial motion and bending accelerations that tend to better align features with the inner jet.
We investigate the dependence of gamma-ray brightness of blazars on intrinsic properties of their parsec-scale radio jets and the implication for relativistic beaming. By combining apparent jet speeds derived from high-resolution VLBA images from the
MOJAVE program with millimetre-wavelength flux density monitoring data from Metsahovi Radio Observatory, we estimate the jet Doppler factors, Lorentz factors, and viewing angles for a sample of 62 blazars. We study the trends in these quantities between the sources which were detected in gamma-rays by the Fermi Large Area Telescope (LAT) during its first three months of science operations and those which were not detected. The LAT-detected blazars have on average higher Doppler factors than non-LAT-detected blazars, as has been implied indirectly in several earlier studies. We find statistically significant differences in the viewing angle distributions between gamma-ray bright and weak sources. Most interestingly, gamma-ray bright blazars have a distribution of comoving frame viewing angles that is significantly narrower than that of gamma-ray weak blazars and centred roughly perpendicular to the jet axis. The lack of gamma-ray bright blazars at large comoving frame viewing angles can be explained by relativistic beaming of gamma-rays, while the apparent lack of gamma-ray bright blazars at small comoving frame viewing angles, if confirmed with larger samples, may suggest an intrinsic anisotropy or Lorentz factor dependence of the gamma-ray emission.
We discuss acceleration measurements for a large sample of extragalactic radio jets from the MOJAVE program which studies the parsec-scale jet structure and kinematics of a complete, flux-density-limited sample of Active Galactic Nuclei (AGN). Accele
rations are measured from the apparent motion of individual jet features or components which may represent patterns in the jet flow. We find that significant accelerations are common both parallel and perpendicular to the observed component velocities. Parallel accelerations, representing changes in apparent speed, are generally larger than perpendicular acceleration that represent changes in apparent direction. The trend for larger parallel accelerations indicates that a significant fraction of these changes in apparent speed are due to changes in intrinsic speed of the component rather than changes in direction to the line of sight. We find an overall tendency for components with increasing apparent speed to be closer to the base of their jets than components with decreasing apparent speed. This suggests a link between the observed pattern motions and the underlying flow which, in some cases, may increase in speed close to the base and decrease in speed further out; however, common hydro-dynamical processes for propagating shocks may also play a role. About half of the components show non-radial motion, or a misalignment between the components structural position angle and its velocity direction, and these misalignments generally better align the component motion with the downstream emission. Perpendicular accelerations are closely linked with non-radial motion. When observed together, perpendicular accelerations are usually in the correct direction to have caused the observed misalignment.
We discuss the jet kinematics of a complete flux-density-limited sample of 135 radio-loud active galactic nuclei (AGN) resulting from a 13 year program to investigate the structure and evolution of parsec-scale jet phenomena. Our analysis is based on
new 2 cm Very Long Baseline Array (VLBA) images obtained between 2002 and 2007, but includes our previously published observations made at the same wavelength, and is supplemented by VLBA archive data. In all, we have used 2424 images spanning the years 1994-2007 to study and determine the motions of 526 separate jet features in 127 jets. The data quality and temporal coverage (a median of 15 epochs per source) of this complete AGN jet sample represents a significant advance over previous kinematics surveys. In all but five AGNs, the jets appear one-sided, most likely the result of differential Doppler boosting. In general the observed motions are directed along the jet ridge line, outward from the optically thick core feature. We directly observe changes in speed and/or direction in one third of the well-sampled jet components in our survey. While there is some spread in the apparent speeds of separate features within an individual jet, the dispersion is about three times smaller than the overall dispersion of speeds among all jets. This supports the idea that there is a characteristic flow that describes each jet, which we have characterized by the fastest observed component speed. The observed maximum speed distribution is peaked at ~10c, with a tail that extends out to ~50c. This requires a distribution of intrinsic Lorentz factors in the parent population that range up to ~50. We also note the presence of some rare low-pattern speeds or even stationary features in otherwise rapidly flowing jets... (abridged)
In its first three months of operations, the Fermi Gamma-Ray Observatory has detected approximately one quarter of the radio-flux-limited MOJAVE sample of bright flat-spectrum active galactic nuclei (AGNs) at energies above 100 MeV. We have investiga
ted the apparent parsec-scale jet speeds of 26 MOJAVE AGNs measured by the Very Long Baseline Array (VLBA) that are in the LAT bright AGN sample (LBAS). We find that the gamma-ray bright quasars have faster jets on average than the non-LBAS quasars, with a median of 15 c, and values ranging up to 34 c. The LBAS AGNs in which the LAT has detected significant gamma-ray flux variability generally have faster jets than the nonvariable ones. These findings are in overall agreement with earlier results based on nonuniform EGRET data which suggested that gamma-ray bright AGNs have preferentially higher Doppler boosting factors than other blazar jets. However, the relatively low LAT detection rates for the full MOJAVE sample (24%) and previously known MOJAVE EGRET-detected blazars (43%) imply that Doppler boosting is not the sole factor that determines whether a particular AGN is bright at gamma-ray energies. The slower apparent jet speeds of LBAS BL Lac objects and their higher overall LAT detection rate as compared to quasars suggest that the former are being detected by Fermi because of their higher intrinsic (unbeamed) gamma-ray to radio luminosity ratios.
We report the results of parsec-scale, multi-frequency VLBA observations of the core region of 3C 279 in Stokes I, linear polarization, and circular polarization. These full polarization spectra are modeled by radiative transfer simulations to constr
ain the magnetic field and particle properties of the parsec-scale jet in 3C 279. The polarization properties of the core region, including the amount of linear polarization, the amount and sign of Faraday rotation, and the amount and sign of circular polarization can be explained by a consistent physical picture. The base of the jet is modeled as an inhomogeneous Blandford-Konigl style conical jet dominated by a vector-ordered poloidal magnetic field along the jet axis, and we estimate its net magnetic flux. This poloidal field is responsible for the linear and circular polarization from this inhomogeneous component. Farther down the jet the magnetic field in two homogeneous features is dominated by local shocks and a smaller fraction of vector-ordered poloidal field remains along the jet axis. In this picture, we find the jet to be kinetically dominated by protons with the radiating particles being dominated by electrons at an approximate fraction of >~ 75%. Based on the amounts of Faraday conversion deduced for the homogeneous components, we find a plausible range for the lower cutoff in the relativistic particle energy spectrum to be 5 <~ gamma_l <~ 35. The physical picture described here is not unique if the observed Faraday rotation and depolarization occur in screens external to the jet; however, we find the joint explanation of linear and circular polarization observations from a single set of magnetic fields and particle properties internal to the jet to be compelling evidence for this picture. (Abridged)
