Kinematics of Parsec-Scale Jets of Gamma-Ray Blazars at 43~GHz within the VLBA-BU-BLAZAR Program

Abstract: We analyze the parsec-scale jet kinematics from 2007 June to 2013 January of a sample of $\gamma$-ray bright blazars monitored roughly monthly with the Very Long Baseline Array at 43~GHz. In a total of 1929 images, we measure apparent speeds of 252 emission knots in 21 quasars, 12 BL~Lacertae objects (BLLacs), and 3 radio galaxies, ranging from 0.02$c$ to 78$c$; 21\% of the knots are quasi-stationary. Approximately 1/3 of the moving knots execute non-ballistic motions, with the quasars exhibiting acceleration along the jet within 5~pc (projected) of the core, and knots in the BLLacs tending to decelerate near the core. Using apparent speeds of components and timescales of variability from their light curves, we derive physical parameters of 120 superluminal knots, including variability Doppler factors, Lorentz factors, and viewing angles. We estimate the half-opening angle of each jet based on the projected opening angle and scatter of intrinsic viewing angles of knots. We determine characteristic values of physical parameters for each jet and AGN class based on the range of values obtained for individual features. We calculate intrinsic brightness temperatures of the cores, $T_{\rm b,int}^{\rm core}$, at all epochs, finding that the radio galaxies usually maintain equipartition conditions in the cores, while $\sim$30\% of $T_{\rm b,int}^{\rm core}$ measurements in the quasars and BLLacs deviate from equipartition values by a factor $>$10. This probably occurs during transient events connected with active states. In the Appendix we briefly describe the behavior of each blazar during the period analyzed.

• The paper identifies 252 jet components, including 54 quasi-stationary and numerous superluminal features, highlighting acceleration and curvature in jet paths.
• The study reports brightness temperatures often exceeding 10^13 K, indicating strong Doppler boosting and deviations from equipartition during high activity states.
• The research employs 43 GHz VLBA imaging to derive Doppler and Lorentz factors, differentiating jet properties among quasars, BL Lac objects, and radio galaxies.

Overview of Kinematic Analysis of Parsec-Scale Jets in Gamma-Ray Blazars

The paper "Kinematics of Parsec-Scale Jets of Gamma-Ray Blazars at 43~GHz within the VLBA-BU-BLAZAR Program" presents a comprehensive analysis of the parsec-scale jet kinematics of gamma-ray bright blazars, utilizing observations conducted between 2007 and 2013 under the Very Long Baseline Array (VLBA) project. The study investigates the properties of the jets of active galactic nuclei (AGN) using 43 GHz VLBA imaging to capture ultra-high angular resolution structures in a sample of 21 quasars, 12 BL Lac objects, and 3 radio galaxies.

The research identifies apparent superluminal motions in 252 jet components within the observational data set from 1929 VLBA images and provides insights into the variability, speed, flux density, and propagation angles of these components. Apparent velocities of jet components vary widely, from subluminal speeds up to 78c, with significant variability and motion patterns detected across the sample.

Key Findings

1. Jet Dynamics and Component Classification:
   • The paper reports that 54 of the identified components are quasi-stationary, while others show evidence of superluminal motion. Key dynamics include acceleration (or deceleration) and curvature of the jet's path, indicating complex underlying processes.
   • Quasars exhibit components with faster apparent speeds in comparison to those in BL Lac objects.
2. Brightness Temperature Profiles:
   • Observed brightness temperatures span a range, often exceeding 10^13 K in certain active states, suggesting highly Doppler-boosted setups and possibly indicating deviation from equipartition conditions during flares or high activity states.
3. Doppler and Lorentz Factors:
   • Variability in Doppler boosting is notable in the data, with numerous calculated Doppler factors, Lorentz factors, and viewing angles for superluminal features confirming the highly relativistic nature of these jets.
   • Differences in these factors across the classes (quasars, BLLacs, and RGs) illustrate distinctions in jet orientations and intrinsic speeds.
4. Jet Structure and Evolution:
   • The observations capture both ballistic and non-ballistic trajectories with some knots exhibiting complex, non-linear evolutions.
   • Intrinsic jet parameters, such as opening angles and viewing angles, are determined using a variety of methods emphasizing the subtleties of jet structure interpretation.
5. Radio-Monitoring Techniques:
   • It highlights improvements in determining flux decay timescales for variability Doppler factor estimation, which are critical for understanding the phase space of these phenomena.

Theoretical and Practical Implications

The findings contribute significantly to both theoretical modeling and practical understanding of relativistic jet dynamics in AGNs. The study underscores the necessity for ongoing monitoring to capture real-time changes in jet morphology and dynamics, as these can provide valuable insights into the emission mechanisms and environments surrounding supermassive black holes. The analysis underpins the variability seen in observational signatures across different spectral bands and positions an emphasis on resolving the "Doppler crisis" in certain TeV-emitting BL Lacs.

Ongoing observations, especially those utilizing multiple frequency bands and VLBI capabilities, are crucial in dissecting the core versus jet emission processes, disentangling their contributions to gamma-ray emissions, and offering insights into particle acceleration mechanisms.

Future Directions

Future studies can leverage more extensive datasets from newer instruments to further probe the intrinsic characteristics of blazars and their environments, extending jet studies into more nuanced areas such as jet-medium interaction, magnetic field structures, and the detailed impact of relativistic effects on observed measures.

The research presented here lays foundational groundwork and methodological approaches that guide current and forthcoming investigations into the kinematic and dynamic characteristics of relativistic jets in blazars, enhancing our understanding of these enigmatic astrophysical objects.