MOJAVE XIX: Brightness Temperatures and Intrinsic Properties of Blazar Jets

TL;DR

This study analyzes multi-epoch observations of 447 AGN jets to determine their brightness temperatures, intrinsic properties, and relativistic effects, revealing correlations with gamma-ray detection and differences among BL Lac subclasses.

Contribution

It provides the first comprehensive measurement of median intrinsic brightness temperatures and Doppler factors for a large AGN jet sample, linking jet physics to gamma-ray emission.

Findings

Median intrinsic brightness temperature is (4.1 +- 0.6) x 10^10 K.

Jets with higher Doppler factors are more likely to be gamma-ray detected.

HSP BL Lacs have lower Doppler and Lorentz factors, larger viewing angles.

Abstract

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 and 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.