Relativistic beaming and gamma-ray brightness of blazars

TL;DR

This study examines how relativistic beaming influences the gamma-ray brightness of blazars by analyzing jet properties and viewing angles, revealing that gamma-ray bright blazars tend to have higher Doppler factors and specific viewing angle distributions.

Contribution

It provides the first detailed analysis linking gamma-ray brightness of blazars to their jet Lorentz factors and viewing angles using VLBA and Fermi data.

Findings

Gamma-ray bright blazars have higher Doppler factors.

Gamma-ray bright blazars have narrower viewing angle distributions.

Lack of gamma-ray bright blazars at large comoving angles explained by beaming.

Abstract

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.