Journal
ASTRONOMY & ASTROPHYSICS
Volume 512, Issue -, Pages -Publisher
EDP SCIENCES S A
DOI: 10.1051/0004-6361/200913740
Keywords
galaxies: active; galaxies: jets; quasars: general; BL Lacertae objects: general
Categories
Funding
- National Science Foundation [AST-0807860, AST-0707693]
- NASA [NNX08AV67G]
- Academy of Finland [120516]
- Alexander von Humboldt
- Russian Foundation for Basic Research [08-02-00545]
- Academy of Finland (AKA) [120516, 120516] Funding Source: Academy of Finland (AKA)
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Aims. 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. Methods. 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. Results. 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.
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