Relativistic beaming and gamma-ray brightness of blazars
Max-Planck-Institut für Radioastronomie, Auf dem
Hügel 69, 53121 Bonn, Germany e-mail: firstname.lastname@example.org
2 Department of Physics and Astronomy, Denison University, Granville, OH 43023, USA
3 Department of Physics, Purdue University, 525 Northwestern Avenue, West Lafayette, IN 47907, USA
4 Metsähovi Radio Observatory, Helsinki University of Technology TKK, Metsähovintie 114, 02540 Kylmälä, Finland
5 Dr. Karl Remeis-Observatory & ECAP, Friedrich-Alexander University Erlangen-Nuremberg, Sternwartstr. 7, 96049 Bamberg, Germany
6 CRESST/NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
7 Universities Space Research Association, 10211 Wincopin Circle, Suite 500 Columbia, MD 21044, USA
8 Astro Space Center of Lebedev Physical Institute, Profsoyuznaya 84/32, 117997 Moscow, Russia
9 Departament d'Astronomia i Astrofísica, Universitat de València, 46100 Burjassot, Valencia, Spain
Accepted: 12 January 2010
Aims. We investigate the dependence of γ-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 Metsähovi 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 γ-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 γ-ray bright and weak sources. Most interestingly, γ-ray bright blazars have a distribution of comoving frame viewing angles that is significantly narrower than that of γ-ray weak blazars and centred roughly perpendicular to the jet axis. The lack of γ-ray bright blazars at large comoving frame viewing angles can be explained by relativistic beaming of γ-rays, while the apparent lack of γ-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 γ-ray emission.
Key words: galaxies: active / galaxies: jets / quasars: general / BL Lacertae objects: general
© ESO, 2010