Relativistic beaming and gamma-ray brightness of blazars

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: tsavolainen@mpifr-bonn.mpg.de
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

Received: 25 November 2009
Accepted: 12 January 2010

Abstract

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.