Constraining the location of rapid gamma-ray flares in the flat spectrum radio quasar 3C 273

¹ Max-Planck-Institut für Radioastronomie (MPIfR), Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: brani@mpifr-bonn.mpg.de
² Université Bordeaux 1, CNRS/IN2p3, Centre d’Études Nucléaires de Bordeaux Gradignan, 33175 Gradignan, France

Received: 8 March 2013
Accepted: 2 July 2013

Abstract

We present a γ-ray photon flux and spectral variability study of the flat-spectrum radio quasar 3C 273 over a rapid flaring activity period between September 2009 to April 2010. Five major flares were observed in the source during this period. The most rapid flare observed in the source has a flux doubling time of 1.1 hr. The rapid γ-ray flares allow us to constrain the location and size of the γ-ray emission region in the source. The γγ-opacity constrains the Doppler factor δ_γ ≥  10 for the highest energy (15 GeV) photon observed by the Fermi-Large Area Telescope (LAT). Causality arguments constrain the size of the emission region to 1.6 × 10¹⁵ cm. The γ-ray spectra measured over this period show clear deviations from a simple power law with a break in the 1−2 GeV energy range. We discuss possible explanations for the origin of the γ-ray spectral breaks. Our study suggests that the γ-ray emission region in 3C 273 is located within the broad line region (< 1.6 pc). The spectral behavior and temporal characteristics of the individual flares indicate the presence of multiple shock scenarios at the base of the jet.