Constraining the location of rapid gamma-ray flares in the FSRQ 3C 273

Abstract

We present a \gamma-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 are 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 \gamma-ray flares allow us to constrain the location and size of the \gamma-ray emission region in the source. The \gamma \gamma-opacity constrains the Doppler factor, $\delta_{\gamma} \geq$ 10 for the highest energy (15 GeV) photon observed by the {\it Fermi}-Large Area Telescope (LAT). Causality arguments constrain the size of the emission region to 1.6$\times 10^{15}$ cm. The \gamma-ray spectra measured over this period show clear deviations from a simple power law with a break in 1-2 GeV energy range. We discuss possible explanations for the origin of the \gamma-ray spectral breaks. Our study suggests that the \gamma-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.