Fermi-LAT observations of the exceptional gamma-ray outbursts of 3C 273 in September 2009

TL;DR

This paper reports on the unprecedented gamma-ray outbursts of 3C 273 observed by Fermi-LAT in September 2009, revealing detailed spectral and temporal characteristics of these luminous flares.

Contribution

It provides the first detailed analysis of the spectral and temporal structures of the most luminous gamma-ray flares from 3C 273, including their asymmetry and spectral hardness variations.

Findings

Flares had asymmetric rise and decay times of about 6 hours.

Spectral index was significantly harder during flares.

Short, intense flares released comparable energy to longer, less intense ones.

Abstract

We present the light curves and spectral data of two exceptionally luminous gamma-ray outburts observed by the Large Area Telescope (LAT) experiment on board Fermi Gamma-ray Space Telescope from 3C 273 in September 2009. During these flares, having a duration of a few days, the source reached its highest gamma-ray flux ever measured. This allowed us to study in some details their spectral and temporal structures. The rise and decay are asymmetric on timescales of 6 hours, and the spectral index was significantly harder during the flares than during the preceding 11 months. We also found that short, very intense flares put out the same time-integrated energy as long, less intense flares like that observed in August 2009.