Study of the variability of Blazars gamma-ray emission

TL;DR

This study analyzes gamma-ray variability in bright blazars using Fermi data, constraining the size of emission regions and jet dynamics, and suggesting complex emission mechanisms beyond single blob models.

Contribution

It provides new constraints on the size and dynamics of blazar emission regions based on variability timescales and gamma-ray transparency.

Findings

Emission regions are smaller than ~10^-3 parsec.

Outbursts are likely due to standing shocks or multiple flares.

Lower limits on Lorentz factors ensure gamma-ray transparency.

Abstract

The gamma-ray emission of blazar jets shows a pronounced variability and this feature provides limits to the size and to the speed of the emitting region. We study the gamma-ray variability of bright blazars using data from the first 18 months of activity of the Large Area Telescope on the Fermi Gamma-Ray Space Telescope. From the daily light-curves of the blazars characterized by a remarkable activity, we firstly determine the minimum variability time-scale, giving an upper limit for the size of the emitting region of the sources, assumed to be spheroidal blobs in relativistic motion. These regions must be smaller than ~10^-3 parsec. Another interesting time-scale is the duration of the outbursts. We conclude that they cannot correspond to radiation produced by a single blob moving relativistically along the jet, but they are either the signature of emission from a standing shock extracting energy from a modulated jet, or the superposition of a number of flares occurring on a shorter time-scale. We also derive lower limits on the bulk Lorentz factor needed to make the emitting region transparent for gamma-rays interacting through photon-photon collisions.