L. Stawarz, T. Takahashi (ISAS/JAXA), Y. Tanaka (Hiroshima Univ.), on behalf of the Fermi-LAT Collaboration
We present a new approach to study blazar variability, making full use of the available gamma-ray data. Since most of the radiative power of FSRQs is released in GeV gamma-rays, rapid and high-amplitude variability in this energy range is of particular importance for investigating energy dissipation processes and energetics of relativistic jets in this type of sources. We selected ten bright flares with the best photon statistics from all the LAT-detected FSRQs amounting to ∼400 objects for this work. In our systematic analysis of short-timescale variability, we discovered very rapid flux changes with doubling timescale as short as 1 hour, which was the shortest variability timescale in the GeV range found for all the Active Galactic Nuclei (AGN) so far (e.g. PKS1510-089; Saito et al. 2013). We modeled time dependent spectra of the brightest blazar flares with a numerical simulation assuming an internal shock scenario (Moderski et al. 2003, 2005), and obtained an indication that gamma-ray emission takes place at ~10^18 cm from the central SMBH. We found that IR photons in a dusty torus as well as broad line region photons constitute a significant fraction of the observed MeV/GeV spectra as a result of inverse Compton radiation. Our detailed modelling of flaring light curves also implies that the non-uniformity of the Dopper factor across the jet causes significant time distortion in the observed gamma-ray light curves. Since jet components with different observation angles would have different Doppler factors, the observed time profile from these different components would also be different even if the bulk Lorentz factor is uniform across the jet.