First minute-scale variability in Fermi-LAT blazar observations during the giant outburst of 3C279 in 2015 June

Abstract

The Flat Spectrum Radio Quasar 3C 279 underwent several outbursts in the past, having flared with a peak γ-ray flux above 100 MeV (FE>100 MeV) exceeding 10−5 ph cm−2 s−1, in 2013 December, 2014 April, and 2015 June. The 2013 December outburst showed an unusually hard power-law γ-ray spectrum (index∼1.7), and an asymmetric light curve profile with few-hour time scale variability. This could be successfully explained using our second order Fermi acceleration model. The outburst in 2015 June was even more powerful, with FE>100 MeV ∼ 4×10−5 ph cm−2 s−1, the historically highest even when the EGRET era is included. For the first time, significant flux variability at sub-orbital timescales was found in blazar observations by Fermi-LAT, with flux doubling times of less than 5 minutes. In the standard external-Compton scenario with conical jet geometry, the minute-scale variability requires a very high bulk jet Lorentz factor (> 50) and extremely low magnetization even at the jet base (∼ 100RS). However, such a high bulk jet Lorentz factor and low magnetization at the jet base pose challenges to standard models of electromagnetically driven jets. Alternately, we consider a synchrotron origin scenario for the GeV γ-ray outburst, which would work in a magnetically dominated jet. In this contribution, we present observational results of the outbursts in detail and discuss current problems on relativistic jets inferred from γ-ray observations.