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

The Flat Spectrum Radio Quasar 3C 279 underwent several outbursts in the past, having flared with a peak gamma-ray flux above 100 MeV (F-E>100MeV) 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 gamma-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 F-E>100mey 4 x 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 (similar to 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 gamma-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 gamma-ray observations.