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On 2015 June 16, Fermi-LAT observed a giant outburst from the flat spectrumradio quasar 3C 279 with a peak $>100$ MeV flux of $\sim3.6\times10^{-5}\;{\rmphotons}\;{\rm cm}^{-2}\;{\rm s}^{-1}$ averaged over orbital period intervals.It is the historically highest $\gamma$-ray flux observed from the sourceincluding past EGRET observations, with the $\gamma$-ray isotropic luminosityreaching $\sim10^{49}\;{\rm erg}\;{\rm s}^{-1}$. During the outburst, the Fermispacecraft, which has an orbital period of 95.4 min, was operated in a specialpointing mode to optimize the exposure for 3C 279. For the first time,significant flux variability at sub-orbital timescales was found in blazarobservations by Fermi-LAT. The source flux variability was resolved down to2-min binned timescales, with flux doubling times less than 5 min. The observedminute-scale variability suggests a very compact emission region at hundreds ofSchwarzschild radii from the central engine in conical jet models. A minimumbulk jet Lorentz factor ($\Gamma$) of 35 is necessary to avoid both internal$\gamma$-ray absorption and super-Eddington jet power. In the standardexternal-radiation-Comptonization scenario, $\Gamma$ should be at least 50 toavoid overproducing the synchrotron-self-Compton component. However, thispredicts extremely low magnetization ($\sim5\times10^{-4}$). Equipartitionrequires $\Gamma$ as high as 120, unless the emitting region is a smallfraction of the dissipation region. Alternatively, we consider $\gamma$ raysoriginating as synchrotron radiation of $\gamma_{\rm e}\sim1.6\times10^6$electrons, in magnetic field $B\sim1.3$ kG, accelerated by strong electricfields $E\sim B$ in the process of magnetoluminescence. At such short distancescales, one cannot immediately exclude production of $\gamma$ rays in hadronicprocesses.

Minute-Timescale >100 MeV gamma-ray variability during the giant outburst of quasar 3C 279 observed by Fermi-LAT in 2015 June