Detection of minute-timescale gamma-ray variability in BL Lacertae by Fermi-LAT

Abstract: BL Lacertae, the prototype of the BL Lacertae (BL Lac) category of blazars, underwent a giant $\gamma-$ray flare in April 2021. The Large Area Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope (hereafter Fermi-LAT) observed a peak $\gamma-$ray (0.1$-$500 GeV) flux of $\sim$2 $\times$ 10$^{-5}$ photons cm$^{-2}$ s$^{-1}$ within a single orbit on 2021 April 27, which is historically the brightest $\gamma-$ray flux ever detected from the source. Here, we report, for the first time, the detection of significant minute-timescale GeV $\gamma-$ray flux variability in the BL Lac subclass of blazars by the Fermi-LAT. We resolved the source variability down to 2-min binned timescales with a flux halving time of $\sim$1 minute, which is the shortest GeV variability timescale ever observed from blazars. The detected variability timescale is much shorter than the light-crossing time ($\sim 14$ minutes) across the central black hole of BL Lac indicating a very compact $\gamma-$ray emission site within the outflowing jet. Such a compact emitting region requires the bulk Lorentz factor of the jet to be larger than 16 so that the jet power is not super Eddington. We found a minimum Doppler factor $\delta_{min}$ of 15 using the $\delta$ function approximation for the $\gamma\gamma$ opacity constraint. For a conical jet geometry, considering $\Gamma = \delta_{min}$, the observed short variability timescale suggests the very compact emission region to lie at a distance of about 8.62 $\times$ 10$^{14}$ cm from the central engine of BL Lac.