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.

arXiv comment6 pages, 5 figures, 2 tables, accepted for publication in the Astronomy & Astrophysics (A&A) main journal

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