Detection of minute-timescale γ-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 γ-ray flare in April 2021. The Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope (hereafter Fermi-LAT) observed a peak γ-ray (0.1−500 GeV) flux of ∼2 × 10−5 ph cm−2 s−1 within a single orbit on 2021 April 27, which is historically the brightest γ-ray flux ever detected from the source. Here, we report, for the first time, the detection of significant minute-timescale GeV γ-ray flux variability in the BL Lac subclass of blazars by the Fermi-LAT. We resolved the source variability down to two-minute binned timescales with a flux halving time of ∼1 min, which is the shortest GeV variability timescale ever observed from blazars. The detected variability timescale is much shorter than the light-crossing time (∼14 min) across the central black hole of BL Lac, indicating a very compact γ-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 δmin of 15 using the δ function approximation for the γγ opacity constraint. For a conical jet geometry, considering Γ = δmin, the observed short variability timescale for BL Lac suggests that the very compact emission region lies at a distance of about 8.62 × 1014 cm from its central engine.