Fermi-LAT light curves of BL Lac covering a period of ∼35 days. Panel a: one-day binned γ-ray (0.1−500 GeV) light curve. Panel b: variation of γ-ray photon index with time; the blue and red horizontal lines indicate the average PL index during the period considered and the PL index mentioned in the 4FGL catalogue, respectively. Panel c: three-hour binned γ-ray light curve of BL Lac. Panel d: arrival time distribution of photons with energies greater than 10 GeV. The vertical red lines indicate the period of the major γ-ray flare.