Minute time scale variability in $\gamma$-ray flare of BL Lacertae

TL;DR

This study reports minute-scale gamma-ray variability in BL Lacertae during a major flare, revealing extremely rapid changes that suggest a very compact emission region near the supermassive black hole.

Contribution

First detailed analysis of sub-minute gamma-ray variability in BL Lacertae, constraining the size and location of the emission region during a major flare.

Findings

Detected variability on 1-minute timescale.

Estimated emission region size of 10^13 cm.

Located close to the supermassive black hole, inside the broad-line region.

Abstract

In October 2024, The object BL Lac experienced a brightest flaring event in gamma-ray ($>$100 MeV) with a historical $\gamma$-ray flux of $\sim$10$^{-5}$ erg cm$^{-2}$ s$^{-1}$. Soon after the event was followed across the waveband and in X-ray (0.3-10 keV) it was also found to be flaring with the maximum flux achieved during this event as 8.30$\times$10$^{-11}$ erg cm$^{-2}$ s$^{-1}$. The high gamma-ray significance enables us to probe the shortest time scale variability possible and for that, we produced the orbital binned light curve, 5 minutes binned light curve, and the 2 minutes binned light curve. A clear variation is seen in the 5-minute light curve and is fitted with the sum of exponentials to derive the rise and decay time scale which ranges between 3 to 12 minutes. The fastest variability time is also estimated to be an order of 1 minute from 2 minute. The estimated size of the emission region is very small (10$^{13}$ cm) compared to the size of the black hole event horizon. The location of the emission region is estimated to be very close to the supermassive black hole (10$^{14}$ cm) and much inside the BLR (0.1 pc). We discussed the possible way to explain this fast-flux variability in BL Lac.