BL Lacertae under the Flare of 2024: Probing Temporal and Spectral Dynamics

Abstract

In October 2024, the object BL Lacertae experienced the brightest flaring event in gamma-ray ($>$100 MeV) with a historically bright $γ$-ray flux of $\sim$2.59 $\times 10^{-5}$ erg cm$^{-2}$ s$^{-1}$ with a detection of a 175.7 GeV photon with Fermi-LAT. This event was also followed by very high-energy $γ$-ray detection with LHAASO, VERITAS, and MAGIC. Soon after, Swift-XRT and Swift-UVOT follow-up confirmed the concurrent flare in X-ray, UV, and optical bands. A minimum flux doubling/halving time of 1.06 $\pm$ 0.26 hour with 4$σ$ significance has been observed with the Fermi-LAT orbit binned light curve. No compelling correlation has been found between $γ$-ray spectral indices and fluxes. The log-normal $γ$-ray flux distribution during the flare confirms the multiplicative nature of the non-linear perturbation causing the flare. We applied a one-zone leptohadronic model to fit the broadband SED during the flaring period. The broadband SED modeling reveals that the sudden enhancement of the magnetic field and bulk factor might promote the flare. The SED modeling also suggested a more compact emission region, which may be described by a shorter variability time than the observed one. The hadronic part best fitted the high energy part of the spectrum, suggesting the jets of BL Lac could provide a promising environment to accelerate the cosmic ray particles, such as protons. The jets of BL Lacertae could also be the possible source of astrophysical neutrinos, as an upper limit on neutrinos has already been reported from IceCube.