Rapid Variability and Broadband Spectral Modeling in the Flaring Activity of BL Lacertae

Le résumé fourni par la source

Abstract We report a multiwavelength study of two flaring episodes of the blazar BL Lacertae during MJD 60500–60800 (2024 July 9–2025 May 5). The source reached a daily averaged γ -ray flux of (1.03 ± 0.05) ×10 −5 photons cm −2 s −1 ( E > 100 MeV) on MJD 60588 (2024 October 5). Using orbit-binned data from the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope, we identify a minimum flux halving timescale of τ = 1.33 ± 0.29 hr. This constrains the upper limit on the size of the γ -ray-emitting region to R ≤ 2.0 × 10 15 cm, as well as its distance from the central supermassive black hole to R H ≤ 5.9 × 10 16 cm, assuming a Doppler factor of δ = 14.8 derived from the spectral energy distribution (SED) modeling. We find tentative evidence for subminute γ -ray variability with a minimum doubling time of 0.7 ± 0.2 min ( p -value = 0.03). This may originate from an extremely compact region with a size of R ≤ 1.8 × 10 13 cm, suggesting that the emission arises from magnetohydrodynamic substructures, such as plasmoids within a magnetic reconnection zone. Spectral analysis reveals a significant “softer-when-brighter” trend ( r = 0.96, p = 4.5 × 10 −4 ) during the minute-scale flare peaks, indicating a complex interplay between particle acceleration and radiative cooling. The SED is reproduced using a one-zone leptonic model, in which synchrotron self-Compton (SSC) and external Compton (EC) scattering effectively account for the high-energy emissions. The reduced magnetic field strengths and hard electron injection spectral indices observed during the flaring states suggest enhanced particle acceleration efficiency, possibly associated with relativistic magnetic reconnection.

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