Systematic Search and Study of Short-Timescale Flare Structures in BL Lac object Gamma-ray Emission

Abstract

We present here the first systematic search of short timescale $\gamma$-ray flares from 29 high Galactic latitude BL Lac objects over 14 years of Fermi Large Area Telescope data. Using a combined Bayesian Blocks and HOP algorithm, we identified seven high-quality orbital timescale flare segments from three sources and quantified 24 short-timescale flare structures. We then performed a comprehensive analysis of flare symmetry, power spectral density (PSD) of variability, and flux-photon index relation. The main results are as follows: (1) The flare symmetry parameter $A$ shows a "U-shaped" distribution. Short timescale flares are symmetric while long timescale flares are asymmetric. The number of fast-rise slow-decay and slow-rise fast-decay type flares are equal. No correlation is found between $A$ and peak/integral flux. No parameter evolution is seen between consecutive flares either. The observations support a scenario where longer timescale flares originate from superposition of short, symmetric sub-hour flares. (2) PSD from yearly to hourly timescales is modeled using the CARMA process. At lower frequencies, the PSD follows the typical broken power-law form. The high-frequency region of the PSD exhibits a continuous power-law shape, indicating that $\gamma$-ray variability originates from a single physical process across all probed timescales. (3) The flux-photon index distribution shows a pattern of "harder-when-brighter" or "softer-when-brighter," but becomes flat above a certain critical flux, with $\Gamma$ $\approx$ 2. This behavior cannot be simply explained by a two-component or blazar sequence model, and we speculate it may be related to complex interplay between electron acceleration and cooling.