2024ApJ...974..233K

The origin of fast flux variability in blazars is a long-standing problem, with many theoretical models proposed to explain it. In this study, we focus on BL Lacertae to model its spectral energy distribution (SED) and broadband light curves using a diffusive shock acceleration process involving multiple mildly relativistic shocks, coupled with a time-dependent radiation transfer code. BL Lacertae was the target of a comprehensive multiwavelength monitoring campaign in early 2021 July. We present a detailed investigation of the source's broadband spectral and light-curve features using simultaneous observations at optical-UV frequencies with the Swift Ultraviolet/Optical Telescope, in X-rays with the Swift X-Ray Telescope and AstroSat-SXT/LAXPC, and in gamma rays with Fermi-LAT, covering the period from 2021 July to August (MJD 59400-59450). A fractional variability analysis shows that the source is most variable in gamma rays, followed by X-rays, UV, and optical. This allowed us to determine the fastest variability time in gamma rays to be on the order of a few hours. The AstroSat-SXT and LAXPC light curves indicate X-ray variability on the order of a few kiloseconds. Modeling simultaneously the SEDs of low- and high-flux states of the source and the multiband light curves provided insights into the particle acceleration mechanisms at play. This is the first instance of a physical model that accurately captures the multiband temporal variability of BL Lacertae, including the hour-scale fluctuations observed during the flare.