Multiple Components and Spectral Evolution of BL Lacertae as Revealed by Multiwavelength Variability and SED Modeling

Abstract

BL Lac has entered an active state since 2020, with multiwavelength observations revealing intense flares. In this study, we conducted 12-night multicolor optical monitoring using an 85 cm telescope from 2020 September to 2024 June and collected long-term broad-band archived data from radio to $\gamma$-rays. Intraday variabilities were detected on four nights, and most of them exhibited a bluer-when-brighter behavior. Both clockwise and counterclockwise spectral hysteresis loops were found within a single night. However, no reliable intraband time lag was detected for the intranight variabilities. On long timescales, the cross-correlation analysis shows that the variations of the optical, X-ray, and $\gamma$-ray bands do not reveal an obvious time delay, while the variations in the radio bands lagged them by about 370 days. The measured time lags suggest two distinct emission regions respectively responsible for the optical to $\gamma$-ray radiation and for the radio radiation, with a spatial separation of approximately $4.50\times10^{19}\ \rm cm$. We modeled the broad-band spectral energy distributions during four flaring epochs and one quiescent epoch, and found evidence for the possible persistent existence of a very high energy emission region. We also confirmed a spectral evolution of the source from an intermediate synchrotron peaked BL Lac object to a low synchrotron peaked BL Lac object.