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

TL;DR

BL Lacertae’s prolonged activity is analyzed with 12 nights of optical IDV data and extensive multiwavelength archives to map variability and SED evolution. The work identifies intraday variability with a pronounced BWB trend and rare spectral hysteresis loops, while long-term correlations show optical–γ-ray–X-ray variations are virtually simultaneous and radio variations lag by ~$370$ days, signaling two spatially separated emission zones. SED modeling with a two-zone leptonic (SSC+EC) framework reveals a persistent VHE emission region beyond the BLR, and the source shifts from an intermediate BL Lac (IBL) state during quiescent/high-energy flares to a low BL Lac (LBL) state during radio flares, indicating dynamic jet structure and particle acceleration regimes. Collectively, the results constrain jet geometry, emission site separations of about $Δd≈$\$4.50\times10^{19}$ cm, and the role of VHE processes in BL Lacertae, offering insight into multi-zone leptonic models for blazar variability.

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 $γ$-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 $γ$-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 $γ$-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.

Figures (10)

• Figure 1: Finding chart of BL Lac in the $V$-band. The labels “T” and “1–5” represent BL Lac, two comparison stars, the check star, and two field stars, respectively.
• Figure 2: The intraday light curves of BL Lac and the check star are displayed in the top and bottom panels of each subfigure, respectively. The $B$- and $V$-band light curves are shifted for clarity. On MJDs 59568 and 59888, the monitoring interruptions occurred temporarily due to bad weather. From MJD 59565 to 59568 and on MJD 59888, unfavorable weather conditions led to a relative large standard deviation for the magnitude of the check star, as shown in Table \ref{['tab1']}.
• Figure 3: The overall $B$-, $V$-, and $R$-band light curves of BL Lac.
• Figure 4: CMDs of BL Lac for the nights exhibiting IDV. The color gradient from blue to red indicates the progression of observation time. The solid lines indicate the linear fitting results. The observation date (MJD), the linear regression equation, the correlation coefficient $r$, and the $p$-value are shown in the upper-left corner. The vertical dashed lines in the four subplots mark the data segments associated with spectral hysteresis loops.
• Figure 5: Examples of lags measured by three approaches on MJD 59888. The left, center, and right columns represent lags in the $B-V$, $B-R$, and $V-R$ bands, respectively. Lags and uncertainties are given in all panels and are labeled by the solid and dotted lines, respectively. Top two rows: the CCCD/PD obtained from ICCF/PyROA. The dashed lines are the KDE of the weighted CCCD/PD and the histograms are the unweighted ones. The shaded regions are the primary peaks. The x-axes have been zoomed in for clarity. Bottom row: the discrete correlation coefficient (points) and the likelihood (curves) obtained from ZDCF and PLIKE, respectively.