Modeling the Multiwavelength Spectra and Variability of BL Lacertae in 2000

BL Lacertae was the target of an extensive multiwavelength monitoring campaign in the second half of 2000. The campaign had revealed optical and X-ray intraday variability on time scales of ∼ 1.5 hours and evidence for significant spectral variability both at optical and X-ray frequencies. During the campaign, BL Lacertae was observed in two different activity states: A quiescent state with relatively low levels of optical and X-ray fluxes and a synchrotron cutoff at energies below the X-ray regime, and a flaring state with high levels of optical and X-ray emission and a synchrotron cutoff around or even beyond ∼ 10 keV. In this paper, we are using both leptonic and hadronic jet models to fit the broadband spectra and spectral variability patterns observed in both activity states in 2000. We start out with global spectral models of both activity states. Subsequently, we investigate various flaring scenarios for comparison with the observed short-term variability of BL Lacertae in 2000. For our leptonic jet model, we find that the short-term variability, in particular the optical and X-ray spectral variability, can be best represented with a flaring scenario dominated by a spectral-index change of the spectrum of ultrarelativistic electrons injected into the jet. Based on this result, a detailed model simulation of such a flaring scenario, reproducing the observed optical and X-ray spectral variability and broadband SED of BL Lacertae during the BeppoSAX pointing around Nov. 1, 2000, simultaneously, is presented. Our leptonic modeling results are compared to fits using the hadronic synchrotron-proton blazar (SPB) model. That model can reproduce the observed SEDs of BL Lacertae in a scenario with µ-synchrotron dominated high-energy emission. It requires a significantly higher magnetic field than the leptonic model – 2 – (∼ 40 G vs. ∼ 2 G in the leptonic model) and a lower Doppler factor associated with the bulk motion of the emission region (D ∼ 8 vs. D ∼ 18 in the leptonic model). The hadronic model predicts a significantly larger 100 GeV flux than the leptonic models, well within the anticipated capabilities of VERITAS and MAGIC.