CAJ | 학술논문

The multi-frequency light curves of BL Lacertae during 1997.5 - 1999.5 have been modeled by four outbursts, each having a 3-stage evolution in the (S_m, v_m) plane with distinct rising-plateau-decaying phases. It is shown that the observed light curves can be well fitted for the eight frequencies from 350GHz to 4.8 GHz. The main characteristics of the model-fitting are: (1) the outbursts are found to have very flat spectra with an optically thin spectral index α (defined as S_v α ∝ v~(-α)) of about 0.15. This is consistent with the results previously obtained by Valtaoja et al. (1992); (2)it is found that there is no spectral flattening between the rising-plateau phase and the decay phase. In other words, the optically thin spectral index does not change from the rising-plateau phase to the decay phase. These features are in contrast to the 3-stage shocked-in-jet model proposed by Marscher & Gear (1985) for submm-IR-optical flares, in which a spectral flattening of △α = 0.5 is predicted when a transition occurs from the Compton/synchrotron phase (or rising-plateau phase) to the adiabatic phase (or decay phase) with α_~~>0.5 for the shock being non-radiative. We propose a new model to interpret the fitting results, suggesting that the 3-stage evolution of the mm--cm outbursts in BL Lacertae may be related to the process of shock formation and propagation in a highly collimated jet (for example, a 'parabolic' jet). In particular, during the rising phase, the thickness of the synchrotron-radiating region created by the shock may rapidly increase with time (relative to the jet width) due to the rapid injection of relativistic electrons and a magnetic field, and this leads to the observed behavior that the turnover flux density S_m rapidly increases while the turnover frequency v_m decreases. In the decay phase, the emitting plasma enters into a free expansion regime without further injection of relativistic electrons and a magnetic field (for example, when a transition from a collimated regime into a conical regime occurs). The plateau phase is a short period between the two regimes with no distinct features determined.