Theoretical study of multifrequency emission variability in blazars.

摘要

Relativistic jets produced in the nuclei of active galaxies emit highly variable nonthermal emission spanning the entire electromagnetic spectrum. Recent observations of flares and outbursts in blazars have revealed that there is a strong correlation between the emission variability observed in different spectral bands. Furthermore, variability of the X-ray emission is frequently delayed with respect to the flares at lower frequencies, in the optical, infrared, and radio bands. To probe physical properties of the emitting plasma in these blazars, I have developed a model of emission variability that allows a rigorous study of the time lags. The lags can be understood if the emission observed at lower frequencies is dominated by synchrotron radiation, while inverse Compton scattering of the local synchrotron and external photons provides the emission at higher frequencies. This model takes into account the internal structure of the emitting medium determined by (1) energy stratification of relativistic electrons, which are accelerated at a shock front and subsequently cool via synchrotron and inverse Compton emission and (2) light travel delays of the photons traveling across the emitting volume. The properties of the emitting plasma are constrained by relativistic shock jump conditions that are applied to the gas dynamical system of two colliding relativistic shocks. The shocks provide the injection of highly relativistic electrons that emit synchrotron radiation and are involved in inverse Compton scattering. The model takes into account light travel delays of the synchrotron photons that provide the seeds for inverse Compton scattering, an effect neglected in previous studies. The observed synchrotron and inverse Compton emission is determined by solving time-dependent radiative transfer equations through an inhomogeneous, rapidly evolving medium energized by relativistic shocks. I present extensive numerical simulations based on this theoretical approach, which show that the inverse Compton flares lag their synchrotron counterparts because of (1) light travel delays of the seed photons or (2) prolonged evolution of the injected electrons at the low energy cut-off of the electron distribution. I establish the emission properties that distinguish different emission models and discuss application of the model to emission variability seen in the quasar 3C 273.