Variability In Active Galactic Nuclei

Active galactic nuclei, or AGN, are peculiar galaxies which have energetic phenomena in the central regions. Some of the most important properties of AGN are that they are star-like objects, have time-variable continuum flux, and the spectra include broad emission lines. Variability is a common phenomenon in AGN and is observed over wide range from radio to X-ray. It provides a powerful tool for constraining the physical mechanisms and models of the central engine of AGN. There are at least two variability classes of AGN. The first includes BL Lac objects and optical violent variable (OVV) quasars, which have short time-scales and large amplitudes. The second class includes Seyfert galaxies and normal QSOs, which show smaller amplitudes on time-scales of days to years. Many important informations of variability were found by single-epoch multi-wavelength observations of different AGN samples, such as anti-correlation between variability and luminosity, positive correlation between variability and redshift, and anti-correlation between variability and wavelength. The multi-band photometry monitorings also show that the common behavior in the optical band is that AGN become bluer when they brighten. At present, there are at least three basic models for explaining the observed AGN variability. They are disk-instability model, starburst model and microlensing model respectively.In this paper, using disk-instability model and starburst model, the AGN optical variability and spectral slope variability are simulated and confronted with observations. We get reasonable light curves in B and V bands and find a positive correlation between variability and spectral slope. The results show that these two models can qualitatively explain the AGN variability in the optical band.In addition, with a three-year optical spectroscopic monitoring data of 12 QSOs, we analyze the flux variability and spectral slope variability in the observation frame. The correlation between the flux variability and redshift, the flux variability andspectral slope variability are also discussed. Large amplitudes of variability and obvious spectral slope variability in all QSOs are found in the observation frame. We do not discover any significant correlation between variability and redshift, spectral slope and flux. But we find media strong anti-correlation between variability of spectral slope and variability of log-flux.