To Explore The Spectral Index Of The Active Galactic Nuclei

Variability is a common phenomenon in AGNs. Structure function and auto-correlation function are major tools for studying the variability properties of AGN samples. There are three basic models to interpret the observed AGN variability. They are disc instability model, starburst model and microlensing model. Variability provides a powerful tool for constraining the physical models of the central engine of AGN. QSOs often become bluer when they brighten.In this paper, using 7.5-year spectroscopic monitoring data of a sample of 17 PG QSOs, we study the optical spectral slope variance. Most of these 17 PG QSOs show anti-correlation between the spectral slope and the rest-frame 5100(?) continuum flux. For the ensemble of these 17 PG QSOs, a strong anti-correlation between the average spectral slope and the average luminosity is found while a median anti-correlation is found between spectral slope changes and continuum flux variations, indicating a hardening of the spectrum during bright phases. The disc instability model can qualitatively explain the anti-correlation between the spectral slope and the continuum flux for individual QSOs.AGNs are sources of luminous X-ray emission. However, the X-ray and optical emission of AGNs can be absorbed by obscuring columns of dust and gas. Mid-infrared observations provide an opportunity to study the properties of obscured AGNs. At these wavelengths, the emission is less affected by dust extinction. The mid-infrared continuum of AGNs is dominated by a power-law. The characteristic SEDs can be used to distinguish AGNs from normal galaxies.In this paper, we use IRAC photometry to select AGN candidates in the GOODS-N field. Source detection and photometry are carried out with DAOPHOT package. We fit the IRAC spectra with a power-law. We select as AGN candidates that are detected in all four IRAC bands and whose spectral indicesα≤-0.5. 39 AGN candidates are found. The optical detection fraction in the GOODS-N field is 69% while the X-ray detection fraction is 41%.