Optical spectral index–luminosity relation for the 17 mapped Palomar–Green quasars

Abstract

In this paper, the optical spectra index - luminosity relationship is checked for the well-known 17 individual mapped QSOs, in order to give one more clearer conclusion on the so far conflicting dependence of the spectral index on the luminosity for AGN. Different from the global relationships based on the color difference (photometry parameters) for samples of AGN, the more reliable relationship is determined for the multi-epoch observed individual mapped QSOs with no contamination from the host galaxies, the line variabilities and the much different central properties. The final confirmed results are as follows. (1): No strong dependence of the optical spectral index on the continuum luminosity can be found for all the 17 QSOs, besides two objects (PG 0026 and PG 1613) having some weak trends (with $3\sigma$ confidence level) for the relationship. In other words, the common sense 'AGNs get bluer when they get brighter' is not so common. (2): There are much different damped intrinsic variability time scales for the variability modes of the optical spectral index and the continuum emission, through the well applied Damped Random Walk method for the AGN variability. In other words, there are some different intrinsic mechanisms controlling the variabilities of the optical spectral index and the power law AGN continuum emission. Therefore, the much weak dependence of the optical spectral index on the continuum luminosity can be further confirmed.