The Optical Spectral Slope Variability for 17 Palomar-Green QSOs

Using 7.5 yr spectroscopic monitoring data of a sample of 17 Palomar-Green quasars, we obtain the optical spectral slope for each object at all epochs by a power-law fit to the spectra in continuum bands. We study the spectral slope changes of each object and the correlation between the spectral slope and the rest-frame 5100À continuum flux. The results show that all of 17 PG QSOs exhibit apparent spectral slope variability and seven of the 17 objects become bluer when they brighten and the average spectral slope of each QSO becomes smaller for brighter ones. We also find that there exists an anti-correlation between the spectral slope changes and the rest-wavelength 5100À continuum flux variations. These are consistent with previous spectral studies.