The relation between the optical spectral slope and the luminosity for   17 Palomar-Green QSOs
  Using 7.5-year spectroscopic monitoring data of a sample of 17 Palomar-Green QSOs (PG QSOs) (z=0.061-0.371), we obtain the optical spectral slope for each object at all epochs by a power-law fit to the spectra in continuum bands. All of these 17 PG QSOs exhibit obvious spectral slope variability. Most of the 17 objects show anti-correlation between the spectral slope and the rest-frame 5100$\AA$ continuum flux while five of them exist strong anti-correlation (correlation coefficient R larger than 0.5). For the ensemble of these 17 PG QSOs, a strong anti-correlation between the average spectral slope and the average rest-frame 5100$\AA$ luminosity is found while no correlation is found between the spectral slope and the Eddington ratio. A median anti-correlation between spectral slope changes and continuum flux variations is also found which indicates a hardening of the spectrum during bright phases. Accretion disk (jet) instability models with other mechanisms associated with changes in the accretion processes are promising. 
X. Pu, W. Bian & K. Huang (Department of Physics and Institute of   Theoretical Physics, Nanjing Normal University, Nanjing,...