2006MNRAS.372..246P

Using 7.5-yr spectroscopic monitoring data of a sample of 17 Palomar-Green quasi-stellar objects (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 anticorrelation between the spectral slope and the rest-frame 5100-Å continuum flux, while five of them exhibit strong anticorrelation (correlation coefficient R larger than 0.5). For the ensemble of these 17 PG QSOs, a strong anticorrelation between the average spectral slope and the average rest-frame 5100-Å luminosity is found, while no correlation is found between the spectral slope and the Eddington ratio. A median anticorrelation between spectral-slope changes and continuum-flux variations is also found which indicates a hardening of the spectrum during bright phases. Accretion disc (jet) instability models with other mechanisms associated with changes in the accretion processes are promising.