An accretion disc model for quasar optical variability

Abstract

Some different correlations between optical-UV variability and other quasar properties, such as luminosity, black hole mass and rest-frame wavelength, were discovered. The positive correlation between optical-UV variability amplitude and black hole mass was first found by Wold et al., and this was confirmed by Wilhite et al. We suggest that the accretion disk model can explain these correlations, provided the optical-UV variability is triggered by the change of accretion rate. The disk temperature of accretion discs decreases with increasing black hole mass, which leads to systematical spectral shape difference with black hole mass even if the black hole is accreting at the same rate m_dot (m_dot = M_dot / M_dotEdd). The observed positive correlation between optical-UV variability and black hole mass can be well reproduced by our model calculations, if the mean accretion rate m_dot0 ~ 0.1 with variation of m_delta ~ 0.4 - 0.5 m_dot0. We also found that the observed correlations of optical-UV variability amplitude with luminosity or rest-frame wavelength can be qualitatively explained by this accretion disc model.