Constraints on the temperature inhomogeneity in quasar accretion discs
  from the ultraviolet-optical spectral variability
  The physical mechanisms of the quasar ultraviolet (UV)-optical variability
are not well understood despite the long history of observations. Recently,
Dexter & Agol presented a model of quasar UV-optical variability, which assumes
large local temperature fluctuations in the quasar accretion discs. This
inhomogeneous accretion disc model is claimed to describe not only the
single-band variability amplitude, but also microlensing size constraints and
the quasar composite spectral shape. In this work, we examine the validity of
the inhomogeneous accretion disc model in the light of quasar UV-optical
spectral variability by using five-band multi-epoch light curves for nearly 9
000 quasars in the Sloan Digital Sky Survey (SDSS) Stripe 82 region. By
comparing the values of the intrinsic scatter $\sigma_{\text{int}}$ of the
two-band magnitude-magnitude plots for the SDSS quasar light curves and for the
simulated light curves, we show that Dexter & Agol's inhomogeneous accretion
disc model cannot explain the tight inter-band correlation often observed in
the SDSS quasar light curves. This result leads us to conclude that the local
temperature fluctuations in the accretion discs are not the main driver of the
several years' UV-optical variability of quasars, and consequently, that the
assumption that the quasar accretion discs have large localized temperature
fluctuations is not preferred from the viewpoint of the UV-optical spectral
variability.
