Dust and the intrinsic spectral index of quasar variations: hints of
  finite stress at the innermost stable circular orbit
  We present a study of 9242 spectroscopically-confirmed quasars with
multi-epoch ugriz photometry from the SDSS Southern Survey. By fitting a
separable linear model to each quasar's spectral variations, we decompose their
five-band spectral energy distributions into variable (disc) and non-variable
(host galaxy) components. In modelling the disc spectra, we include attenuation
by dust on the line of sight through the host galaxy to its nucleus. We
consider five commonly used attenuation laws, and find that the best
description is by dust similar to that of the Small Magellanic Cloud, inferring
a lack of carbonaceous grains from the relatively weak 2175AA absorption
feature. We go on to construct a composite spectrum for the quasar variations
spanning 700 to 8000AA. By varying the assumed power-law
$L_{\nu}\propto\nu^\alpha$ spectral slope, we find a best-fit value
$\alpha=0.71\pm0.02$, excluding at high confidence the canonical
$L_{\nu}\propto\nu^{1/3}$ prediction for a steady-state accretion disc with a
$T\propto r^{-3/4}$ temperature profile. The bluer spectral index of the
observed quasar variations instead supports the model of Mummery & Balbus in
which a steeper temperature profile, $T\propto r^{-7/8}$, develops as a result
of finite magnetically-induced stress at the innermost stable circular orbit
extracting energy and angular momentum from the black hole spin.
