The Sloan Digital Sky Survey Reverberation Mapping Project:
  Accretion-Disk Sizes from Continuum Lags
  We present accretion-disk structure measurements from continuum lags in the
Sloan Digital Sky Survey Reverberation Mapping (SDSS-RM) project. Lags are
measured using the \texttt{JAVELIN} software from the first-year SDSS-RM $g$
and $i$ photometry, resulting in well-defined lags for 95 quasars, 33 of which
have lag SNR $>$ 2$\sigma$. We also estimate lags using the \texttt{ICCF}
software and find consistent results, though with larger uncertainties.
Accretion-disk structure is fit using a Markov Chain Monte Carlo approach,
parameterizing the measured continuum lags as a function of disk size
normalization, wavelength, black hole mass, and luminosity. In contrast with
previous observations, our best-fit disk sizes and color profiles are
consistent (within 1.5~$\sigma$) with the \citet{SS73} analytic solution. We
also find that more massive quasars have larger accretion disks, similarly
consistent with the analytic accretion-disk model. The data are inconclusive on
a correlation between disk size and continuum luminosity, with results that are
consistent with both no correlation and with the \citet{SS73} expectation. The
continuum lag fits have a large excess dispersion, indicating that our measured
lag errors are underestimated and/or our best-fit model may be missing the
effects of orientation, spin, and/or radiative efficiency. We demonstrate that
fitting disk parameters using only the highest-SNR lag measurements biases
best-fit disk sizes to be larger than the disk sizes recovered using a Bayesian
approach on the full sample of well-defined lags.
