Solving the puzzle of discrepant quasar variability on monthly
  time-scales implied by SDSS and CRTS data sets
  We present an improved photometric error analysis for the 7,100 CRTS
(Catalina Real-Time Transient Survey) optical light curves for quasars from the
SDSS (Sloan Digital Sky Survey) Stripe 82 catalogue. The SDSS imaging survey
has provided a time-resolved photometric data set which greatly improved our
understanding of the quasar optical continuum variability: Data for monthly and
longer time-scales are consistent with a damped random walk (DRW). Recently,
newer data obtained by CRTS provided puzzling evidence for enhanced
variability, compared to SDSS results, on monthly time-scales. Quantitatively,
SDSS results predict about 0.06 mag root-mean-square (rms) variability for
monthly time-scales, while CRTS data show about a factor of 2 larger rms, for
spectroscopically confirmed SDSS quasars. Our analysis has successfully
resolved this discrepancy as due to slightly underestimated photometric
uncertainties from the CRTS image processing pipelines. As a result, the
correction for observational noise is too small and the implied quasar
variability is too large. The CRTS photometric error correction factors,
derived from detailed analysis of non-variable SDSS standard stars that were
re-observed by CRTS, are about 20-30%, and result in reconciling quasar
variability behaviour implied by the CRTS data with earlier SDSS results. An
additional analysis based on independent light curve data for the same objects
obtained by the Palomar Transient Factory provides further support for this
conclusion. In summary, the quasar variability constraints on weekly and
monthly time-scales from SDSS, CRTS and PTF surveys are mutually compatible, as
well as consistent with DRW model.
