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We present an improved photometric error analysis for the 7,100 CRTS(Catalina Real-Time Transient Survey) optical light curves for quasars from theSDSS (Sloan Digital Sky Survey) Stripe 82 catalogue. The SDSS imaging surveyhas provided a time-resolved photometric data set which greatly improved ourunderstanding of the quasar optical continuum variability: Data for monthly andlonger time-scales are consistent with a damped random walk (DRW). Recently,newer data obtained by CRTS provided puzzling evidence for enhancedvariability, compared to SDSS results, on monthly time-scales. Quantitatively,SDSS results predict about 0.06 mag root-mean-square (rms) variability formonthly time-scales, while CRTS data show about a factor of 2 larger rms, forspectroscopically confirmed SDSS quasars. Our analysis has successfullyresolved this discrepancy as due to slightly underestimated photometricuncertainties from the CRTS image processing pipelines. As a result, thecorrection for observational noise is too small and the implied quasarvariability is too large. The CRTS photometric error correction factors,derived from detailed analysis of non-variable SDSS standard stars that werere-observed by CRTS, are about 20-30%, and result in reconciling quasarvariability behaviour implied by the CRTS data with earlier SDSS results. Anadditional analysis based on independent light curve data for the same objectsobtained by the Palomar Transient Factory provides further support for thisconclusion. In summary, the quasar variability constraints on weekly andmonthly time-scales from SDSS, CRTS and PTF surveys are mutually compatible, aswell as consistent with DRW model.

Solving the puzzle of discrepant quasar variability on monthly time-scales implied by SDSS and CRTS data sets