In this paper, standard accretion disk models of AGNs are tested using light curves of 26 objects well observed for reverberation mapping. Time scales of variations are estimated by the most common definition of the variability time scale and the zero-cros

theZDCFmethodforfouronesoutofthese26objects.Theanalyzedresultsshowthattheharderandsofterphotonsattheoptical/UVbandsarecorrelatedwiththeharderphotonsleadingthesofteronesforNGC4151andNGC7469,andwiththeharderphotonslaggingthesofteronesforNGC5548andFairall9(seeTable3).ForNGC7469andNGC4151,themeasuredtimelagsaremarginallyconsistentwiththetimelagsestimatedinthecaseofcontinuumthermalreprocessing.Itispossiblethattheoptical/UVemissionsofNGC4151andNGC7469arethereprocessedX-raysthatarecommonlyattributedtoComptonup-scatteringofthermalUVseedphotonsbyhotelectrons.ForNGC5548andFairall9,theUVphotonsareunlikelytobefromthecontinuumthermalreprocessingintheaccretiondisk.Ourinvestigationsonthevariabilitytimescales,therelationofthevariabilitytimescaleswiththeblackholemasses,andthetimelagsbetweendi erentbandsareunlikelytobeinconsistentwithorarelikelytobeconditionallyinfavorofthestandardaccretiondiskmodelsofAGNs.

Wearegratefultotheanonymousrefereeforhis/herconstructivecommentsandsug-gestionsleadingtosigni cantimprovementofthispaper.WearealsogratefultoProf.S.Kaspiforhisconstructivecommentsandsuggestionsthathelpedtosigni cantlyimprovethispaper.Prof.T.AlexanderisthankedforkindlyprovidinghisZDCFcode.HTLthanksfor nancialsupportbyNationalNaturalScienceFoundationofChina(Grant10778702).JMBissupportedbyNSFC(Grant10573030)and(Grant10778726).

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