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

diationsarethereprocessedX-raysthatarecommonlyattributedtoComptonup-scatteringofthermalUVseedphotonsbyhotelectronsinacorona(e.g.,Sunyaev&Titarchuck1980;Haardt&Maraschi1991),theopical/UVandX-raylightcurvesareexpectedtobecorrelatedwiththeX-raysleadingtheopitcal/UVradiation,andthentheharderandsofterphotonsinoptical–UVregimearecorrelatedwiththeharderphotonsleadingthesofterones.Theopitcal/UVemissionsinNGC7469andNGC4151probablybelongtothiscase.Ifthebulkoftheobservedoptical/UVcontinuumarisesfromtheviscousdissipationinaccretiondisk,theresultinglightcurveswouldbecorrelatedbuttheUVradiationsshouldleadtheX-rays.ThisscenarioissupportedbyobservationsoftheSeyfertgalaxyMCG–6-30-15(Ar´evaloetal.2005).Inthisscenario,theobservedUVandtheseed-photon-emittingregionsarecon-nectedbyperturbationsoftheaccretion owtravellinginwardsthroughtheaccretiondisk,a ecting rstthemainUVemittingradiiandthentheinnermostregionwherethebulkoftheseedphotonsisexpectedtobeproduced(e.g.,Ar´evaloetal.2005).Weanalyzedthe uxvariationsin1–2KeV(Leighlyetal.1997)and1855 A(O’Brienetal.1998)for3C390.3,andasimilarbehaviortoMCG–6-30-15isfound.ThetimelagestimatedbytheZDCF

1.28obcentroidfor3C390.3isτcent= 4.01

+0.77dayswiththeX-rayslaggingtheUVradiation.

TheUVradiationemittedbyNGC5548andFairall9mightbelongtothethermalradia-tionfromtheviscousdissipation,andperturbationsoftheaccretion owtravellinginwardsthroughtheaccretiondiskresultinthesofterphotonsleadingtheharderones.Ourresultsmaysupportthatthesignsoftimelagsdi erfromcasetocase(e.g.,Maozetal.2002).Theexistencesofnegativeaswellaspositivetimelagsimplythatdi erentprocessescouldbedominatingtheemissionsatdi erentcases,andgenerallydon’tindicateanysimplerelationbetweentheenergybands.

Inthispaper,asampleof26objectswellobservedforreverberationmappingisusedtotestthestandardaccretiondiskmodelsacceptedwidelybycomparingthetheoreticalexpectationstothemeasuredtimescalesofopticalvariations,theobservedrelationoftheblackholemasseswiththemeasuredtimescales,andthemeasuredtimelagsbetweentheoptical/UVbands.Thetimescalesmeasuredbyboththemostcommonde nitionofthevariabilitytimescaleandthezero-crossingtimeoftheACFareconsistentwitheachother(seeFig.1).Theobservedvariabilitytimescalesarelinearlycorrelatedwiththeblackholemasses(seeFig.6),andthislinearrelationisconditionallyconsistentwithexpectationforthethermaltimescalesandtheblackholemassesinequation(3).Adoptingtheviscosityparametertypicallyofα～0.1(Shakura&Sunyaev1973),thethermaltimescalesaremostclosetothemeasuredtimescalesofopticalvariations.Thecombinationsofα～0.03–0.2anda =0.5–0.998couldresultinthethermaltimescalesthatareinwellagreementwiththeopticalvariabilitytimescalespresentedinTable2.Thenitislikelythattheopticalvariationsresultfromtheaccretiondiskthermalinstability.Thetimelagsaremeasuredby