Tests for Standard Accretion Disk Models by Variability in A(2)

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

1.INTRODUCTION

Large uxvariationsontimescalesfromyearstohoursarecommoninactivegalacticnuclei(AGNs),andlongertimescalevariationsoftheorderofmonthstoyearsmayberelatedtothepropagationoftheshortertimescalevariations(e.g.,Ulrichetal.1997).Thecombinationofhigh uxvariabilityandshortvariabilitytimescalesimpliesthattheenergyconversioninAGNsismoree cientthantheordinarystellarprocesses.Accretionofmatterontoablackholecanhavehighenergyreleasee ciency(Reesetal.1982;Rees1984).TheevidencethatAGNssuchasquasarsandSeyfertgalaxiesarepoweredbygravitationalaccretionofmatterontosupermassiveblackholesisnowquiteconvincing.Certainlytherehasbeennode nitivedetectionoftherelativistice ectsthatwouldberequiredforunam-biguousidenti cationofasingularity,althoughstudiesoftheironKαemissionlineintheX-rayspectraofAGNscurrentlyprovidessomepromise(e.g.,Reynolds&Nowak2003).

Ingeneraloptical–UVradiationsofmostnon-blazartypesourcesarewithintheso-calledBigBlueBump.Theopticalvariabilityischaracterizedaspoorlyunderstood,butisneverthelessrecognizedasameansofprobingphysicalscalesthatcannotberesolvedspatiallybyanytelescopeorinstrument(e.g.,Netzer&Peterson1997;Petersonetal.2004;Woldetal.2007).Anumberofmodelshavebeenproposedtoexplainoptical–UVquasarvariability.Onewayofattemptingtohelpconstrainingtheproposedmodelsisto ndrelationshipsbetweenvariabilityandotherparametersofAGNs,suchasblackholemass.TheblackholemassisafundamentalparameterofAGNs,andthediscoveryofsucharelationship–orlackthereof–mayprovideusefulcluestothephysicalmechanismsbehindthevariability(e.g.,Woldetal.2007).Processesintrinsictothecentralengineitselfcoulddominate.Woldetal.(2007)investigatethedependenceofquasarvariabilityonblackholemass,and ndthatblackholemasscorrelateswiththemeasuredvariabilityamplitude.Anumberofmodelsforquasaropticalvariabilityexistbuttherearenoclearpredictionsrelatingblackholemassandvariabilityamplitude.Di erentsourcesofopticalvariationscanbeassociatedwithdi erentcharacteristictimescales,andmanyofthesetimescalesdependonblackholemass.Collier&Peterson(2001)attempttode nearelationshipbetweenblackholemassandcharacteristicvariabilitytimescale.Studying10well-monitoredAGNs,theyreportevidenceofblackholemasscorrelatingwithcharacteristicopticalvariabilitytimescalesthatareroughlyconsistentwithaccretiondiskthermaltimescales.

Thestandardaccretiondiskisthebasicmodelforaradiativelye cient,geometricallythin,opticallythickdisk(Shakura&Sunyaev1973).Inthestandardpicturethisaccretiondiskradiatesthermallymainlyintheoptical/UVbandsforAGNswithblackholemassesof～106–109M⊙.AGNswithblackholemassesof～107–109M⊙wouldbeexpectedtohaveaccretiondiskthermalcharacteristictimescalesoftheorderofmonthstoyears.Many