Disk Instabilities and Cooling Fronts(3)

FUTUREPROSPECTS

Whatcanweexpectbywayofprogressinthisarea?Ourabilitytosetlimitsontheallowedformsofαislimitedbytherangeofluminositieswecanobserveandthepresenceofnon-exponentialfeaturesinaccretiondisklightcurves,e.g.re aresandsecondarymaxima.Aslongasthesefeaturesremainpoorlyunderstooditwillbedi culttopindowntherequiredformofα.However,evenwithinthelimitsimposedbyourcurrentstateofunderstandingthecomputationalmodelscouldbemadetoyieldsigni cantlymoreinformation.Forexample,wehavenopreciseideaoftheappropriateerrorbarsareontheparametersoftheproposedfunctionalformofα.CCLoriginallyestimatedthatthevalueofnallowedbyobservational

Accretion disk outbursts, and their subsequent decline, offer a unique opportunity to constrain the physics of angular momentum transport in hot accretion disks. Recent work has centered on the claim by Cannizzo et al. that the exponential decay of luminos

constraintswasprobablywithin15%ofthe‘exponential’valueof1.5,butthevalueofnwhichactuallyyieldsaperfectexponentialdecayis10%higher,andtheavailablegridofmodelshasnotyetnaileddowntheallowedrangeofn.

Amoreseriousdangeristhepossibilityisthatthecoolingwavemodelusedhereneglectssomeimportantphysicale ects.Therelaxationofthemagnetic eldasthecoolingwaveapproachesisdi culttocalculate,sincetheactualdynamomechanismis,asnotedabove,acontroversialtopic.However,weexpectthetypicalrateforthisrelaxationtobecomparabletothedynamogrowthrate,whichshouldbe～α .Thisisalsothethermalrelaxationrateinanopticallythickdisk.Thediskstructurewillevolveatslowerratesuntiltheonsetofrapidcooling,whichistoolatetoa ectthecoolingwavearguments.Moreover,evenifweassumethatthemagnetic eldisfrozenintotheplasmafromtheverybeginningoftherarefactionwavewecanshowthatithasnoperceptiblee ectonourresults.

Another,andpotentiallymoreseriousproblemisthatthistreatmentassumesthatthedisktemperatureisdeterminedbylocaldissipation,thatis,itignoresirradiation.AttemptstoaccountfortheopticallightfromBHsystemssuggestthatirradiationmaybeassigni cantaslocalenergydissipation(Cannizzo1997

[12]),althoughitprobablydoesnotdominatebyalargefactor.Ifwerestrictourattentiontodwarfnovaesystems,whereirradiationislessimportant,thenwe ndsimilar,butlessprecise,constraintsonα(Canizzo1996[9]).Inanycase,thee ectofmoderateirradiationonthecoolingwavestructureneedstobeexplored.

Arethereacceptablealternativestothecoolingwavemodelforthethesesystems,perhapsonesthatnotinvolveavaryingα?Atpresenttheanswerisno,althoughathoroughsearchhasnotyetbeendone.Ifwewishtoconsideralternativeex-planationsfortheexponentialdecayofX-rayemissionfromblackholeaccretiondisksystems,weneedmodelsthatmaintainaconstantaccretiontimescaleintherelevantmassreservoir,thatis

1τacc=αc2

s

Accretion disk outbursts, and their subsequent decline, offer a unique opportunity to constrain the physics of angular momentum transport in hot accretion disks. Recent work has centered on the claim by Cannizzo et al. that the exponential decay of luminos

anddensityincreasessmoothlyasLxdecreases.Itremainstobeseenwhetherornotaphysicallyreasonablemodelcanbeconstructedalongtheselines.ThisworkhasbeensupportedinpartbyNASAgrantNAG5-2773.

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