Energetics of a Black Hole-Accretion Disk System with Magnetic Connection: Limit of Low Accretion Rate

摘要

Weconsidertheenergeticsofablackhole-accretiondisksystemwithmagneticconnection:aKepleriandiskisconnectedtoaKerrblackholebyalarge-scalemagneticfield.Weassumethat1)themagneticfieldislockedtotheinnerboundaryofthediskandcorotateswithit,and2)theaccretionrateislow,buttheaccretionfromthediskcanstillprovideasufficientamountofcoldplasmaparticlesinthetransitionregionsothatthemagnetohydrodynamicsapproximationisvalid.Byfindingsolutionsthatsmoothlypassthefastcriticalpointneartheequatorialplane,wefindthatasystemwithafastrotatingblackholeandthatwithaslowrotatingblackholebehaveverydifferently.Forablackholewith$$agta_mathrm{cr}equiv0.3594,Maa_mathrm{cr}equiv0.3594,M$$,where$$M$$isthemassand$$a$$isthespecificangularmomentumoftheblackhole,thespinningenergyoftheblackholeisefficientlyextractedandtransportedtothedisk,thusincreasingtheradiationefficiencyofthediskbyordersofmagnitude.Forablackholewith$$0leqaa_mathrm{cr}$$,theinnerregionofthediskisdisruptedbythemagneticfieldandtheinnerboundarymovesouttoaradiuswheretheangularvelocityofthediskisequaltothespinningangularvelocityoftheblackhole.Asaresult,thediskmayhaveanextremelylowradiationefficiencyif$$0leqa/Mll1$$.