SPRINGER BRIEFS IN ASTRONOMY Laura Brenneman Measuring the Angular Momentum of Supermassive Black Holes 123 SpringerBriefs in Astronomy SeriesEditors MartinRatcliffe WolfgangHillebrandt MichaelInglis Forfurthervolumes: http://www.springer.com/series/10090 Laura Brenneman Measuring the Angular Momentum of Supermassive Black Holes 123 LauraBrenneman SmithsonianAstrophysicalObservatory Cambridge,MA,USA ISSN2191-9100 ISSN2191-9119(electronic) ISBN978-1-4614-7770-9 ISBN978-1-4614-7771-6(eBook) DOI10.1007/978-1-4614-7771-6 SpringerNewYorkHeidelbergDordrechtLondon LibraryofCongressControlNumber:2013941131 ©LauraBrenneman2013 Thisworkissubjecttocopyright.AllrightsarereservedbythePublisher,whetherthewholeorpartof thematerialisconcerned,specificallytherightsoftranslation,reprinting,reuseofillustrations,recitation, broadcasting,reproductiononmicrofilmsorinanyotherphysicalway,andtransmissionorinformation storageandretrieval,electronicadaptation,computersoftware,orbysimilarordissimilarmethodology nowknownorhereafterdeveloped.Exemptedfromthislegalreservationarebriefexcerptsinconnection with reviews or scholarly analysis or material supplied specifically for the purpose of being entered and executed on a computer system, for exclusive use by the purchaser of the work. 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Printedonacid-freepaper SpringerispartofSpringerScience+BusinessMedia(www.springer.com) Contents 1 Introduction ................................................................... 1 2 MeasuringBlackHoleSpin.................................................. 5 3 ApplyingtheReflectionModel .............................................. 11 3.1 Requirements............................................................. 11 3.2 SpectralModels.......................................................... 12 3.3 Time-Averagedvs.Time-ResolvedSpectra ............................ 15 4 CaseStudies:MCG–6-30-15,NGC3783andFairall9 ................... 17 4.1 MCG–6-30-15............................................................ 17 4.2 NGC3783................................................................ 24 4.3 Fairall9................................................................... 33 5 MeasuringtheSMBHSpinDistribution................................... 39 5.1 SourcesofSystematicError............................................. 39 5.2 TheCurrentSpinSample................................................ 40 6 ConclusionsandFutureDirections ......................................... 45 7 Epilogue:NuSTARValidatesInnerDiskReflectioninNGC1365 ...... 47 References.......................................................................... 51 v Chapter 1 Introduction Blackholesrepresenttheultimatefrontierinastrophysics:theone-waypassageto the unknown and the unknowable. These exotic objects are defined by a charac- teristic radius known as the event horizon: the radius from the central collapsed remnant (or singularity) at which the escape velocity of the black hole equals the speed of light. Black holes therefore emit no light themselves, and we can only observe them indirectly by analyzing the electromagnetic (e/m) radiation released fromthegastheyaccrete.Thisaccretiontypicallytakestheformofageometrically- thin, optically-thick disk (Shakura and Sunyaev 1973) for black holes which are actively accreting gas (L /L ≥ 0.001, Miller 2007). The finite value of the X Edd speedoflightrendersallmaterialandspacetimewithintheeventhorizoncausally separated from the Universe in which we live; at present, there is no known way toaccessinformationfrombeyondtheeventhorizon.1 Duetothislimitation,allof our knowledge of black hole systems comes from Einstein’s Special and General theoriesofRelativity,andfrome/mobservationsofaccretiondisksaroundknown orsuspectedblackholesystemswhichare,almostinvariably,brightand/ornearby. In spite of their enigmatic nature, black holes are arguably the simplest objects in the Universe, possessing only three fundamental properties by which they can be completely defined: (1) mass, (2) spin, and (3) electric charge. In practice, the electric charge of a black hole in any environment other than a pure vacuum is assumedtobenegligible,astheblackholewouldrapidlyaccreteoppositelycharged particles and neutralize itself. Mass and spin—or angular momentum—are thus theonlytwomeaningfulpropertiesthatdescribeanastrophysicalblackhole(Kerr 1963). The mass of a black hole determines the degree to which the spacetime in 1Quantummechanical“tunneling”theoreticallyenablesblackholestoemitthermalradiationat a very slow rate. This is known as “Hawking Radiation,” (Hawking 1974) and can eventually evaporateablackhole.However,ablackholewithsolarmasswouldtake∼1066yearstoevaporate viathisprocess.Asupermassiveblackholewouldtakeconsiderablylonger. L.Brenneman,MeasuringtheAngularMomentumofSupermassiveBlackHoles, 1 SpringerBriefsinAstronomy,DOI10.1007/978-1-4614-7771-6 1, ©LauraBrenneman2013 2 1 Introduction whichitresidesiswarped(asintheclassic“bowlingballonatrampoline”analogy), whereas spin determines the degree to which that spacetime is twisted (much like beatersinthickbatter). The masses of stellar-mass black holes within our own galaxy are typically determined by examining the orbital and radiative properties of their companion stars. Measuring the masses of SMBHs can be more difficult, however. Several methodshavebeendevelopedtoestimatethemassesofSMBHs:e.g.,reverberation mapping (Blandford and McKee 1982), stellar velocity dispersion (Ferrarese and Merritt2000),tracingofstellarorbits(Genzeletal.2000;Ghezetal.2000),maser observations(WatsonandWallin1994),andgravitationallensing(Silvestro1974). Most of these methods rely on measuring radiation emitted relatively far from the black hole. Observationally, black holes range in size from ∼3to1010M(cid:2), with most stellar-mass black holes clustered in the 5–20M(cid:2) range and most SMBHs with masses of 106–108M(cid:2). Over the last decade there has been some evidence to support the existence of intermediate mass black holes with masses of order 102–104M(cid:2)(e.g.,MillerandColbert2004). Though black hole mass is by no means trivial to measure, spin is the more challengingpropertytoascertain.Incontrasttoconstrainingmass,measuringspin requiresprobingthenatureofthespacetimewithinafewgravitationalradiiofthe event horizon (where the gravitational radius is defined as r ≡ GM/c2; G is g Newton’s constant, M is the mass of the black hole and c is the speed of light). The angular momentum of a black hole only manifests through Lense-Thirring precession,alsoknownasframe-dragging,whichoccursonlyintheinnermostpart of the accretion disk where relativistic effects cause the spacetime in this region tobecometwistedinthesamedirectionthattheblackholeisrotating.Toobserve this effect, observations of the innermost disk must be made in X-rays, given the energetic processes at work in the cores of actively-accreting black holes. Current X-ray telescopes lack the spatial resolution necessary to resolve the innermost regions of the accretion disk, even in bright, nearby AGN. As such, X-ray spectra takenfromthecoreoftheAGNarethetoolofchoiceforexaminingthespacetime oftheinnerdisk. Spin (in dimensionless form) is defined as a ≡ cJ/GM2, where cosmic censorshipwithintheframeworkofGeneralRelativitydictatesthat−1 ≤ a ≤ +1 (negative spin values represent retrograde configurations in which the black hole spins in the opposite direction to the disk, positive values denote prograde spin configurations, and a = 0 implies a non-spinning black hole), and J represents the black hole angular momentum (Bardeen et al. 1972; Thorne 1974). If spin is known to within Δa ≤ 10%, then meaningful correlations can be drawn between spin and other environmental variables, e.g., the history of the accretion flow and thepresenceandpowerofrelativisticjetsinthesystem. Supermassive black holes are particularly interesting to examine, given that their masses and spins have likely evolved considerably in the billions of years since their formation. SMBHs grow by either merging with other black holes or accreting gas, most often by a combination of the two processes (e.g., Volonteri et al. 2005). Additionally, as a SMBH grows, it can go through periods where it 1 Introduction 3 producespowerfuloutflowsofkineticandradiativeenergythroughtheproduction ofwindsandjets(Fabian2012),seedingthesurroundingenvironmentwithmatter and energy. Such heating of the ambient gas in and around the host galaxy may ultimatelyplayasignificantroleinregulatingitsrateofstarformation.Thistypeof “feedback”processhasbeencitedasapotentialexplanationforthefamedM −σ relation linking the mass of the SMBH to the velocities of the stars in the central bulgeofitshostgalaxy,aswellastothemassofthebulgeitself(e.g.,Ferrareseand Merritt 2000; Gu¨ltekin et al. 2009). Given that jets are thought to be launched by themagneticextractionofrotationalenergyfromtheergosphereoftheblackhole (BlandfordandZnajek1977)whentheblackholespingetssufficientlylarge(a ≥ 0.93;AgolandKrolik2000),spinmayplayasignificantroleinregulatinggalaxy growthonscalesfarbeyondthegravitationalsphereofinfluenceoftheblackhole. Put simply, measurements of the spins of SMBHs in AGN can contribute to our understanding of these complex and energetic environments in three principalways: • They offer a rare probe of the nature of the spacetime proximal to the event horizon of the black hole, well within the strong-field gravity regime (Fabian etal.1989;Laor1991); • They can shed light on the relation of a black hole’s angular momentum to its outflow power in the form of jets (e.g., Narayan and McClintock 2012; Steiner etal.2012forstellar-massblackholes); • They can also inform us about the relative role of gas accretion vs. mergers in recent epochs of the life of the host galaxy and its AGN (Berti and Volonteri 2008). For these reasons, developing a theoretical and observational framework in which tomeasureblackholespinaccuratelyandpreciselyisofcriticalimportancetoour understandingofhowgalaxiesformandevolveovercosmictime. Advances in theoretical modeling as well as observational sensitivity in the Chandra/XMM-Newton/Suzaku era are finally producing robust constraints on the spins of a handful of SMBHs. Computationally, new algorithms developed within the past decade (Dovcˇiak et al. 2004; Beckwith and Done 2005; Brenneman and Reynolds 2006; Dauser et al. 2010, 2013) have made it possible to perform fully relativistic ray-tracing of photon paths emanating from the accretion disk close to the black hole, keeping the black hole spin as a variable parameter in the model. When such models are fit to high signal-to-noise (S/N) X-ray spectra from the innermost accretion disk, they yield vital physical information about the black hole/disk system, including constraints on how fast—and in what direction—the blackholeisrotating. Inthiswork,IdiscussourcurrentknowledgeofthedistributionofSMBHspins in the local universe and future directions of black hole spin research. I begin in Chap.2 with an examination of the spectral modeling techniques used to measure blackholespin,focusingonthosemosteffectiveinconstrainingspininAGN.Ithen discuss the models involved, reviewing the caveats that must be considered in the process in Chap.3. In Chap.4 I demonstrate the application of these techniques 4 1 Introduction to deep observations of the nearby, type 1 AGN MCG–6-30-15, NGC 3783 and Fairall9.IexaminingourcurrentknowledgeofthespindistributionoflocalSMBHs inChap.5,alongwithitsimplications.Futuredirectionsforthisfieldofresearchare presentedinChap.6.