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Advection-Dominated Accretion and the Black Hole Event Horizon

Ramesh Narayan. Advection-Dominated Accretion and the Black Hole Event Horizon. Energy Equation. Per Unit Volume.

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Advection-Dominated Accretion and the Black Hole Event Horizon

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  1. Ramesh Narayan Advection-Dominated Accretion and theBlack Hole Event Horizon

  2. Energy Equation Per Unit Volume Accreting gas is heated by viscosity (q+) and cooled by radiation (q-). Any excess heat is stored in the gas and transported with the flow. This represents “advection” of energy (qadv), or “advective cooling”

  3. Thin Accretion Disk (Shakura & Sunyaev 1973; Novikov & Thorne 1973;…) Most of the viscous heat energy is radiated Advection-Dominated Accretion Flow (ADAF) (Narayan & Yi 1994, 1995ab; Abramowicz et al. 1995; Chen et al. 1995;…) Most of the heat energy is advected with the gas Energy Equationq+ = q-+ qadv

  4. Two Kinds of ADAFs • Advection dominates under two conditions • Radiation is trapped in the gas and cannot diffuse out before gas falls into the BH. “Slim Disk” solution (Abramowicz, Czerny, Lasota & Szuszkiewicz 1988). L  LEdd • Gas is very dilute and cannot radiate its thermal energy before it falls into the BH. Radiatively Inefficient -- “RIAF” (Ichimaru 1977; NY 1994,1995; Abramowicz et al. 1995). L  (0.01-0.1)LEdd

  5. Properties of ADAFs/RIAFs: 1 • Very hot:Ti ~ 1012K/r,Te ~ 109-11K(virial, since ADAF loses very little heat) • Large pressure: cs ~ vK • Geometrically thick: H/R ~ 1 • Optically thin (because of low density) • Expect Comptonized spectrum: kT100 keV • It is a stable solution • Explains low hard stateof XRBs Esin et al. (1998,2001)

  6. Properties of ADAFs/RIAFs: 2 • Thin disk to ADAF/RIAF boundary occurs at Mdotcrit ~ 0.01—0.1 MdotEdd (for reasonable  ~ 0.1) • Location of the boundary is nicely consistent with Lacc at which: • BH XRBs switch from the high soft state to the low hard state (Esin et al. 1997) • AGN switch from quasar mode to LINER mode (Lasota et al. 1996; Quataert et al. 1999; Yuan & Narayan 2004) Yuan & Narayan (2004)

  7. BH Accretion Paradigm: Thin Disk + ADAF Slim Disk State? (Kubota, Makishima) Slim Disk RIAF Narayan & Quataert (2005) Narayan (1996); Esin et al. (1997)

  8. Properties of ADAFs/RIAFs: 3 • By definition, an ADAF has low radiative efficiency • Roughly, we expect a scaling (Narayan & Yi 1995) • Extreme inefficiency of Sgr A* and other quiescent SMBHs is explained (N, Yi & Mahadevan 1995) • Quiescent XRBs explained (N, McClintock & Yi 1996; N, Barret & McClintock 1997) Narayan & Yi (1995) Esin et al. (1997)

  9. Properties of ADAFs: 4 Winds and Jets Narayan & Yi (1994, Abstract): … the Bernoulli parameter is positive, implying that advection-dominated flows are susceptible to producing outflows … We suggest that advection-dominated accretion may provide an explanation for … the widespread occurrence of outflows and jets in accreting systems Narayan & Yi (1995, Title): “Advection-Dominated Accretion: Self-Similarity and Bipolar Outflows” Strong outflows confirmed in numerical simulations ADAFs  JETS, WINDS

  10. Recent Developments • Fender, Belloni & Gallo (2003): • paradigm on accretion flows and jets • steady jets found in hard state • hysteresis • SMBH accretion and galaxy formation • Effect of “feedback” on galaxy formn & SMBH growth • “radio mode” of accretion • It all comes down to ADAFs-outflows-jets

  11. BH Accretion Paradigm: Thin Disk + ADAF + Jet Narayan 1996; Esin et al. (1997) Fender, Belloni & Gallo (2003) ADAF model provides theoretical underpinning for jet paradigm Hysteresis in low-high-low state transitions not yet understood

  12. Jean-Pierre, We Need You!! • ADAFs have always been strongly attacked • Now ADAFs are being forgotten • Things were okay so long as Jean-Pierre was our spokesman ! • Then he lost interest in ADAFs … • Now, the ADAF-Bashers are running wild • The ADAF clan is getting absolutely killed Jean-Pierre, please come back!

  13. Are Black Hole Candidates Really Black Holes? • We know that BH candidates are • Compact: R  few RS • Massive: M  3M(not neutron stars) • But how sure are we that they are really BHs? • Can we find independent evidence that our BH candidates actually possess Event Horizons ? • This is a basic and important question

  14. Accretion and the Event Horizon • Accretion flows are very useful, since inflowing gas reaches the center and “senses” the nature of the central object • X-ray binaries have an additional advantage --- we can compare NS and BH systems

  15. Signatures of the Event Horizon • Differences in quiescent luminosities of XRBs(Narayan, Garcia & McClintock 1997; Garcia et al. 2001; McClintock et al. 2003;…) • Differences in variability power spectra of XRBs(Sunyaev & Revnivtsev 2000) • Differences in Type I X-ray bursts between NSXRBs and BHXRBs (Narayan & Heyl 2002; Tournear et al. 2003; Remillard et al. 2006) • Differences in X-ray colors of XRBs(Done & Gierlinsky 2003) • Differences inthermal surface emission of NSXRBs and BHXRBs (McClintock, Narayan & Rybicki 2004) • IR fluxofSgr A* (Broderick & Narayan 2006, 2007)

  16. Basic Idea Lstar Ldisk • Accretion releases energy ~(GM/R) per gram accreted • Typically, 50% is released in the disk, Ldisk~0.1(Mdot)c2, and 50% at the stellar surface, Lstar~0.1(Mdot)c2 • For a given Mdot, predicts some difference in luminosity between a NS and a BH • Usually, luminosity difference is modest (order unity)

  17. Enter ADAFs ! Lstar Ldisk • A low-mdotADAF/RIAF system has Ldisk  0.1(Mdot)c2 • Most of the gravitational energy is stored in the gas as thermal energy and released only when the gas hits the stellar surface • With stellar surface: L = Ldisk+ Lstar ~ 0.2 (Mdot)c2 • With event horizon:L=Ldisk (Mdot)c2 • Expect huge deficit inL :- L(Mdot)c2 – robust test possible • But we need an independent estimate of MdotNS control sample

  18. Surface

  19. The First Study • Narayan, Garcia & McClintock (1997) • Considered NS and BHXRB transients in quiescence -- very sub-Eddington accn • Found evidence for a large luminosity difference • But only a few systems … • Much better evidence now

  20. Better Way to Plot the Data • Our original idea was too simple – we just compared luminosity swings • But different SXTs will have different mdot values in quiescence • Better to plot Eddington-scaled quiescent luminosities vs orbital period • Lasota & Hameury (1998) • Menou et al. (1999)

  21. 1997 2000 2002 2007

  22. Transient XRBs in quiescence have ADAFs (N, M & Yi 96) • Binary period Porbdetermines Mdotin quiescence (Lasota & Hameury 1998; Menou et al. 1999) • At each Porb, we see that L/LEddis much lower for BH systems. True also for raw Lvalues. (Garcia et al. 2001; McClintock et al. 2003; …)

  23. Transient XRBs in quiescence have ADAFs (N, M & Yi 96) • Binary period Porb determines Mdot in quiescence (Lasota & Hameury 1998; Menou et al. 1999) • At each Porb, we see that L/LEdd is much lower for BH systems. True also for raw Lvalues. (Garcia et al. 2001; McClintock et al. 2003; …)

  24. Independent Confirmation • Radiation from the surface of a star is expected to be thermal • X-ray spectra of BH XRBs in quiescence have power-law shape • We can set a stringent limit on thermal component in the BH system XTE J1118+480 (McClintock et al. 2004)  no surface McClintock et al. (2003)

  25. Can Strong Gravity Provide a Loophole? • Suppose our BHs do have surfaces, but at a radius VERY SLIGHTLY outside the horizon (gravastar, dark energy star): Rstar=RS+R • Extreme relativistic effects are expected: • Radiation may take forever to get out • Surface emission may be redshifted away • Emission may be in particles, not radiation • Surface may not have reached steady state • None of these can explain the observations

  26. Takes Forever for Signals to Get Out • In terms of time as measured by an observer at infinity, the world line of infalling matter never crosses the horizon, and • Signals emitted by the matter take “forever” to get out

  27. But How Much Extra Delay? • The extra delay relative to the Newtonian case is TINY • At most it is 10 ms (for R ~ Planck scale) --- no big deal

  28. Gravitational Redshift • Looks serious, especially if redshift is large • But energy has to be conserved • A calculation shows that Lloc exceeds Mdot c2by precisely a factor (1+z)2such that L = Mdot c2

  29. Avery Broderick’s Argument L L/(1+z)2 L L

  30. Summary • ADAFs are found all over the place • >99% of BHs in the universe have ADAFs ! • Strong connection between ADAFs and Jets • ADAFs provide compelling evidence for the existence of BHEvent Horizons • Jean-Pierre playedamajor role in all this

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