ADVECTION-DOMINATED

ACCRETION AND THEBLACK HOLE EVENT

HORIZON

ADVECTION-DOMINATED

ACCRETION AND THEBLACK HOLE EVENT

HORIZONRamesh Narayan

Energy EquationEnergy Equation

adv

adv

Tds dQ dQ dQ

dsT q q q

dtq q q

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”

PerUnit

Volume

Energy Equationq+ = q- + qadv

Energy Equationq+ = q- + qadv

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

adv

2rad

q q q

L 0.1Mc

:

adv

2rad

2adv

q q q

L 0.1Mc

L 0.1Mc

:

Two Kinds of ADAFsTwo 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

Properties of ADAFs/RIAFs: 1Properties 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 state of XRBs

Esin et al. (1998,2001)

Properties of ADAFs/RIAFs: 2Properties 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)

BH Accretion Paradigm: Thin Disk

+ ADAF

BH Accretion Paradigm: Thin Disk

+ ADAF

Narayan (1996); Esin et al. (1997)

Slim Disk State? (Kubota, Makishima)

Narayan & Quataert (2005)

Slim Disk

RIAF

Properties of ADAFs/RIAFs: 3Properties 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)

2

ADAF ADAF

crit crit

M M0.1 ; L

M M

Narayan & Yi (1995)

Esin et al. (1997)

Properties of ADAFs: 4 Winds and Jets

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

Recent DevelopmentsRecent 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

BH Accretion Paradigm: Thin

Disk + ADAF + Jet

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

Jean-Pierre, We Need You!!

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!

Are Black Hole Candidates Really Black

Holes?

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

Accretion and the Event Horizon

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

Signatures of the Event Horizon

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 in thermal surface emission of NSXRBs and

BHXRBs (McClintock, Narayan & Rybicki 2004)

IR flux of Sgr A* (Broderick & Narayan 2006, 2007)

Basic IdeaBasic Idea

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)

LdiskLstar

Enter ADAFs !Enter ADAFs !

A low-mdot ADAF/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 in L :- L (Mdot)c2 – robust test possible

But we need an independent estimate of Mdot NS control

sample

LdiskLstar

Surface

The First Study

The First Study

Narayan, Garcia & McClintock (1997)

Considered NS and BH XRB transients in quiescence -- very sub-Eddington accn

Found evidence for a large luminosity difference

But only a few systems … Much better evidence now

Better Way to Plot the Data

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)

1997

2000

2002

2007

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 L values. (Garcia et al. 2001; McClintock et al. 2003; …)

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 L values. (Garcia et al. 2001; McClintock et al. 2003; …)

Independent ConfirmationIndependent 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)

Can Strong Gravity Provide a Loophole?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

Takes Forever for Signals to Get Out

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

22 2

2

2

2 2 2 2

21

21

sin

GM drds dt

GMc rc r

r d d

But How Much Extra Delay?

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

star starGR

star

ln 0.1 ln msS

S

R R Rt

c R R R

Gravitational RedshiftGravitational Redshift

Looks serious, especially if redshift is large But energy has to be conserved A calculation shows that Lloc exceeds Mdot c2

by precisely a factor (1+z)2 such that L =

Mdot c2

1/2 1/2

locloc2

1 1

1

S S

S

R Rz

R R

L RL L

Rz

Avery Broderick’s Argument

Avery Broderick’s Argument

1/2star

/1 S

S

R

R R R

z R

L

L/(1+z)2

L

L

SummarySummary

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 BH Event Horizons

Jean-Pierre played a major role in all this