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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