WINDS AND
JETS FROM
ACCRETION FLOWS
Ramesh Narayan
Pre-ADAF History




Shapiro, Lightman & Eardley (1976):
hot 2T solution – thermally unstable
Ichimaru (1977): Hint that there are
two hot 2T solutions
Rees et al. (1982): Ion torus model –
unclear which 2T solution (unstable?)
Narayan & Yi (1995), Abramowicz et al.
(1995): ADAF, topology of solutions,
stability, etc.
ADAFs, Winds and Jets
Narayan & Yi (1994, Abstract):
… the Bernoulli parameter is positive, implying that advectiondominated 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 simulation
ADAFs  WINDS, JETS
Steady One-Dimensional
Adiabatic Flow
P  K
vR

2
 cs ,
d
dvR
w   P / (  1 )
d
d w 
 

 



dR
dR
 dR
dR
dR   
1 dP
d 1 2
w 

 vR   
  0
dR
dR  2
 
dBe
Be 
1
2

2
vR   

 1
2
c s  C o n s ta n t
Be: Bernoulli parameter
Bernoulli Parameter

Be is conserved in a steady adiabatic flow

For the self-similar ADAF solution,
Be 
1
2
 2

2
vR 
1
2
2
2
 R 
 5  3  2
vK
 9  5 
GM
R



 1
2
cs
2
 0 .1 2 v K
Be is positive (for  < 5/3) which means that the gas is not
bound to the BH – it can expand to infinity and flow out

Hence strong outflows/winds are expected in an ADAF

Outflow speed: v ~ (2Be)1/2 ~ 0.3vK

In contrast, gas in a thin disk is tightly bound: Be ~ -vK2/2
Convection
Narayan & Yi (1994, Abstract):
… Convection is likely in many of these flows and, if present,
will tend to enhance the above effects (winds, outflows)…
Narayan & Yi (1995, Abstract):
… In addition, all the solutions are convectively unstable,
and the convection is particularly important along the
rotation axis…we suggest that a bipolar flow will develop
along the axis of these flows, fed by material from the
surface layers of the equatorial inflow.
ADAFs  WINDS, JETS
Why is there Convection?




Accreting gas is steadily heated by
viscous dissipation
But it is not radiating any of the energy
Entropy increases with decreasing R:
P/ ~ R-(5-3)/2 ~ R-1/4
Satisfies the classic Schwarzschild
criterion for convective instability
Outflows/Convection in
Viscous Rotating Flows


Numerical simulations of
viscous rotating radiatively
inefficient hydro flows reveal
considerable convective activity
(Igumenshchev et al. 1996,
2001; Stone et al. 1999;
Igumenshchev & Abramowicz
1999, 2000)
These flows are called
convection-dominated
accretion flows (CDAFs)
Abramowicz et al. (2001)
Computer
Simulations
of ADAFs
2D MHD: Stone & Pringle (2001)
3D MHD: Igumenshchev et al. (2003)
3D hydro: Igumenshchev et al. (2000)
GRMHD
Simulation
of a
Magnetized
ADAF
The simulation spontaneously
generates:
1. geometrically thick flow
2. strong wind
3. magnetized relativistic jet
McKinney & Gammie (2004)
Mass Loss in the Wind

If mass is injected at a rate Mdotinj at some
outer radius Rinj, accretion rate decreases
with decreasing R
s
 R 
M ( R )  M inj 
 , 0  s 1
R 
 inj 

Less mass reaches the center than is
supplied on the outside
How Much Mass Does the
BH Actually Accrete?




Less than what is supplied
SMBH: Assuming Bondi flow on the
outside, which circularizes at some
radius rcirc  rBondi, then
MdotBH ~ MdotBondi/rcircs
BHXRB: Mdot is set by transition radius:
MdotBH ~ Mdot(rtr)/rtrs
The value of s is highly uncertain…
Geometry of ADAF Model
Cooling
External
Flow
Medium
ADAF
rcirc
ADAF
rtr
rBondi
Why are Quiescent BHs
Extraordinarily Dim?

Why are quiescent XRBs and quiescent
SMBHs like Sgr A* so dim?

Is it because they have

Low radiative efficiency?

Low mass accretion rate?

Both?
Radiatively Inefficient vs Mass
Outflow


Sgr A* is extremely underluminous
because of 3 (roughly equal) factors
(Yuan et al. 2003):

Low mass supply: MdotBondi ~ 10-4 MdotEdd

Mass Outflow:
MdotBH ~ 10-2.5MdotBondi

Low Rad. eff.:
Lacc ~ 10-2 (0.1 MdotBH c2)
All part of the ADAF paradigm (e.g., if
radiatively efficient, MdotBH=MdotBondi)
Nuclear SMBHs and Feedback

Bright AGN have thin disks, LLAGN have ADAFs

SMBHs produce most of their luminosity in the thin disk
phase (quasars, bright AGN)

SMBHs spend most of their time (90-99%) in the ADAF
phase (quiescence)

SMBHs accrete most of their mass in the thin disk phase
(Hopkins et al. 2005)

SMBHs probably produce a lot of their outflow energy in
the ADAF phase – 100% coupled to the external medium
Energy Output in the Wind

The wind will carry substantial
kinetic energy which might have an
important effect on the
 R 
M ( R )  M inj 

R 
 inj 
surroundings

Energy is of order a few percent of
the outflow mass energy

AGN could modify mass supply from
external medium (AGN feedback)

Disk outflow during core collapse
may drive SNe (Kohri et al. 2005)
dM
w

s M inj


s
1 s
 R inj
R

dL w  B e d M
w
s

 dR


 GM 

d M
 R 
 s M inj c  R inj 
Lw 


2(1  s )  R S 
2
s
 RS 


 R in 
w
1 s
ADAFs and Feedback






Mechanical feedback from SMBH during superEddington accretion phase
Radiative feedback from AGN during bright
quasar phase
Mechanical feedback through winds (and jets)
during ADAF phase
Causes reduced accretion – important for
understanding AGN evolution
Strongly affects galaxy formation
“Radio mode” is related to ADAF physics
ADAFs and Jets
Narayan & Yi (1994, Abstract):
… the Bernoulli parameter is positive, implying that advectiondominated 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
The connection to outflows/winds was obvious
The connection to jets was a wild guess!!
Relativistic Jets




The power in an accretion flow is
~ 0.1 Mdot c2
If a substantial fraction of this energy
goes into a substantial fraction of the
mass, expect only subrelativistic outflow
To get a relativistic jet, we have to
concentrate the accretion energy in a
small fraction of the mass
Even better: extra source of energy
Relativistic Jets
“Superluminal” Motion
3C273
GRS
1915+105
Two Kinds of Jets

BH XRBs have two kinds of jets:



Radio-loud quasars come in two types




Steady low-power jet in the hard state
Impulsive high-power jet ejections
FRI sources: steady low-power
FRII sources: blobby(?) high-power
Perhaps the physics is the same for
both classes of objects
ADAF connection for Hard State/FRI
BH Accretion
Paradigm: Thin Disk
+ ADAF + Jet
Narayan 1996; Esin et al. (1997)
Fender, Belloni & Gallo (2003)
BH XRBs: strong connection between ADAFs and jets
Hysteresis in low-high-low state transitions not fully understood
ADAFs/Jets in LLAGN


Enhanced Radio
emission/Jet activity
seen in low-luminosity
AGN (LLAGN) 

 = L/LEdd

R’ = 6 cm /B band
Radio-quiet AGN
probably have no
ADAFs, only thin disks
Ho (2002)
ADAF vs Jet




ADAFs are clearly associated with Jets
Observed radiation is a combination of
emission from ADAF and Jet
Radiation from thermal electrons likely
to be from the ADAF
Radiation from power-law electrons
likely to be from the Jet
Radiation: ADAF vs Jet




Radio emission is almost always from
PL relativistic electrons in the jet
X-rays in the hard state look very
thermal, and must be from the ADAF
But, at lower accretion rates, the jet
may dominate even in X-rays
IR/optical could be from outer thin disk,
or from ADAF, or from jet…
Ingredients Needed for
Relativistic Jets




Impressive observational evidence for a
connection between ADAFs and
relativistic jets
At the same time there is considerable
evidence that thin disks are not
conducive to producing jets
Therefore, the accretion mode is clearly
one major factor behind jet activity
What about BH spin?





Horizon shrinks: e.g., RH=GM/c2 for a*=1
Singularity becomes ring-like
Particle orbits are modified
Frame-dragging --- Ergosphere
Energy can be extracted from BH
Free Energy

Area Theorem: The surface area of a BH
can never decrease

A BH of mass M and spin a* has less area
than a non-spinning BH of the same mass

Therefore, by reversible processes, this BH
can be converted to a non-spinning BH of
lower mass, thereby releasing energy
A  8 M  M   M

How
Much
Energy?
 8 M
2
 1 6 M
1 
2
2
1 a
 if
a
2
*
2

1/ 2



a*  0 
M ax im u m E n erg y A vailab le
E   M in itial  M
0
fin al
c
2
(if a *  0 )
 0 .2 9 M c
2
(if a *  1)
Spinning Black Hole as an
Energy Source





A spinning BH has free energy that can
in principle be extracted (Penrose 1969)
Can be done with specially designed
particles (Penrose 1969), but this is
unlikely to happen in a real system
Is there a natural way to “grip” a BH to
extract the free energy?
Magnetic fields are promising
Magnetic Penrose Process (Meier 2000)
MHD Jet Simulations
Numerical MHD simulations of ADAFs around rotating BHs produce
impressive jets/outflows (Koide et al. 2002; de Villiers et al. 2003;
McKinney & Gammie 2004; Komissarov 2004; Semenov et al. 2004;
McKinney 2006; …)
JET
OUTFLOW
40M
400M
a*=
0.94
McKinney & Gammie (2004), McKinney (2006)
Semenov et al. (2004)
Other papers: De Villiers et al. (2003); McKinney & Gammie (2004);
Komissarov et al. (2004), Tchekhovskoy et al. (2008)…
Jets from Spinning Black Holes
Semenov et al. (2004)
Role of the Black Hole



The accretion disk produces a massloaded outflow with only mildly
relativistic speed even from inner edge
Field lines from the ergosphere region
inside the disk inner edge are much
cleaner and are magnetically dominated
(Poynting-dominated)
Rotation of these field lines is favorable
for producing a relativistic jet
Magnetic Hoop Stress and
Jet Collimation


A popular picture of jet collimation
is that the hoop stress of a helical
magnetic field provides the inward
collimating force
But this does not really work for
relativistic jets, especially in the
force-free regime
Force-Free
Magnetodynamics



Force-Free: An approximation in which
we have charges, currents and strong
magnetic fields, but no mass
density/inertia
That is, we assume that the charged
particles are massless
This is a reasonable first approximation
for studying ultra-relativistic jets
Spinning Split Monopole
Michel (1973) derived an
exact solution for a
spinning split monopole
with a force-free
magnetosphere
Strong acceleration
But no collimation!
Field lines are swept back,
but they do not collimate
in the poloidal plane

How are Jets Collimated?




Self-collimation is apparently not feasible
with relativistic jets
We need some external medium to
collimate the spinning magnetic fields
In the case of a Gamma-Ray Burst, the
envelope of the star provides collimation
For other accreting BHs, the accretion
disk has to do it  Strong Outflow
Cartoon of a Jet System
Gamma-Ray Burst
XRB or AGN
Necessary Ingredients:
A Proposal

Powerful jet requires






Spinning BH/Star
Magnetic field
Currents (conducting)
Low inertia
Confining medium
 ADAF (disk wind)
(Tchekhovskoy
et al. 2008)
Axisymmetric force-free jet from a spinning magnetized star surrounded
by a magnetized disk (Tchekhovskoy et al. 2008)
Toy Model: Numerical simulation of a force-free jet surrounded by a stellar
envelope or a disk wind
Near Zone:
80
2
~10 r
BH
5
Lorentz factor
increases
steadily as jet
moves out:
4

jet ~ z1/2
3
Rotation
hardly affects
the poloidal
structure of
the field even
tho’ B  Bz
2
1
-40
0
40
2x106
Far Zone: ~106rBH
Lorentz factor
continues to
increase and
reaches ~103
by a distance
of 106rBH
log10 
3
2
106
Jet is naturally
collimated:
jet ~ few
degrees
1
0
-5x104
0
5x104
Main Results



Acceleration and collimation of a forcefree jet depend on the radial profile of
the confining external pressure
A profile P ~ r-5/2, as expected for a
stellar envelope or an ADAF wind,
seems to be favorable
Terminal Lorentz factor depends on
how far out the confinement operates:
max ~ (rmax)1/2
ADAF vs Thin Disk





Nearly all simulation results to date are
for non-radiative flows: ADAFs
Produce strong outflows and jets
What kind of jets/winds do thin disks
produce?
Preliminary indication is that the jet is
absent and the wind is relatively weak
(e.g., Shafee et al. 2008)
Consistent with observations…
Unresolved Issues



How different are mass-loaded jets
compared to force-free jets?
Are their terminal Lorentz factors and
collimation angles very different?
Given that B  Bz, why are jets stable
over such enormous distances (e.g.,
Kruskal-Shafranov criterion)?
BH Spin and Jets

There has been much speculation that jets are powered
by BH spin

Microquasar GRS 1915+105 has remarkable relativistic
jets:  ~ 2.7 (Mirabel & Rodriguez) --- and it has a*1
--- looks like evidence for spin-jet connection…

GRO J1655-45 is also a microquasar:  ~ 2.7 --- but it
has a more modest spin: a* ~ 0.65 – 0.75

So, is there really a connection between rapid BH spin
and powerful jets? Not clear …
BH Masses, Spins and Jets
Source Name
BH Mass (M)
BH Spin (a*)
Jets?
LMC X-3
5.9—9.2
~0.25
X
XTE J1550-564
8.4—10.8
(~0.5)

GRO J1655-40
6.0—6.6
0.7 ± 0.05

M33 X-7
14.2—17.1
0.77 ± 0.05
X
4U1543-47
7.4—11.4
0.8 ± 0.05
X
GRS 1915+105
10--18
0.98—1

Summary




Strong theoretical link between ADAFs
and strong outflows
Strong observational link between
ADAFs and relativistic jets
Plausible scenario: ADAF wind helps to
collimate and accelerate the jet
Role of BH spin is unclear