(a * ) Jets?

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15 Νοε 2013 (πριν από 3 χρόνια και 6 μήνες)

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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 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 simulation


ADAFs


WINDS, JETS

Steady One
-
Dimensional
Adiabatic Flow



2
s
R
R
2
R
2 2
R s
P K c,w P/( 1)
dv
d 1 dP d d w
v
dR dR dR dR dR
dBe d 1 w
v 0
dR dR 2
1
Be v c Constant
2 1

   
 
 





   
 
     
 
 
 
   
 
 
   

Be
: Bernoulli parameter

Bernoulli Parameter


Be

is conserved in a steady adiabatic flow


For the self
-
similar
ADAF
solution,






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
~ 0.3v
K


In contrast, gas in a
thin disk

is tightly bound:
Be ~
-
v
K
2
/2







2 2 2 2
R s
2 2
K K
1 1 GM
Be v R c
2 2 R 1
5 3
2 v 0.12v
9 5





   


 

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
Mdot
inj

at some
outer radius
R
inj
, accretion rate decreases
with decreasing
R






Less mass reaches the center than is
supplied on the outside

inj
inj
( ),0 1
s
R
M R M s
R
 
  
 
 
 
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
r
circ



r
Bondi
, then


Mdot
BH

~ Mdot
Bondi
/r
circ
s


BHXRB
:
Mdot

is set by transition radius:


Mdot
BH

~ Mdot(r
tr
)/r
tr
s


The value of
s

is highly uncertain…

Geometry of ADAF Model

ADAF

Cooling


Flow

External

Medium

ADAF

r
circ

r
Bondi

r
tr

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:

Mdot
Bondi
~ 10
-
4

Mdot
Edd


Mass Outflow:
Mdot
BH

~ 10
-
2.5
Mdot
Bondi


Low Rad. eff.:
L
acc

~ 10
-
2

(0.1 Mdot
BH

c
2
)


All part of the
ADAF

paradigm (e.g., if
radiatively efficient,
Mdot
BH
=Mdot
Bondi
)

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
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)

inj
inj
inj
1
inj
1
2
inj
inj
in
( )
Be
2(1 )
s
w
s s
w w
w
s
s
S
w
S
R
M R M
R
s M
d M dR
R R
GM
dL d M d M
R
R
R
s M c
L
s R R





 

 
 
 
 
 

 
 
 
 
 
 
 
 

 
 

 
 
ADAFs and Feedback


Mechanical feedback from
SMBH

during super
-
Eddington 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 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


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 c
2


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:


Steady low
-
power jet in the hard state


Impulsive high
-
power jet ejections


Radio
-
loud quasars
come in two types


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/L
Edd


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.,
R
H
=GM/c
2

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

How
Much
Energy?







1/2
2 2
2 2
*
2
*
8
8 1 1
16 if 0
A M M M a
M a
M a



 
  
 
 
  
 


2
initial final
*
2
*
Maximum Energy Available
0 (if 0)
0.29 (if 1)
E M M c
a
Mc a
 
 
 
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; …)

OUTFLOW

JET

McKinney & Gammie (2004), McKinney (2006)

40M

400M

a*=
0.94

Other papers:
De Villiers et al. (2003); McKinney & Gammie (2004);
Komissarov et al. (2004), Tchekhovskoy et al. (2008)…

Semenov et al. (2004)

Jets from Spinning Black Holes

Semenov et al. (2004)

Role of the Black Hole


The accretion disk produces a mass
-
loaded 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:
~
10
2
r
BH



5

2

4

3

1



0

40

-
40

80

Lorentz factor
increases
steadily as jet
moves out:



jet

~ z
1/2



Rotation
hardly affects
the poloidal
structure of
the field even
tho’
B




B
z

Far Zone:
~
10
6
r
BH

log
10


3

2

1

0



Lorentz factor
continues to
increase and
reaches
~10
3

by a distance
of
10
6
r
BH


Jet is naturally
collimated:

jet

~ few
degrees

5x10
4

-
5x10
4


2x10
6

10
6


0

Main Results


Acceleration and collimation of a
force
-
free 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

~ (r
max
)
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




B
z
, 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