Accretion disk winds in Active Galactic Nuclei: X-ray observations ...
Winds and Jets from accretion flows
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Transcript of Winds and Jets from accretion flows
WINDS AND JETS FROM
ACCRETION FLOWS
WINDS AND JETS FROM
ACCRETION FLOWS
Ramesh Narayan
Pre-ADAF HistoryPre-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 JetsADAFs, 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
Steady One-Dimensional Adiabatic
Flow
2s
RR
2R
2 2R s
P K c , w P/ ( 1)
dv d 1 dP d d wv
dR dR dR dR dR
dBe d 1 wv 0
dR dR 2
1Be v c Constant
2 1
Be: Bernoulli parameter
Bernoulli ParameterBernoulli 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 ~ 0.3vK
In contrast, gas in a thin disk is tightly bound: Be ~ -
vK2/2
2 2 2 2R s
2 2K K
1 1 GMBe v R c
2 2 R 1
5 32 v 0.12v
9 5
ConvectionConvectionNarayan & 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?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 FlowsOutflows/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
Computer Simulations
of ADAFs2D MHD: Stone & Pringle (2001)
3D MHD: Igumenshchev et al. (2003)
3D hydro: Igumenshchev et al. (2000)
GRMHD Simulation
of a Magnetize
d ADAF
GRMHD Simulation
of a Magnetize
d ADAFThe simulation spontaneously generates:1.geometrically thick flow2.strong wind3.magnetized relativistic jet
McKinney & Gammie (2004)
Mass Loss in the WindMass Loss in the Wind
If mass is injected at a rate Mdotinj at some outer radius Rinj, accretion rate decreases with decreasing R
Less mass reaches the center than is supplied on the outside
inj
inj
( ) , 0 1
s
RM R M s
R
How Much Mass Does the BH Actually
Accrete?
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)/rtr
s
The value of s is highly uncertain…
Geometry of ADAF Model
Geometry of ADAF Model
ADAF
Cooling
Flow
External
Medium
ADAF
rcirc rBondi
rtr
Why are Quiescent BHs Extraordinarily Dim?
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
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
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
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
1inj
12inj inj
in
( )
Be
2(1 )
s
w s s
w ww
s s
Sw
S
RM R M
R
sMd M dR
R R
GMdL d M d M
R
R RsM cL
s R R
ADAFs and FeedbackADAFs 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 JetsADAFs 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 JetsRelativistic 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
Relativistic Jets
“Superluminal” Motion“Superluminal” Motion
3C273 GRS 1915+105
Two Kinds of JetsTwo 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
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 LLAGNADAFs/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 JetADAF 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 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
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 EnergyFree 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?
How Much
Energy?
1/22 2
2 2*
2*
8
8 1 1
16 if 0
A M M M a
M a
M a
2initial 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
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 SimulationsMHD Jet SimulationsNumerical 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 HolesJets from Spinning Black Holes
Semenov et al. (2004)
Role of the Black HoleRole 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
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 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 MonopoleSpinning Split MonopoleMichel (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?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 SystemCartoon of a Jet System
Gamma-Ray Burst XRB or AGN
Necessary Ingredients: A Proposal
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: ~102rBHNear Zone: ~102rBH
5
2
4
3
1
0 40-40
80
Lorentz factor increases steadily as jet moves out:
jet ~ z1/2
Rotation hardly affects the poloidal structure of the field even tho’ B Bz
Far Zone: ~106rBHFar Zone: ~106rBHlo
g10
3
2
1
0
Lorentz factor continues to increase and reaches ~103 by a distance of 106rBH
Jet is naturally collimated: jet ~ few degrees
5x104-5x104
2x106
106
0
Main ResultsMain 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 ~ (rmax)1/2
ADAF vs Thin DiskADAF 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 IssuesUnresolved 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 JetsBH 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
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
SummarySummary 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