Winds and Jets from accretion flows

transcript

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

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

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 ~ -

2 2 2 2R s

2 2K K

1 1 GMBe v R c

2 2 R 1

5 32 v 0.12v

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

( ) , 0 1

RM R M s

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

The value of s is highly uncertain…

Geometry of ADAF Model

Cooling

External

Medium

rcirc 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?

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

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)

12inj inj

RM R M

sMd M dR

GMdL d M d M

R RsM cL

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

“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

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

16 if 0

A M M M a

2initial final

Maximum Energy Available

0 (if 0)

0.29 (if 1)

E M M c

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

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

McKinney & Gammie (2004), McKinney (2006)

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

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 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

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

0 40-40

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

Lorentz factor continues to increase and reaches ~103 by a distance of 106rBH

Jet is naturally collimated: jet ~ few degrees

5x104-5x104

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

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

Winds and Jets from accretion flows

Documents