AGN jet launch scenarios

(1)

AGN jet launch scenarios

Rony Keppens

Centre for mathematical Plasma Astrophysics Department of Mathematics, KU Leuven

(2)

Astrophysical Jets

• astrophysical jets: ubiquitous presence of accretion disks

⇒ Young Stellar Objects (YSO)

⇒ compact objects in binaries

⇒ Active Galactic Nuclei (AGN)

• collimated, reach high velocities (up to c)

⇒ mass source of the jet?

⇒ how to collimate and keep collimated?

⇒ how to launch and accelerate mass in jet?

Rony Keppens (KU Leuven) Jet launch Nov. 2013, IAC winter school 2 / 48

(3)

Protostar environments

• Forming stars: T-Tauri with mass M∗≤ M

– optical HST image (Burrows et al. 1996)

⇒ edge-on flaring disk, reflection nebula, jets

⇒ collimated emission-line jets from center

⇒ jet-knots move at few hundred km/s (supersonic!)

(4)

• Link between accretion disk and jet?

⇒ observed proportionality jet/disk luminosity

⇒ Jet collimation: magnetic?

• Rotation measured at 20-90 AU above disk

⇒ Bacciotti et al 2002, Dougados: ApSS 293, 45 (2004)

⇒ resolved v_ϕ profiles O(10) kms⁻¹, jet width 30 AU

⇒ consistent with magneto-centrifugal launch

• Direct detectionB in inner disk region

⇒ FU Ori: Donati et al, Nature 438, 466 (2005)

⇒ 1 kG field at ∼ 0.05 AU, T ≈ 10000 K, n ≈ 10²³ m⁻³

⇒ significant ratio vertical/azimuthal field [O(2)], β = O(1)!

• role B in accretion, AM transport, disk/jet instabilities?

(5)

• Link between accretion disk and jet?

⇒ observed proportionality jet/disk luminosity

⇒ Jet collimation: magnetic?

• Rotation measured at 20-90 AU above disk

⇒ Bacciotti et al 2002, Dougados: ApSS 293, 45 (2004)

⇒ resolved v_ϕ profiles O(10) kms⁻¹, jet width 30 AU

⇒ consistent with magneto-centrifugal launch

• Direct detectionB in inner disk region

⇒ FU Ori: Donati et al, Nature 438, 466 (2005)

⇒ 1 kG field at ∼ 0.05 AU, T ≈ 10000 K, n ≈ 10²³ m⁻³

⇒ significant ratio vertical/azimuthal field [O(2)], β = O(1)!

• role B in accretion, AM transport, disk/jet instabilities?

(6)

Magnetic Accretion-Ejection structure

• key ingredients are: presence of accretion disk +B

⇒ observed versus ‘artist impression’ (for AGN)

(7)

Magnetic Accretion-Ejection structure

• key ingredients are: presence of accretion disk +B

⇒ observed versus ‘artist impression’ (for XRB SS433)

(8)

Plotkin et al, MNRAS 2012

• fundamental plane BH activity: link radio, X-ray luminosities, BH mass log L_x = ξ_Rlog L_R+ ξ_Mlog M_BH+B

⇒ from galactic BH up to supermassive (BL Lac) AGN cores

• converting accretion flow energy to radiation is universal!

(9)

Casse & Keppens, ApJ 581, 988 (2002)

• perform axially symmetric 2.5D MHD simulations

⇒ disk with initial vertical B:

self-consistently forms collimated jet

⇒ 15 % of accreted mass persistently ejected

(10)

MAES

• mechanism for jet launch and acceleration

⇒ mass source: accretion disk

⇒ collimation and acceleration of jet: B

• identicalfor YSO, compact objects, AGN

⇒ MAES model: gravitational influence on disk material

⇒ does not require specific disk/magnetosphere interaction

⇒ object (+ magnetosphere or event horizon): point source

⇒ consistent with observed jet radii at ‘origin’

(11)

MAES: Streamlines

• in jet launch region: accretion and ejection

⇒ resistive disk treatment allows accretion

⇒ accretion rate as BC: mimics outer regions

(12)

MAES: persistent launch

• spatio-temporal resistivity only in disk

⇒ accretion flow slips across vertical B

⇒ region above disk: ideal MHD (frozen-in)

• jet launch, once initiated, persists:

⇒ material ejected from disk: 15 % of ˙M_A

(13)

MAES: hollow jets

• since jet launched from disk: hollow jet

⇒ reaches super-fastmagnetosonic speeds

⇒ accelerated while ejected, magneto-centrifugally

(14)

MAES: Force analysis

• Jet ejection mechanism: axial force analysis

⇒ thermal pressure gradient lifts matter

⇒ magnetic torque brakes in disk, spins up jet

(15)

• Jet ejection mechanism: axial (vertical) force analysis

⇒ thermal pressure gradient lifts matter out of disk at surface

• magnetic torque (J × B)_θchanges sign at disk surface:

⇒ brakes disk matter, gravity wins from centrifugal: accrete

⇒ spins up jet: magnetocentrifugal acceleration of jet matter

⇒ AM is transported by magnetic field

⇒ AM flux purely parallel to poloidal flow vp/B_pconfiguration

(16)

(17)

MAES: Angular Momentum

• Angular Momentum: channeled byB

⇒ in disk: magnetic torque brakes azimuthally

⇒ gravity wins from centrifugal: accretion

⇒ AM flux parallel to poloidal flow/Bp

• torque (J × B)_ϕ changes sign at disk surface:

⇒ magnetocentrifugal acceleration of jet

• starts and stays collimated by magnetic hoop force

⇒ B_ϕcreated by rotating disk

(18)

MAES Jet extent

• radial extension of jet launching region

⇒ equipartition field region

⇒ sufficiently bent poloidal B(cold jet)

(19)

MAES Energetics

• Jet energetics: hot jet emerges

⇒ disk material heats: compression & Ohmic

• jet luminosity ∝ energy liberated by accretion GM∗M˙_A/2R_I

(20)

MAES Summary

• Mechanism for launch:

– magnetic torque brakes disk material azimuthally and spins up jet

– mass source for jet: disk – B collimates

– B accelerates

• Launching jets – comoving view

(21)

Jet launching: improving the models

• Numerical ‘proof of principle’ (2.5D VAC simulations)

⇒ Jet Launch: ApJ 581, 988 (2002)

⇒ Energetics: ApJ 601, 90 (2004)

• MAES model explains

⇒ how jets are launched and accelerated

⇒ why start and remain collimated

⇒ underluminous disks and hot jets

• improvements:

⇒ higher resolution (grid-adaptive) studies Zanni et al., A&A 469, 811 (2007)

⇒ including stellar outflows, viscosity in the disk Meliani et al., A&A 460, 1 (2006)

(22)

Zanni et al., A&A 469, 811 (2007)

• FLASH code (with AMR); emphasize role of ano-

malous resistivity prescription in disk (not always steady jet launch)

(23)

Two-component outflows

[Meliani et al.: A&A 460, 1 (2006)]

• ‘turbulence’ by enhanced visco-resistive α-prescription

⇒ Poynting flux of disc-jet: removes most AM from thin disc

⇒ β ∼ 1 in disc scale height ⇒ effective torque

• wind region: hot corona (turbulent heating near axis)

⇒ numerical sink (0.1 AU) → mass source along polar axis

(24)

Two-component outflows

• collimation differs for wind versus jet

⇒ wind region forces: thermal + Lorentz (pinch)

⇒ in turn collimated by disc-driven jet

• all axisymmetric, aimed at stationary endstates, unanswered:

⇒ 3D jet stability and termination

⇒ multi-component jets: interface dynamics?

⇒ near star accretion dynamics or inner near BH regime

(25)

• So far: considered how to launch jets, aided byB

⇒ launch/propagation/termination should be studied jointly

• helically magnetized jets propagation

⇒ hot jet, penetrating molecular cloud

(26)

• observed fine structure along jet axis: shocks?

• MHD wave propagation for radio jets

⇒ study m = 0 perturbations for compressible magnetic jet

⇒ linear wave mode dispersion relation

⇒ integrodifferential equation for weakly nonlinear waves:

possible observed jet structure as evidence of soliton propagation along jet axis?Roberts, ApJ 318, 590 (1987)

• consider stability aspects for jet segment first

(27)

(28)

(29)

• Jet variability (knots):

– internal or disk instabilities?

– non-straight: precess or deformed helically?

• Jet collimation: magnetically?

• Idealized studies in MHD framework

(30)

3D jet configurations

• [astrophysical] jet of radius R_jet:

⇒ 3D Kelvin-Helmholtz case study

⇒ cylindrical cross-section

⇒ shear flow (width 2a) across its circumference

⇒ t = 0 parallel uniform B, V₀=0.645, a = 0.05, B₀=0.129

⇒ reference parameters Ms =0.5, Ma=5, β = 120

• 3D perturbation wavenumber n along jet, m about jet axis

⇒ sideways ‘kink’ perturbation m = 1 = n

(31)

• quasi-linear analytical prediction:

⇒ (m, n) = (1, 1) excitation leads to (0, 2), (2, 2), and (2, 0)

⇒ check for t < 0.5: poloidal magnetic/kinetic energy evolution

(32)

• in horizontal cross-cut: doubled 2D result: ρ at t = 4

(33)

• high ρ isosurface and v_x =0 jet surface colored by p_th

(34)

• p_th gradient induces wavenumber doubling on top/bottom

(35)

• low ρ: 3D fibril and sheet structures: cospatial with high B_pol

(36)

• localized 3D high B_pol regions control jet deformation

(37)

• Nonlinear evolution for varying initial 3D perturbation

⇒ field lines, density evolution

(38)

Kelvin-Helmholtz and Current-Driven modes

• 3D MHD simulations of cylindrical jets

⇒ supersonic M = 1.26 jet segment with axial β = 32

⇒ unstable surface-type Kelvin-Helmholtz instabilities

⇒ uniform B: nonlinear evolution as before

• astrophysical jet collimation → azimuthal field components

⇒ collimation by jet pinching (tension in B_ϕ)

⇒ helical fields: current-carrying jets

⇒ possibility for both KH and current-driven kink instabilities

• studyinterplay between KH and CD instabilities in jets

(39)

• Magnetic Lorentz force = tension + pressure

• Flux tube with twisted field lines:

⇒ stabilizing axial B_Z tension

⇒ destabilizing Bϕ pressure in ‘kink’

⇒ pure MHD kink instability when twist q = B_ϕ/RB_Z >q_cr

(40)

• magnetized cylindrical jets

⇒ axial flow profile V_Z(R) ∝ tanh[(R_j− R)/a]

⇒ 3 magnetic configurations: Uniform → twisted fields

• For sufficiently twistedB fields: both KH and kink unstable

⇒ what about their mutual interaction?

(41)

• current-carrying HEL2 (highest twist) case: eigenfunctions

⇒ linear radial displacements show different character

0.0 0.5 1.0 1.5 2.0

r

−0.10

−0.05 0.00 0.05 0.10

ξ

⇒ m = −1 CD is localized centrally

⇒ m = ±1 KH is localized at jet radius R = R_j =1

(42)

• saturation-disruption compare UNI (a), HEL1 (b), HEL2 (c)

⇒ density structure in jet cross-section at t = 14

⇒ development fine structure less when twisted

⇒ decrease of axial kinetic energy: less in HEL2 case

(43)

• jet coherency maintained due to KH-CD interaction

⇒ magnetic deformation due to centrally developing CD

⇒ increases Bϕ & saturates KH vortices at jet surface

• 3D impression of jet after 14 transit times: UNI versus HEL2

⇒ increased (nonlinear) stability due to interacting instabilities!

(44)

Moll ’09

• non-relativistic 3D MHD jets, decay of toroidal field by kink

(45)

Rossi et al. ’08

• relativistic 3D HD jets, mix in ISM: entrainment

⇒ self-consistent ‘spine-layer’ develops (starts as underdense jet)

(46)

Mizuno et al. ’07

• relativistic 3D MHD jets, with spine-sheath structure

⇒ distortions from boundary perturbations when weakly magnetized, surrounding wind stabilizes

(47)

• Jets (+disks) at galactic scales, with central supermassive BH

⇒ what about relativistic to GR effects?

• close to BH: gravity wins, Bondi accretion (supersonic inflow, reverse of solar Parker wind)

⇒ where inflow supersonic sets accretion radius

⇒ rotation + AM conservation halts inflow (disk/torus forms)

⇒ disk turbulence: α prescription for thin disk

• inner regions: radiation pressure can repel mass supply

• MHD processes needed for jet production:

⇒ magnetocentrifugal (cold) jets: Blandford-Payne

⇒ Blanford-Znajek: EM energy from rotating BH ergosphere

(48)

(49)

(50)

• near BH structure: without/with accretion in plunging region

⇒ [Komissarov, Ch. 4: Relativistic jets from AGN, Wiley, ’12]

Figure 4.5The magnetic flux trapping effect. Left panel: Structure of the magnetosphere in the model where the disk is terminated at the last stable orbit. Right panel: Structure of the magnetosphere with included accretion flow in the plunging region (light shadow of grey ).

Notice that in this case the magnetic flux threading the black hole is higher.

(51)

Meliani et al. ’06

• analytic stationary GRMHD solutions, Schwarzschild BH metric

⇒ efficient magnetic rotator collimates to cylindrical jet by magnetic pinching, hot corona drives thermally, low Poynting flux, near axis solution by construction

(52)

Fendt 1997

• numerical stationary GRMHD solutions, Kerr BH metric

⇒ force-free electromagnetic solution: ρcE + (1/c)j × B = 0

⇒ gives cylindrically collimated, pure Poynting flux jet, internal force-balanced (finite element code)

(53)

• McKinney et al, MNRAS 2012: 3D GRMHD for rotating BH

⇒ density, field lines and fluxes on BH

(54)

• McKinney et al, Science 2013: 3D GRMHD for rotating BH

⇒ self-alignmentbetween inner disk and BH spin axis, outer disk controls further jet out direction