Magnetic Fields and Jet Formation John F. Hawley University of Virginia Workshop on MRI Turbulence June 18 th 2008

Collaborators : Kris Beckwith (UVa) Julian H. Krolik (JHU) Scott Noble (JHU) Jake Simon (UVa)

Jet Formation Young stellar objects X-ray binaries – accreting NS or BH Symbiotic stars – accreting WD Supersoft X-ray sources – accreting WD AGN – accreting supermassive BH Gamma ray burst systems The Ubiquity of Jets suggests that they are produced under general conditions. Gravity + Rotation (disk and/or central star) + Magnetic fields

Jet Theory Disk rotation + vertical field: Blandford-Payne type wind/jet Black Hole rotation + vertical field: Blandford-Znajek Poynting flux jet Past axisymmetric simulations with initial vertical fields have demonstrated efficacy of these mechanisms. Under what circumstances will a large-scale poloidal field be present? Is such a field always required for jet formation? Can such a field be generated in the disk by a dynamo process, or is it brought in from outside?

Simulating Black Hole Accretion Disks Black hole accretion process require us to describe the behaviour of matter & magnetic fields (& radiation!) in strong gravity Solution of set of equations for GRMHD necessary + inversion method + scheme for preserving divB=0 (constrained transport): see De Villiers et al. (2003), Gammie et al. (2004), Anninos (2005) Advection: Momentum: Internal Energy: Induction:

Simulations of accretion into a Kerr hole from an Initial Magnetized Gas Torus Ensemble of black hole spins: a/M = 0, 0.5, 0.9, -0.9, 0.93, 0.95, 0.99, Colors indicate density Initial gas pressure supported orbiting torus Initial magnetic field configurations: high  dipole and quadrupole loops, toroidal field, vertical field

Limitations of Current Global Simulations Global problem difficult to resolve spatially: turbulent scales to parsecs – Need 3 spatial dimensions Wide range of timescales Limited to simple equation of state Dissipation, heating, thermodynamics too limited No radiative losses; no global radiative transfer System scales with M; density set by assumed accretion rate

Side view: log density: a/M=0.9 model

Keplerian Dipole Disk Simulations Evolution: –MRI acts on the initial field, leading to large-amplitude MHD turbulence, which drives the subsequent evolution of the torus End of the simulation: –Quasi-steady-state accretion disk, surrounded by a hot corona –Low density, hot funnel region filled with (predominantly) radial field lines –Material in this region is unbound and with boost factor 2-10 –As black hole spin increases, Poynting flux in jet increases due to dragging of radial field lines anchored in black hole event horizon by rotation of space time

Properties of the Accretion Disk Accretion disk angular momentum distribution near Keplerian After several thousand M of time, models have come into approximate steady state Disk is MHD turbulent; internal stress due to the magnetorotational instability No abrupt changes at marginally stable orbit; density, velocity smooth & continuous Large scale fluctuations and low-m spiral features No stress edge; evidence for transfer of angular momentum from hole to disk Implications for the equilibrium spin of the hole if it has grown from accretion

What about the Poynting flux Jet?

Origin of the poloidal Funnel Field From an initial dipole 2D Simulation – thick torus Color: Plasma Beta White field lines

Field Topologies DipoleQuadrupoleMultiple Loop

3D Simulations: Jet Properties Things in the disk seem pretty much independent of field topology Significant unbound Poynting flux dominated outflow (relativistic jet) present in the dipole case Diagnostic: radial profiles of shell integrated magnetic field in unbound material Things here are very different. Neither the toroidal nor quadrupole fields produce much of a jet dipole: black solid line quadrupole: blue solid line toroidal: purple solid line dashed lines: +/- 1 std. dev. Magnetic Field Strength

Toroidal Field Generation of MHD turbulence from a toroidal field configuration relies on non-axisymmetric modes, i.e. there’s no such thing as a 2D toroidal field simulation No funnel field formation in this case

Jets: a summary Outflow throughout funnel, but only at funnel wall is there significant mass flux Outgoing velocity ~ c in funnel wall jet Poynting flux dominates within funnel Jet luminosity increases with hole spin – Poynting flux jet is powered by the black hole Fraction of jet luminosity in Poynting flux increases with spin Both pressure and Lorentz forces important for acceleration Existence of funnel jet depends on establishing radial funnel field – need to understand when this can happen Large Scale poloidal field in the funnel can produce a jet

Field Topology Properties of magnetized black hole accretion disks seem to be remarkably insensitive to magnetic field topology: the only dependence is in terms of the magnetic field strength. Appearance of disk should be mostly independent of magnetic field topology This is not true for the jet: –Jet formation requires a consistent sense of vertical field to brought down to the event horizon –This occurs readily for dipole, less so for quadrupole, not at all for toroidal initial field topologies –Reconnection events between funnel and disk field determine the variability of the jet

Origin of Large Scale Field Is net vertical flux required, or just large-scale poloidal field? Can significant large-scale poloidal field be generated with MRI turbulent disks? Can net field be advected inward by MRI turbulent disks? Balance magnetic diffusion/reconnection timescale against accretion timescale How does the presence or absence of a jet relate to the overall state of the disk and its magnetic field?

Conclusions Accretion disk structure Accretion efficiency Intrinsic variability Spin of hole Jet formation and power Global simulations are providing information about: But more work is needed to understand: Magnetic turbulence with non-ideal plasmas Thermodynamics and radiative properties of low density and collisionless plasmas Large scale fields and dynamos in accretion systems Details of launching mechanisms for Astrophysical Jets