Simulations of Jets from Black-Hole Accretion Disks

Chris LindnerUT Austin

PI: P. Chris FragileCollege of Charleston

Collaborators: Peter Anninos, Jay Salmonson

Relativistic JetsIm

ages courtesy of NASA and ATN

F

• High Speed• High Energy• Observable in X-Ray and

sometimeseven visible and radio spectrums

M87Hubble Space Telescope

Visible

NGC 4261Radio and visible image

PKS 2356-61Radio and visible image

Relativistic Jets

Minkowski’s ObjectRadio emissions overlaid in red

Jet from an AGN

Crab NebulaJet from a Neutron Star

Active Galaxy Centaurus A

Simulating Tilted Black Hole Accretion disks

• End of a star’s life• Gravity bends light

around it• It bends to the

point where no light can escape!

• Can be found at the center of almostevery galaxy

Simulating Tilted Black Hole Accretion disks

• We can’t “see” black holes…

• …but we can study how their gravity affects the objects around them

Black Hole Accretion Disk Systems• X-ray binary star systems and galaxy nuclei• Black hole accretes matter from donor star• Disk of plasma forms around black hole• Angular momentum is exchanged throughMagnetic fields• Magnetically dominated flux points away

from black hole’s poles, forming jets

What is a jet?

• Poynting Flux Jet – EM jet described byBlandford-Znajek Mechanism located in “evacuated funnel”

• Funnel Wall jet – gas-pressure launchedmaterial jet surrounding the poyting fluxregion

Total Pressure (gas plus magnetic)Hawley & Krolik, 2006

• Magnetic fields are enhanced via angularmomentum transport

• Leads to a strong polar magnetic field

The Magneto Rotational Instability and Blandford-Znajek Mechanism

Blandford and Z

najek 1977• Positron-electron pair creation could

create spark gaps in B fields, and acceleration of these charges could lead to observed emissions

Jets: What we don’t knowWhat powers the jets?

What sets Jet orientation?• Not all jets are perfectly linear

• Some form corkscrew patterns, indicating jet precession

• Binary systems have been observed where jet orientations don’t match the angular momentum of the accreting object

How is the black hole oriented?• Currently, this cannot be determined by

observation aloneBlundell, K. M. & Bowler, M. G., 2004, ApJ, 616, L159

Total intensity image at 4.85 GHz of SS433

Why do Computational Astrophysics?

• Tests the extremes of space that cannot be experimentally recreated

• Many vital parameters cannot be observed

• Many problems have no exploitable symmetry

Finite Volume Simulations• Divide the computational area into

zones• Each zone contains essential data

about the material contained inside

• The simulation is evolved in time through a series of time steps

• As the simulation progresses, cells communicate with each other – calculate

GRMHD Equations in Cosmos++Extended Artificial Viscosity (eAV)

mass conservation

momentum conservation

induction

“divergence cleanser”

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Highlights of Cosmos++

• Developers: P. Anninos, P. C. Fragile, J. Salmonson, & S. Murray– Anninos & Fragile (2003) ApJS, 144, 243– Anninos, Fragile, & Murray (2003) ApJS, 147, 177– Anninos, Fragile & Salmonson (2005) ApJ, 635, 723

• Multi-dimensional Arbitrary-Lagrange-Eulerian (ALE) fluid dynamics code– 1, 2, or 3D unstructured mesh

• Local Adaptive Mesh Refinement (Khokhlov 1998)

Highlights of Cosmos++• Multi-physics code for Astrophysics/Cosmology

– Newtonian & GR MHD– Arbitrary spacetime curvature (K. Camarda -> Evolving

GRMHD)– Relativistic scalar fields– Radiation transport (Flux-limited diffusion -> Monte Carlo)– Equilibrium & Non-Equilibrium Chemistry (30+ reactions)– Radiative Cooling– Newtonian external & Self-gravity

• Developed for large parallel computation– LLNL Thunder, NCSA Teragrid, NASA Columbia, JPL Cosmos,

BSC MareNostrum, UT Lonestar, UT Ranger

Relativistic Jets in Simulation• Angular momentum supported torus

surrounding a rotating black hole

• Weak seed dipole magnetic field (poloidal)

• Low density background

• Minor initial fluctuations to foster instabilities

• Mass disk << Mass BH

• Simulated for low number of orbital periods

Relativistic Jets in Simulation

McKinney 2005

Log Density(~10 orders of magnitude between

dark red and blue)

Magnetic Field Geometry

Simulating Tilted Black Hole Accretion disks

• Black holes spin• Accretion Disks

Spin

• Do they have to spin together?

• Could this explain jet precession?

What determines jet orientation in accretion disk systems?We can answer this question by simulating systems where the angular momentum of the disk is not aligned with the angular momentum ofThe black hole

“Tilted accretion disks”(Fragile, Mathews, & Wilson, 2001, Astrophys. J., 553, 955)

• Can arise from asymmetric binary systems

• Breaks the main degeneracy in the problem

Initial tilted-disk simulations

[Show Movies]

Initial tilted-disk simulations• Standing shock along “line of nodes”

creates accretion streams• Increase in accretion rate• Observable precession• No Bardeen-Petterson effect observed

No Jets!!!

Interesting physics.. but

Spherical-Polar Grid• Most commonly used type of

grid for accretion disk simulations– good angular momentum

conservation– easy to accommodate event

horizon• Not very good for simulating

jets in 3D– zones get very small along

pole forcing a very small integration timestep

– pole is a coordinate singularity

• creates problems, particularly for transport of fluid across the pole

Cubed-Sphere Grid• Common in atmospheric

codes• Not seen as often in

astrophysics• Adequate for simulating

disks– good angular momentum

conservation– easily accommodates event

horizon• Advantages for simulating

jets– nearly uniform zone sizing

over entire grid– no coordinate singularities

(except origin)

The Cubed Sphere

Each block has its own coordinate system

Six cubes are projected into segments of a sphere

Untilted Disk Jets

MagneticField Lines

Unbound Material

Untilted Disk Jets

Scaled as 6 x (Mjet/Mtorus)

MassFluxRMax = BlueUnboundMassFlux = Black

DeVilliers, Hawley & Krolik 2004

(x10^6 – 6x10^6)

Possible Issues

• Unphysical or physical numerical reconnection

• Mass loading

• Lack of angular momentum conservation in funnel region

• … or maybe previous simulations are too symmetric?

Conclusions

• Two types of jets: Poynting flux and matter (funnel wall)

• Jets do form in MHD simulations– Do not require initial large-scale magnetic fields

• Further study is needed in the area of jet orientation and eliminating symmetries (and we’re working on it!)

Late time evolution of Gamma Ray Bursts

• Light curve decays rapidly in Gamma ray burst

• Is it a product of Central engine activity?

• Is there enough material to feed a jet?

Relativistic Jets in Simulation

Beckwith, Hawley, and Krolik 2008 Hawley, and Krolik 2005

Plasma β Magnetosonic Mach Number

Hawley & Krolik 2005

Untilted Disk Jets

Magnetosonic Mach Number

“Late Time”

“Late Time”

Hawley & Krolik 2005