simulations of jets from black-hole accretion disks chris lindner ut austin pi: p. chris fragile...
TRANSCRIPT
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”
2
22
0
0
~
~~~82
4
1
4
10
p
hiiht
kjkj
iiij
ij
t
Bjj
ijijt
ijijt
iit
c
cBc
gVBVBB
QPPggBBg
S
SS
BBgBBgVSS
DVD
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