Connecting Accretion Disk Simulations with Observations

Part II: Ray Tracing

Jason Dexter

10/9/2008

Treu Group Meeting 10/9/2008 2/14

One Slide Part I Summary

• Dramatic advances in theory (GRMHD)– More physics– Much more difficult

• 3D, high resolution short run times• No radiation, may not conserve energy

• Observations still use old models

Need for comparison!

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My research aims to connect contemporary accretion disk

simulations with new observations via ray tracing.

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Ray Tracing

• Method for performing radiative transfer– Turn fluid variables in accretion flow into

observed emission at infinity.– Radiative transfer equationPath integral– Two parts:

1. Calculate light trajectories.

2. Solve radiative transfer equation along ray.

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Ray Tracing• Assume light rays are

geodesics.

• Photographic plate constants of motion

• Trace backwards to ensure that all rays used make it to observer simultaneously.

• Integrate along portions of rays intersecting flow.

• IntensitiesImage, many frequenciesspectrum, many timeslight curve

Schnittman et al (2006)

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New Geodesics Code

• Dexter & Agol (2008), submitted:

– New fast code to compute photon trajectories around spinning black holes.

– Includes time, azimuth dependence for the most part ignored previously.

• Ideal for GRMHD!

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Toy Ray Tracing Problems: Thin Disk

• Mapping of photographic plate to equatorial plane

• Image of Page & Thorne BH

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Toy Ray Tracing Problems: Thin Disk

Line emission Hotspots

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Toy Ray Tracing Problems:Spherical Accretion

Synchrotron spectrum comparison to Shapiro (1973): Including absorption:

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Tilted GRMHD

• “Tilted” GRMHD: Black hole spin axis not aligned with torus axis.

• Shows structure lacking in standard GRMHD.

• Observational signature of tilted disk?

• Collaborating with Chris Fragile, Omer Blaes

Fragile et al (2007)

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Garbage Result

• First result looks like equivalent untilted image.

Trash? Schnittman et al (2006)

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Computational Challenges

• Interpolation– Grid points don’t match geodesic points.– Potentially sharp gradients when geodesics

intersect corners of fluid zones.

• Convergence– Try different interpolation schemes– Different geodesic resolutions

• Keeping things moving

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More physics!

• Add time-dependence– Light delays vary over flow– Interpolation between data sets

• Realistic emission models– Synchrotron– Compton scattering(?)

• Polarization– GR effects on polarization as well

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Applications

• Sgr A*– Compare simulated images from GRMHD to

future VLBI measurements– Test strong field GR– Constraints from polarization calculations

• On possible emission/absorption mechanisms• Accretion models themselves

• Iron lines, QPOs, flares, etc.