Sgr A* from General Relativistic MHD Simulations

Jason DexterUniversity of Washington

With Eric Agol, Chris Fragile and Jon McKinney

Galactic Center Black Hole

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Sagittarius A*

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Jet or nonthermal electrons far from BH

Thermal electrons at BH

Simultaneous IR/x-ray flares close to BH?

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eCharles Gammie

Millimeter VLBI of Sgr A*

• Precision black hole astrophysics

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Doeleman et al. (2008)

Gaussian FWHM ~4 Rs!

Black Hole Shadow

• Sensitive to details of accretion flow– Need accurate theoretical predictions!

Bardeen (1973); Dexter & Agol (2009) Falcke, Melia & Agol (2000)

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GRMHD Models of Sgr A*

• GRMHD perfect for mm Sgr A*– 3D, time-dependent, thick MRI-

driven accretion flow (ADAF/RIAF)– Insignificant cooling(?) – Synchrotron radiation near BH

• Not perfect…– Collisionless plasma (mfp = 104 Rs)– Electrons

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Moscibrodzka et al. (2009)

Ray Tracing

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• Fluid variables emission at infinity

• Calculate light rays assuming geodesics. (ω >> ωp, ωc)

• Observer camera: pixels are rays

• IntensitiesImage, many frequenciesspectrum, many timeslight curve

Schnittman et al. (2006)

Sgr A* Modeling

• Geodesics from geokerr (Dexter & Agol 2009)• Time-dependent, relativistic rad. trans.• Simulations from Fragile et al. (2007, 2009);

McKinney & Blandford (2009)• Joint fits to spectral (Marrone 2006), VLBI

(Doeleman et al. 2008, Fish et al. 2010) data over grid in:– dM/dt, i, a, Ti/Te

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GRMHD Fits to VLBI Data

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Dexter, Agol & Fragile (2009); Doeleman et al. (2008)i=10 degrees i=70 degrees

10,000 km

100 μas

Parameter Estimates• i = 60 degrees

• ξ = -70 degrees

• Te /1010 K = 6 ± 2

• dM/dt = 3 x 10-9 Msun yr-1

• All to 90% confidence

CofC Colloquium 10

+15-15

+86-15

+7-1

Dexter et al. (2010, 2011)

Sky Orientation

Inclination

Electron Temperature

Accretion Rate

All VLBI 2007

Millimeter Flares• Correlation with

accretion rate

• Not caused by magnetic reconnection

• Models reproduce observed mm flares

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Solid – 230 GHz Dotted – 690 GHz

Comparison to Observed Flares

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Eckart et al. (2008)Marrone et al. (2008)

Black Hole Shadow in Sgr A*

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Shadow may be detected on chile-lmt baseline

Shadow

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Additional Applications

• Tilted disks– Same variability, images,

shadows– Precession: time-varying

fit parameters?

• M87– Can’t do “truncated” disk– All jet or mm disk

Dexter, Agol & McKinney (2011)

Dexter, Agol & Fragile (2011)

Shadow

Conclusions

• Fit 3D GRMHD images/light curves of Sgr A* to mm VLBI observations

• Estimates of inclination, sky orientation agree with RIAF fits (Broderick et al. 2009, 2010)

• Electron temperature well constrained• Reproduce observed mm flares• LMT-Chile next best chance for observing

shadow

• Future: polarized emission, complete set of sims.AAS 217 Seattle 15

RIAF Fits

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Spectra

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Visibility Variance

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Accretion Rate Variability

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Event Horizon Telescope

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UV coverage (Phase I: black)

From Shep Doeleman’s Decadal Survey Report on the EHT

Doeleman et al (2009)

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Shadow in Closure Phase