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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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ata
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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
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