GRMHD Models of Sagittarius A*

Sgr A* is so faint that radiation isn't expected to significantly affect the dynamics of its accretion flow. We can then compute synchrotron radiation from simulation data after the fact, and compare the resulting images at millimeter wavelengths to high resolution observations from very long baseline interferometry. Sample movies from this process are shown below at two observer inclinations.

[10 degrees] [70 degrees]

Fitting the images from multiple simulations, we can begin to constrain the parameters of Sgr A*. Best fit inclination and position angles, accretion rate and electron temperature are respectively. The following plots show the probability distributions over these parameters.

[Inclination] [Sky Orientation]  [Electron Temperature] [Accretion Rate]

Our best fit model images are all well described as a superposition of two crescents. The first crescent is due to Doppler beaming, which causes an asymmetry in brightness between the approaching (left) and receeding (right) gas. The second is due to the boundary between light paths that get captured by the black hole, and those which escape to infinity. All images show a clear shadow due to this boundary, which is a signature of the event horizon. If these models accurately represent Sgr A*, the black hole shadow may be detected in the near future by measurements using telescopes in Chile and Mexico.

Best fit images (first and third rows) and visibilities (second and fourth rows) for each simulation (columns). The visibilities are the Fourier transforms of the images and are the observable measured by the very long baseline interferometry experiments. The white triangles show the locations of the measurements so far.

[Best Fit Images and Visibilities]

References: Dexter, J., Agol, E. & Fragile, P. C., ApJL, 703, 142. Dexter, J., Agol, E., Fragile, P. C. & McKinney, J. C., ApJ, 717, 1092.