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Radiative Models of Sagittarius A* and M87 from Relativistic MHD Simulations

Show simple item record Dexter, Jason 2011-08-22T18:33:44Z 2011-08-22T18:33:44Z 2011-07
dc.description.abstract The discovery that the magnetorotational instability (MRI) is likely the mechanism of angular momentum transport and accretion has led to rapid progress in the theory of black hole accretion. General relativistic MHD (GRMHD) simulations currently provide the most realistic physical description of black hole accretion flows, but neglect radiation and are currently only applicable to low luminosity systems, where the radiation can be added in post-processing. To connect simulations with observations, we have developed codes for the semi-analytic computation of Kerr photon orbits (\texttt{geokerr}) and for the calculation of time-dependent, general relativistic radiative transfer via ray tracing (\texttt{grtrans}). The Galactic center black hole candidate, Sgr A*, provides an ideal first problem for study using GRMHD and these radiative transfer methods. Recent very long baseline interferometry (VLBI) observations detected the structure of its accretion flow on event horizon scales, allowing a direct comparison between accretion disk theory and observation. We compare millimeter wavelength images and light curves from a set of three-dimensional GRMHD simulations to VLBI and spectral observations of Sgr A* using general relativistic radiative transfer. The GRMHD models provide an excellent fit to current observations. Magnetic turbulence driven by the MRI naturally explains the observed millimeter variability. Fitting models to observations allows estimates of the inclination and position angles of the black hole, as well as of the median electron temperature in the millimeter emission region and the accretion rate onto the black hole. The black hole shadow, a signature of the event horizon, is unobscured in all models and may be detectable with VLBI experiments in the next 5-10 years. We also consider ``tilted" accretion disk models, where the angular momentum axis of the accretion flow is misaligned from the black hole spin axis. The additional degree of freedom in these models allows a wide range of viable possibilities for Sgr A*. Additional observations both of visibility amplitudes and closure phases will constrain the models further. We also study generic observational consequences of tilt in black hole accretion disks, and find that the radiation edge of these systems is independent of black hole spin; in stark contrast to the untilted disks where it decreases with increasing spin, roughly tracking the marginally stable orbit. Line profiles based on gravitational redshifts and Doppler shifts of these simulations vary strongly with observer azimuth. Coupled with precession, this may lead to strongly time-varying emission lines from tilted, geometrically thick disks. Finally, radiative models of M87 are constructed in a similar fashion to Sgr A*. The M87 spectrum can be fit by either a jet or a jet/disk model. The geometry in M87 can be reasonably constrained by observations of the extended jet emission, and the resulting millimeter images are fairly robust despite considerable model uncertainties. The Gaussian FWHM inferred from mm-VLBI on current telescopes should be 36-41 $\mu$as. Jet images are more compact than disk images, and in either case the shadow in M87 should be accessible to future VLBI observations. en_US
dc.language.iso en_US en_US
dc.subject Astrophysics en_US
dc.subject Relativity en_US
dc.subject Black Holes en_US
dc.subject Sagittarius A* en_US
dc.subject Accretion en_US
dc.subject M87 en_US
dc.subject Radiative Transfer en_US
dc.title Radiative Models of Sagittarius A* and M87 from Relativistic MHD Simulations en_US
dc.type Thesis en_US

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