This is a program of numerical modeling of Sgr A*, the luminous plasma surrounding the black hole at the center of our Galaxy. These models will incorporate self-consistent general relativistic magnetohydrodynamics and general relativistic radiative transfer, including synchrotron emission and absorption by thermal and non-thermal electrons, Compton scattering, bremsstrahlung, and the dynamical effects of the black hole spin. These models will be used to predict the results of imaging Sgr A* by Very Long Baseline Interferometry, and the spectrum, polarization, and variability of Sgr A* from far infrared to gamma-rays. This work will lead to later modeling of systems where cooling and radiation forces are more important than they are in Sgr A*. Accurate models such as these are essential for understanding current and future observational data, and for evaluating the approximations used in present-day studies.
The project will advance discovery and understanding while promoting teaching, training, and learning through the involvement of two students, through the Research Experiences for Undergraduates program. The mixture of graduate and undergraduate students with postdoctoral researchers and the Principal Investigator will benefit everyone involved. Results of this research will be broadly disseminated through public release of code on the web and through public outreach.
In Einstein's theory of gravity, black holes are objects that have collapsed completely under their own gravity. Compact, massive objects that may be black holes inhabit the centers of almost all galaxies. Perhaps the most intriguing of these is Sgr A*, the radio source associated with the black hole candidate at the center of the Milky Way. Sgr A* is well studied at most wavelengths, has a known mass and distance, and will soon be imaged by an alliance of millimeter wavelength antennas called the Event Horizon Telescope. In this grant we developed techniques for modeling and predicting the radio, millimeter, infrared, and x-ray radiation from black holes like Sgr A* assuming that it is a black hole. We sought to develop our model from fundamental physical principles, making as few approximations as modern computational methods permit. In particular, we used a specially developed computational fluid dynamics code to model gas falling into the black hole. The first image shows a color map of gas density in a slice through the equatorial plane of our high resolution, three dimensional models. The gas forms spiral structures, heats up, and falls in toward the black hole event horizon - the hole at the middle of the image. We also developed techniques for estimating how infalling gas would radiate and how this radiation would change over time. By making some assumptions about physical conditions in the gas, we were able to simulate images and predict what the Event Horizon Telescope would see. The second figure shows such an image, with color corresponding to brightness on the sky at a wavelength of 1.3mm. The dark spot in the center is the shadow of the black hole, and it is surrounded by a bright ``photon ring'' characteristic of the optical properties of spacetime close to a black hole. Work done under this grant paves the way for using Event Horizon Telescope, and observations of Sgr A* conducted at other wavelengths, to test our understanding of gravity and of the dynamics and radiative properties of gas falling into black holes. We released our codes publicly on a project website, to enable more rapid progress by other researchers working on the same problem. The grant also supported the training of nine undergraduate research students, six graduate students, and a postdoctoral researcher in advanced computational techniques.
