A black hole is one of the simplest objects in Nature, described only by mass and spin. While many black hole masses have been estimated, spin measurements have only recently become feasible. The largest systematic error in the spin estimates arises from uncertainties in the disk model, particularly the assumption of a vanishing torque at the innermost stable circular orbit. This project will carry out two- and three-dimensional numerical general relativistic magneto-hydrodynamical simulations of the inner regions of accretion disks. It will provide clear guidelines for reliable spin estimation, both by fitting the continuum spectra of X-ray binaries and by fitting relativistically broadened iron lines.
Public interest in black hole research continues high, and this work will showcase the intellectual excitement of science in a way easily appreciated by the public, and by the junior researchers actively involved in the study. The PI has a strong record in such mentoring activities.
This project is co-funded by the NSF Division of Astronomical Sciences and NASA?s Astrophysics Theory Program.
A black hole is an extraordinary object. Even though the smallest black hole known is several times more massive than the Sun, and the largest is as massive as a small galaxy, nevertheless, each black hole is so simple that it is completely described with just two numbers, its mass and its spin. Black hole masses have been successfully measured during the last couple of decades, but until recently black hole spins were beyond reach. The Principal Investigator (PI) and his research group have made the first successul spin measurements. This breakthrough was made possible because the PI and his collaborators have developed over the last several years a technique to measure spins of stellar-mass black holes in X-ray binaries by fitting the X-ray spectra of accretion disks around these black holes. A key ingredient in the method is a theoretical model of accretion disk radiation which was developed in 1973 by Novikov & Thorne. During the current award period, the PI's group carried out large computer simulations of magnetized accretion disks around black holes on national supercomputers and used these to test the reliability of the Novikov-Thorne model. We demonstrated that the Novikov-Thorne model is extremely good for the systems of interest. Hence, there is no loss of accuracy in using this model to estimate black hole spins. This result provides a key validation of our method of measuring spin. In parallel, the PI and his group continued their observational program of measuring spins. We reported spin measurements for three new black hole X-ray binaries: LMC X-1 (spin = 0.85--0.97), A0620-00 (spin = 0--0.3), XTE J1550-564 (spin = 0.35--0.65). Interestingly, these three systems span the entire range of allowed spin values: 0--1. Together with other measurements we have reported in the past, we now have a total of ten black hole spin measurements. In the process of carrying out the above research projects, three graduate students advised by the PI received their PhDs: Dr. Rebecca Shafee (2008), Dr. Alexander Tchekhovskoy (2010), Dr. Roman Shcherbakov (2011). Shafee took up a prize postdoctoral position in the Institute for Brain Studies at Harvard University, Tchekhovskoy became a prize postdoctoral fellow at the Princeton Center for Theoretical Studies. Shcherbakov was awarded a prestigious Hubble Fellowship and moved to the University of Maryland. Two other graduate students, Robert Penna and Yucong Zhu, also contributed to the above research and are currently working towards their PhD under the guidance of the PI. In addition, a postdoctoral fellow, Dr. Akshay Kulkarni (PhD, Cornell University), was supported by funds from this award. At the end of his appointment, Kulkarni took up a position at Microsoft Corporation.
