Session R3: Black Holes: Nature's Ultimate Spinmeisters

Measuring the Spins of Stellar-Mass Black Holes

Starting with Cygnus X-1 in 1972, we now have a good sample of 23 stellar-mass black holes, which are located in X-ray binary systems. During the past five years, we have measured the spins of nine of these black holes. The values we find cover the full allowable range of prograde spins from $a/M=0$ to $>0.98$. These data are used to argue (1) that the high spins of at least some of these black holes are natal, and (2) that the relativistic jets observed for two low-spin black holes are powered largely by the accretion disk, rather than by the spin energy of the black hole. We measure spin by fitting the thermal continuum X-ray spectrum of the black hole to the relativistic accretion-disk model of Novikov and Thorne, thereby determining the radius of the inner edge of the disk. We identify this disk radius with the black hole's innermost stable circular orbit (ISCO). We then trivially obtain the spin from the ISCO radius, which depends only on the spin and mass of the black hole. Strong theoretical evidence that the thin accretion disks we study are sharply truncated at the ISCO, and that they are well-described by the Novikov-Thorne model, is provided by our GR MHD simulations. Likewise, strong empirical support for identifying the measured disk radius with the radius of the ISCO is provided by 26 years of observations showing the extreme stability of the inner-disk radius of LMC X-3. Thus, our measurements of spin are supported by both observational and theoretical evidence.   

Black hole-neutron star mergers: Effects of the orientation of the black hole spin

Mergers of black hole-neutron star (BHNS) binaries offer a remarkable opportunity to study strongly-curved space-time and supernuclear-density matter in the most extreme, dynamical conditions. The gravitational waves emitted as they spiral in and merge should be detectable by Advanced LIGO and VIRGO, while the accretion disc which forms as a result of the disruption of the neutron star is a potential progenitor for short gamma-ray bursts. Recent simulations of BHNS systems in full general relativity have given us an understanding of the influence of the main binary parameters (mass ratio, neutron star radius, magnitude and orientation of the black hole spin) on the gravitational wave signal and the formation of an accretion disc. Additional physical effects, including magnetic fields and nuclear-theory based equations of state, are also beginning to be included in some studies. I will review these results, discussing in more details the influence of the black hole spin (and of the precession of the orbital plane when that spin is misaligned with respect to the orbital angular momentum) on the dynamics of the merger and the characteristics of the resulting accretion disc.