Mergers of neutron stars (NS ϩ NS) or neutron stars and stellar-mass black holes (NS ϩ BH) eject a small fraction of matter with a subrelativistic velocity. Upon rapid decompression, nuclear-density medium condenses into neutron-rich nuclei, most of them radioactive. Radioactivity provides a long-term heat source for the expanding envelope. A brief transient has a peak luminosity in the supernova range, and the bulk of radiation in the UVoptical domain. We present a very crude model of the phenomenon, and simple analytical formulae that can be used to estimate the parameters of a transient as a function of poorly known input parameters. The mergers may be detected with high-redshift supernova searches as rapid transients, many of them far away from the parent galaxies. It is possible that the mysterious optical transients detected by Schmidt et al. are related to neutron star mergers, since they typically have no visible host galaxy.

Based on a spectral analysis of the X-ray continuum that employs a fully relativistic accretion disk model, we conclude that the compact primary of the binary X-ray source GRS 1915+105 is a rapidly rotating Kerr black hole. We find a lower limit on the dimensionless spin parameter of a à > 0:98. Our result is robust in the sense that it is independent of the details of the data analysis and insensitive to the uncertainties in the mass and distance of the black hole. Furthermore, our accretion disk model includes an advanced treatment of spectral hardening. Our data selection relies on a rigorous and quantitative definition of the thermal state of black hole binaries, which we used to screen all of the available RXTE and ASCA data for the thermal state of GRS 1915+105. In addition, we focus on those data for which the accretion disk luminosity is less than 30% of the Eddington luminosity. We argue that these low-luminosity data are most appropriate for the thin -disk model that we employ. We assume that there is zero torque at the inner edge of the disk, as is likely when the disk is thin, although we show that the presence of a significant torque does not affect our results. Our model and the model of the relativistic jets observed for this source constrain the distance and black hole mass and could thus be tested by determining a VLBA parallax distance and improving the measurement of the mass function. Finally, we comment on the significance of our results for relativistic jet and core-collapse models and for the detection of gravitational waves.

We use a ray-tracing technique to compute the observed spectrum of a thin accretion disk around a Kerr black hole. We include all relativistic effects such as frame-dragging, Doppler boost, gravitational redshift, and bending of light by the gravity of the black hole. We also include self-irradiation of the disk as a result of light deflection. Assuming that the disk emission is locally blackbody, we show how the observed spectrum depends on the spin of the black hole, the inclination of the disk, and the torque at the inner edge of the disk. We find that the effect of a nonzero torque on the spectrum can, to a good approximation, be absorbed into a zero-torque model by adjusting the mass accretion rate and the normalization. We describe a computer model, called KERRBB, which we have developed for fitting the spectra of black hole X-ray binaries. Using KERRBB within the Xray data reduction package XSPEC, and assuming a spectral hardening factor f col = 1.7, we analyze the spectra of three black hole X-ray binaries: 4U1543-47, XTE J1550-564, and GRO J1655-40. We estimate the spin parameters of the black holes in 4U1543-47 and GRO J1655-40 to be a/M ∼ 0.6 and ∼ 0.6 − 0.7, respectively. If f col ∼ 1.5 − 1.6, as in a recent study, then we find a/M ∼ 0.7 − 0.8 and ∼ 0.8 − 0.9, respectively. These estimates are subject to additional uncertainties in the assumed black hole masses, distances and disk inclinations.

We fit X-ray spectral data in the thermal-dominant, or high-soft, state of two dynamically confirmed black holes, GRO J1655Ϫ40 and 4U 1543Ϫ47, and estimate the dimensionless spin parameters { a/M of the two holes. a * For GRO J1655Ϫ40, using a spectral hardening factor computed for a non-LTE relativistic accretion disk, we estimate ∼ 0.75 and ∼ 0.65-0.75, respectively, from ASCA and RXTE data. For 4U 1543Ϫ47, we estimate ∼ a a a * * * 0.75-0.85 from RXTE data. Thus, neither black hole has a spin approaching the theoretical maximum p 1. a *