Person:

McClintock, Jeffrey

We present an empirical model of Comptonization for fitting the spectra of X-ray binaries. This model, "simpl", has been developed as a package implemented in XSPEC. With only two free parameters, simpl is competitive as the simplest empirical model of Compton scattering. Unlike other empirical models, such as the standard power-law model, simpl incorporates the basic physics of Compton scattering of soft photons by energetic coronal electrons. Using a simulated spectrum, we demonstrate that simpl closely matches the behavior of physical Comptonization models which consider the effects of optical depth, coronal electron temperature, and geometry. We present fits to RXTE spectra of the black-hole transient H1743-322 and a BeppoSAX spectrum of LMC X-3 using both simpl and the standard power-law model. A comparison of the results shows that simpl gives equally good fits and a comparable spectral index, while eliminating the troublesome divergence of the standard power-law model at low energies. Importantly, simpl is completely flexible and can be used self-consistently with any seed spectrum of photons. We show that simpl - unlike the standard power law - teamed up with diskbb (the standard model of disk accretion) gives results for the inner-disk radius that are unaffected by strong Comptonization, a result of great importance for the determination of black hole spin via the continuum-fitting method.

We consider a simple Newtonian model of a steady accretion disk around a black hole. The model is based on height-integrated hydrodynamic equations, -viscosity, and a pseudo-Newtonian potential which results in an innermost stable circular orbit ( ISCO) that closely approximates the one predicted by general relativity. We find that, as the disk thickness H/R or the value of increases, the hydrodynamic model exhibits increasing deviations from the standard thin disk model of Shakura and Sunyaev. The latter is an analytical model in which the viscous torque is assumed to vanish at the ISCO. We consider the implications of the results for attempts to estimate black hole spin by using the standard disk model to fit continuum spectra of black hole accretion disks. We find that the error in the spin estimate is quite modest so long as H/R 0:1 and 0:2. At worst, the error in the estimated value of the spin parameter is 0.1 for a nonspinning black hole; the error is much less for a rapidly spinning hole. We also consider the density and disk thickness contrast between the gas in the disk and that inside the ISCO. The contrast needs to be large if black hole spin is to be successfully estimated by fitting the relativistically broadened X-ray line profile of fluorescent iron emission from reflection off an accretion disk. In our hydrodynamic models, the contrast in density and thickness is low when H/Rk0:1, suggesting that the iron line technique may be most reliable in extremely thin disks. We caution that these results have been obtained with a viscous hydrodynamic model. While our results are likely to be qualitatively correct, quantitative estimates of, e.g., the magnitude of the error in the spin estimate, need to be confirmed with MHD simulations of radiatively cooled thin disks.

The first extragalactic X-ray binary, LMC X-1, was discovered in 1969. In the 1980s, its compact primary was established as the fourth dynamical black hole candidate. Recently, we published accurate values for the mass of the black hole and the orbital inclination angle of the binary system. Building on these results, we have analyzed 53 X-ray spectra obtained by RXTE and, using a selected sample of 18 of these spectra, we have determined the dimensionless spin parameter of the black hole to be a∗ = 0.92+0.05 −0.07. This result takes into account all sources of observational and model-parameter uncertainties. The standard deviation around the mean value of a∗ for these 18 X-ray spectra, which were obtained over a span of several years, is only Δa∗ = 0.02. When we consider our complete sample of 53 RXTE spectra, we find a somewhat higher value of the spin parameter and a larger standard deviation. Finally, we show that our results based on RXTE data are confirmed by our analyses of selected X-ray spectra obtained by the XMM-Newton, BeppoSAX, and Ginga missions.