Hydrodynamical Studies of Wind Accretion Onto Compact Objects: Two-Dimensional Calculations

We present the results of hydrodynamical simulations of nonaxisymmetric gas flow past a gravitating compact object in two dimensions. Calculations were performed with uniform flow as well as with transverse velocity and density gradients. We find that the flow is highly nonsteady, exhibiting the ``flip-flop'' behavior seen in previous studies in which accretion disks form with alternating directions of rotation. We investigate the periodicity of the flip-flop behavior, and study the effects of spatial resolution on the results. We find that the flip-flop motion creates accretion torques which, in some cases, may be large enough to explain the erratic spin behavior observed in some massive X-ray pulsars.Comment: 16 pages, PostScript; figures available via anonymous ftp to astro.uchicago.edu in /pub/astro/jeffb/wind2d; mpeg movies available at http://astro.uchicago.edu/home/web/jeffb/wind.html; to be published in Astrophysical Journal (submitted 9/96, accepted 10/96

A Good Long Look at the Black Hole Candidates LMC X-1 and LMC X-3

We present results from 170ksec long RXTE observations of LMC X-1 and LMC X-3, taken in 1996 December, where their spectra can be described by a disc black body plus an additional soft (Gamma~2.8) high-energy power-law (detected up to 50keV in LMC X-3). These observations, as well as archival ASCA observations, constrain any narrow Fe line present in the spectra to have an equivalent width <90eV, broad lines (~150eV EW, \sigma ~ 1keV) are permitted. We also study the variability of LMC X-1. Its X-ray power spectral density (PSD) is approximately f^{-1} between 10^{-3} and 0.3Hz with a rms variability of ~7%. Above 5keV the PSD shows evidence of a break at f > 0.2Hz, possibly indicating an outer disc radius of ~1000GM/c^2 in this likely wind-fed system. Furthermore, the coherence function between variability in the > 5keV band and variablity in the lower energy bands is extremely low. We discuss the implications of these observations for the mechanisms.Comment: MNRAS, in press, clearified discussion, esp. on Fe lin

Numerical hydrodynamics in general relativity

The current status of numerical solutions for the equations of ideal general relativistic hydrodynamics is reviewed. With respect to an earlier version of the article the present update provides additional information on numerical schemes and extends the discussion of astrophysical simulations in general relativistic hydrodynamics. Different formulations of the equations are presented, with special mention of conservative and hyperbolic formulations well-adapted to advanced numerical methods. A large sample of available numerical schemes is discussed, paying particular attention to solution procedures based on schemes exploiting the characteristic structure of the equations through linearized Riemann solvers. A comprehensive summary of astrophysical simulations in strong gravitational fields is presented. These include gravitational collapse, accretion onto black holes and hydrodynamical evolutions of neutron stars. The material contained in these sections highlights the numerical challenges of various representative simulations. It also follows, to some extent, the chronological development of the field, concerning advances on the formulation of the gravitational field and hydrodynamic equations and the numerical methodology designed to solve them.Comment: 105 pages, 12 figures. The full online-readable version of this article, including several animations, will be published in Living Reviews in Relativity at http://www.livingreviews.or