6 research outputs found
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