1,621 research outputs found
The Determination of the `Diffusion Coefficients' and the Stellar Wind Velocities for X-Ray Binaries
The distribution of neutron stars (NS's) is determined by stationary solution
of the Fokker-Planck equation. In this work using the observed period changes
for four systems: Vela X-1, GX 301-2, Her X-1 and Cen X-3 we determined D, the
'diffusion coefficient',-parameter from the Fokker-Planck equation. Using
strong dependence of D on the velocity for Vela X-1 and GX 301-2, systems
accreting from a stellar wind, we determined the stellar wind velocity. For
different assumptions for a turbulent velocity we obtained . It is in good agreement with the stellar wind velocity determined by
other methods. We also determined the specific characteristic time scales for
the 'diffusion processes' in X-ray pulsars. It is of order of 200 sec for
wind-fed pulsars and 1000-10000 sec for the disk accreting systems.Comment: 8 pages, Latex, no figures, accepted for publication to Astronomical
and Astrophysical Transactions (1995). Admin note 20Feb2000: original
(broken) version now paper.tex.orig in source; fixed version with two bad
equations set in verbatim used for PS, paper.tex in sourc
Discovery of orbital decay in SMC X-1
The results are reported of three observations of the binary X ray pulsar SMC X-1 with the Ginga satellite. Timing analyses of the 0.71 s X ray pulsations yield Doppler delay curves which, in turn, provide the most accurate determination of the SMC X-1 orbital parameters available to date. The orbital phase of the 3.9 day orbit is determined in May 1987, Aug. 1988, and Aug. 1988 with accuracies of 11, 1, and 3.5 s, respectively. These phases are combined with two previous determinations of the orbital phase to yield the rate of change in the orbital period: P sub orb/P sub orb = (-3.34 + or - 0.023) x 10(exp -6)/yr. An interpretation of this measurement and the known decay rate for the orbit of Cen X-3 is made in the context of tidal evolution. Finally, a discussion is presented of the relation among the stellar evolution, orbital decay, and neutron star spinup time scales for the SMC X-1 system
Correlation between X-ray flux and rotational acceleration in Vela X-1
The results of a search for correlations between X-ray flux and angular acceleration for the accreting binary pulsar Vela X-1 are presented. Results are based on data obtained with the Hakucho satellite during the interval 1982 to 1984. In undertaking this correlation analysis, it was necessary to modify the usual statistical method to deal with conditions imposed by generally unavoidable satellite observing constraints, most notably a mismatch in sampling between the two variables. The results are suggestive of a correlation between flux and the absolute value of the angular acceleration, at a significance level of 96 percent. The implications of the methods and results for future observations and analysis are discussed
The Distribution of X-ray Dips with Orbital Phase in Cygnus X-1
We present results of a comprehensive study of the distribution of absorption
dips with orbital phase in Cygnus X-1. Firstly, the distribution was obtained
using archival data from all major X-ray observatories and corrected for the
selection effect that phase zero (superior conjunction of the black hole) has
been preferentially observed. Dip occurrence was seen to vary strongly with
orbital phase \phi, with a peak at \phi ~ 0.95, i.e. was not symmetric about
phase zero. Secondly, the RXTE ASM has provided continuous coverage of the Low
State of Cygnus X-1 since Sept. 1996, and we have selected dip data based on
increases in hardness ratio. The distribution, with much increased numbers of
dip events, confirms that the peak is at \phi ~ 0.95, and we report the
discovery of a second peak at \phi ~ 0.6. We attribute this peak to absorption
in an accretion stream from the companion star HDE 226868. We have estimated
the ionization parameter at different positions showing that radiative
acceleration of the wind is suppressed by photoionization in particular regions
in the binary system. To obtain the variation of column density with phase, we
make estimates of neutral wind density for the extreme cases that acceleration
of the wind is totally suppressed, or not suppressed at all. An accurate
description will lie between these extremes. In each case, a strong variation
of column density with orbital phase resulted, similar to the variation of dip
occurrence. This provides evidence that formation of the blobs in the wind
which lead to absorption dips depends on the density of the neutral component
in the wind, suggesting possible mechanisms for blob growth.Comment: 9 pages, Latex, 7 ps figures. accepted by MNRA
Resistively detected nuclear magnetic resonance via a single InSb two-dimensional electron gas at high temperature
We report on the demonstration of the resistively detected nuclear magnetic
resonance (RDNMR) of a single InSb two-dimensional electron gas (2DEG) at
elevated temperatures up to 4 K. The RDNMR signal of 115In in the simplest
pseudospin quantum Hall ferromagnet triggered by a large direct current shows a
peak-dip line shape, where the nuclear relaxation time T1 at the peak and the
dip is different but almost temperature independent. The large Zeeman,
cyclotron, and exchange energy scales of the InSb 2DEG contribute to the
persistence of the RDNMR signal at high temperatures.Comment: 11pages,3figure
Spectral Transition and Torque Reversal in X-ray Pulsar 4U 1626-67
The accretion-powered, X-ray pulsar 4U 1626-67 has recently shown an abrupt
torque reversal accompanied by a dramatic spectral transition and a relatively
small luminosity change. The time-averaged X-ray spectrum during spin-down is
considerably harder than during spin-up. The observed torque reversal can be
explained by an accretion flow transition triggered by a gradual change in the
mass accretion rate. The sudden transition to spin-down is caused by a change
in the accretion flow rotation from Keplerian to sub-Keplerian. 4U 1626-67 is
estimated to be near spin equilibrium with a mass accretion rate Mdot~2x10**16
g/s, Mdot decreasing at a rate ~6x10**14 g/s/yr, and a polar surface magnetic
field of ~2b_p**{-1/2} 10^**12G where b_p is the magnetic pitch. During
spin-up, the Keplerian flow remains geometrically thin and cool. During
spin-down, the sub-Keplerian flow becomes geometrically thick and hot. Soft
photons from near the stellar surface are Compton up-scattered by the hot
accretion flow during spin-down while during spin-up such scattering is
unlikely due to the small scale-height and low temperature of the flow. This
mechanism accounts for the observed spectral hardening and small luminosity
change. The scattering occurs in a hot radially falling column of material with
a scattering depth ~0.3 and a temperature ~10^9K. The X-ray luminosity at
energies >5keV could be a poor indicator of the mass accretion rate. We briefly
discuss the possible application of this mechanism to GX 1+4, although there
are indications that this system is significantly different from other
torque-reversal systems.Comment: 10 pages, 1 figure, ApJ
Resolving the Fe xxv Triplet with Chandra in Cen X-3
We present the results of a 45 ks Chandra observation of the high-mass X-ray
binary Cen X--3 at orbital phases between 0.13 and 0.40 (in the eclipse
post-egress phases). Here we concentrate on the study of discrete features in
the energy spectrum at energies between 6 and 7 keV, i.e. on the iron
K line region, using the High Energy Transmission Grating Spectrometer
on board the Chandra satellite. We clearly see a K neutral iron line
at keV and were able to distinguish the three lines of the
\ion{Fe}{25} triplet at 6.61 keV, 6.67 keV, and 6.72 keV, with an equivalent
width of 6 eV, 9 eV, and 5 eV, respectively. The equivalent width of the
K neutral iron line is 13 eV, an order of magnitude lower than
previous measures. We discuss the possibility that the small equivalent width
is due to a decrease of the solid angle subtended by the reflector.Comment: 11 pages, 2 figures, To appear in the Astrophysical Journal Letter
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