2,713 research outputs found
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
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
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
Spectral variations of the X-ray binary pulsar LMC X-4 during its long period intensity variation and a comparison with Her X-1
We present spectral variations of the binary X-ray pulsar LMC X-4 using the
RXTE/PCA observations at different phases of its 30.5 day long super-orbital
period. Only out of eclipse data were used for this study. During the high
state of the super-orbital period of LMC X-4, the spectrum is well described by
a high energy cut-off power-law with a photon index in the range of 0.7-1.0 and
an iron emission line. In the low state, the spectrum is found to be flatter
with power-law photon index in the range 0.5-0.7. A direct correlation is
detected between the continuum flux in 7-25 keV energy band and the iron
emission line flux. The equivalent width of the iron emission line is found to
be highly variable during low intensity state, whereas it remains almost
constant during the high intensity state of the super-orbital period. It is
observed that the spectral variations in LMC X-4 are similar to those of Her
X-1 (using RXTE/PCA data). These results suggest that the geometry of the
region where the iron line is produced and its visibility with respect to the
phase of the super-orbital period is similar in LMC X-4 and Her X-1. A
remarkable difference between these two systems is a highly variable absorption
column density with phase of the super-orbital period that is observed in Her
X-1 but not in LMC X-4.Comment: 7 pages, 5 figures, Accepted for publication in Astronomy and
Astrophysic
Microscopic thickness determination of thin graphite films formed on SiC from quantized oscillation in reflectivity of low-energy electrons
Low-energy electron microscopy (LEEM) was used to measure the reflectivity of
low-energy electrons from graphitized SiC(0001). The reflectivity shows
distinct quantized oscillations as a function of the electron energy and
graphite thickness. Conduction bands in thin graphite films form discrete
energy levels whose wave vectors are normal to the surface. Resonance of the
incident electrons with these quantized conduction band states enhances
electrons to transmit through the film into the SiC substrate, resulting in
dips in the reflectivity. The dip positions are well explained using
tight-binding and first-principles calculations. The graphite thickness
distribution can be determined microscopically from LEEM reflectivity
measurements.Comment: 7 pages, 3 figure
Spectral properties of the X-ray binary pulsar LMC X-4 during different intensity states
We present spectral variations of the binary X-ray pulsar LMC X-4 observed
with the RXTE/PCA during different phases of its 30.5 day long third period.
Only out of eclipse data were used for this study. The 3-25 keV spectrum,
modeled with high energy cut-off power-law and iron line emission is found to
show strong dependence on the intensity state. Correlations between the Fe line
emission flux and different parameters of the continuum are presented here.Comment: 4 pages, 4 figure
High resolution X-ray spectrum of the accreting binary X-ray pulsar GX 1+4
We present here high resolution X-ray spectrum of the accreting binary X-ray
pulsar GX 1+4 obtained with the High Energy Transmission Grating (HETG)
instrument of the Chandra X-ray Observatory. This was supplemented by a
simultaneous observation with the Proportional Counter Array (PCA) of the Rossi
X-ray Timing Explorer (RXTE). During this observation, the source was in a
somewhat low intensity state and the pulse profile with both Chandra and RXTE
shows a narrow dip, characteristic of GX 1+4 in medium and low intensity
states. The continuum X-ray spectrum obtained with the HETG and PCA can be
fitted well with a high energy cutoff power-law model with line of sight
absorption. Interestingly, we find that this low state is accompanied by a
relatively small absorption column density. A 6.4 keV narrow emission line with
an equivalent width of 70 eV is clearly detected in the HETG spectrum. The
fluorescence iron line, or at least part of it is produced in the neutral or
lowly ionized iron in the circumstellar material that also causes most of the
line of sight absorption. In the HETG spectrum, we have found evidence for a
weak (equivalent width ~30 eV) emission line at 6.95 keV. This line is
identified as Ly_alpha emission line from hydrogen-like iron and the spectrum
does not show emission lines from helium-like iron. We discuss various emission
regions for the hydrogen-like iron emission line, like gas diffused into the
Alfven sphere or an accretion curtain flowing from the inner accretion disk to
the magnetic poles.Comment: 15 pages, 4 postscript figures, accepted for publication in The
Astrophysical Journa
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