3,387 research outputs found
Optical Modulation in the X-Ray Binary 4U 1543-624 Revisited
The X-ray binary 4U 1543624 has been provisionally identified as an
ultracompact system with an orbital period of 18~min. We have carried
out time-resolved optical imaging of the binary to verify the ultra-short
orbital period. Using 140\,min of high-cadence -band photometry we recover
the previously-seen sinusoidal modulation and determine a period
\,min. In addition, we also see a 7.0\,mag\,min linear decay, likely related to variations in the
source's accretion activity. Assuming that the sinusoidal modulation arises
from X-ray heating of the inner face of the companion star, we estimate a
distance of 6.0--6.7\,kpc and an inclination angle of
34--61 (90\% confidence) for the binary. Given the
stability of the modulation we can confirm that the modulation is orbital in
origin and 4U 1543624 is an ultracompact X-ray binary.Comment: 6 pages, 3 figures, accepted for publication in Publications of the
Astronomical Society of Australia (PASA
R-Modes on Rapidly Rotating, Relativistic Stars: I. Do Type-I Bursts Excite Modes in the Neutron-Star Ocean?
During a Type-I burst, the turbulent deflagation front may excite waves in
the neutron star ocean and upper atmosphere with frequencies,
Hz. These waves may be observed as highly coherent flux oscillations during the
burst. The frequencies of these waves changes as the upper layers of the
neutron star cool which accounts for the small variation in the observed QPO
frequencies. In principle several modes could be excited but the fundamental
buoyant mode exhibits significantly larger variability for a given
excitation than all of the other modes. An analysis of modes in the burning
layers themselves and the underlying ocean shows that it is unlikely these
modes can account for the observed burst oscillations. On the other hand,
photospheric modes which reside in a cooler portion of the neutron star
atmosphere may provide an excellent explanation for the observed oscillations.Comment: 18 pages, 1 figure, substantial changes and additions to reflect
version to appear in Ap
Variation of D-region nitric-oxide density with solar activity and season at the dip equator
To study the solar control on electron density (N sub e) in the equatorial D region, a program was initiated with Soviet collaboration in 1979. A total of 31 rockets were launched during the high solar activity period, and 47 rockets during the low solar activity period, from Thumba to measure the N sub e profiles. Analysis of the data shows that the average values of N sub e for the high solar activity period are higher by a factor of about 2 to 3 compared to the low solar activity values. It was found that a single nitric oxide density, (NO), profile cannot reproduce all the observed N sub e profiles. An attempt was made to reproduce theoretically the observed N sub e profiles by introducing variation in (NO) for the different solar activity periods and seasons
Instability of Quark Matter Core in a Compact Newborn Neutron Star With Moderately Strong Magnetic Field
It is explicitly shown that if phase transition occurs at the core of a
newborn neutron star with moderately strong magnetic field strength, which
populates only the electron's Landau levels, then in the -equilibrium
condition, the quark core is energetically much more unstable than the neutron
matter of identical physical condition.Comment: Six pages REVTEX file, one .eps file (included
Effect of hyperon-hyperon interaction on bulk viscosity and r-mode instability in neutron stars
We investigate the effect of hyperon matter including hyperon-hyperon
interaction on bulk viscosity. Equations of state are constructed within the
framework of a relativistic field theoretical model where baryon-baryon
interaction is mediated by the exchange of scalar and vector mesons.
Hyperon-hyperon interaction is also taken into account by the exchange of two
strange mesons. This interaction results in a smaller maximum mass neutron star
compared with the case without the interaction. The coefficient of bulk
viscosity due to the non-leptonic weak process is determined by these equations
of state. The interacting hyperon matter reduces the bulk viscosity coefficient
in a neutron star interior compared with the no interaction case. The r-mode
instability is more effectively suppressed in hyperon-hyperon interaction case
than that without the interaction.Comment: 25 pages, 10 figures; two new figures added and results and
discussion section revised; final version to appear in PR
Thermonuclear burst physics with RXTE
Recently we have made measurements of thermonuclear burst energetics and
recurrence times which are unprecedented in their precision, largely thanks to
the sensitivity of the Rossi X-ray Timing Explorer. In the "Clocked Burster",
GS 1826-24, hydrogen burns during the burst via the rapid-proton (rp) process,
which has received particular attention in recent years through theoretical and
modelling studies. The burst energies and the measured variation of alpha (the
ratio of persistent to burst flux) with accretion rate strongly suggests solar
metallicity in the neutron star atmosphere, although this is not consistent
with the corresponding variation of the recurrence time. Possible explanations
include extra heating between the bursts, or a change in the fraction of the
neutron star over which accretion takes place. I also present results from 4U
1746-37, which exhibits regular burst trains which are interrupted by "out of
phase" bursts.Comment: 4 pages, 2 figures, AIP conference proceedings format. To appear in
the proceedings of the "X-ray Timing 2003: Rossi and Beyond" meeting held in
Cambridge, MA, November, 200
High-Resolution X-ray Spectroscopy of the Interstellar Medium: Structure at the Oxygen Absorption Edge
(Abbrev.) We present high-resolution spectroscopy of the oxygen K-shell
interstellar absorption edge in 7 X-ray binaries using the HETGS onboard
Chandra. Using the brightest sources as templates, we found a best-fit model of
2 absorption edges and 5 Gaussian absorption lines. All of these features can
be explained by the recent predictions of K-shell absorption from neutral and
ionized atomic oxygen. We identify the K alpha and K beta absorption lines from
neutral oxygen, as well as the S=3/2 absorption edge. The expected S=1/2 edge
is not detected in these data due to overlap with instrumental features. We
also identify the K alpha absorption lines from singly and doubly ionized
oxygen. The OI K alpha absorption line is used as a benchmark with which to
adjust the absolute wavelength scale for theoretical predictions of the
absorption cross-sections. We find that shifts of 30-50 mA are required,
consistent with differences previously noticed from comparisons of the theory
with laboratory measurements. Significant oxygen features from dust or
molecular components, as suggested in previous studies, are not required by our
HETGS spectra. With these spectra, we can begin to measure the large-scale
properties of the ISM. We place a limit on the velocity dispersion of the
neutral lines of <200 km s^{-1}, consistent with measurements at other
wavelengths. We also make the first measurement of the oxygen ionization
fractions in the ISM. We constrain the interstellar ratio of OII/OI to ~0.1 and
the ratio of OIII/OI to <0.1.Comment: 12 pages, 8 figures, accepted for publication in the Astrophysical
Journal (Vol. 612, September 1 issue
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