987 research outputs found
The energy dependence of burst oscillations from the accreting millisecond pulsar XTE J1814-338
The nature of the asymmetry that gives rise to Type I X-ray burst
oscillations on accreting neutron stars remains a matter of debate. Of
particular interest is whether the burst oscillation mechanism differs between
the bursting millisecond pulsars and the non-pulsing systems. One means to
diagnose this is to study the energy dependence of the burst oscillations: here
we present an analysis of oscillations from 28 bursts observed during the 2003
outburst of the accreting millisecond pulsar XTE J1814-338. We find that the
fractional amplitude of the burst oscillations falls with energy, in contrast
to the behaviour found by Muno et al. (2003) in the burst oscillations from a
set of non-pulsing systems. The drop with energy mirrors that seen in the
accretion-powered pulsations; in this respect XTE J1814-338 behaves like the
other accreting millisecond pulsars. The burst oscillations show no evidence
for either hard or soft lags, in contrast to the persistent pulsations, which
show soft lags of up to 50 s. The fall in amplitude with energy is
inconsistent with current surface mode and simple hot spot models of burst
oscillations. We discuss improvements to the models and uncertainties in the
physics that might resolve these issues.Comment: Some amendments to the discussion section, accepted for publication
in MNRA
Analysis of variability in the burst oscillations of the accreting millisecond pulsar XTE J1814-338
The accreting millisecond pulsar XTE J1814-338 exhibits oscillations at the
known spin frequency during Type I X-ray bursts. The properties of the burst
oscillations reflect the nature of the thermal asymmetry on the stellar
surface. We present an analysis of the variability of the burst oscillations of
this source, focusing on three characteristics: fractional amplitude, harmonic
content and frequency. Fractional amplitude and harmonic content constrain the
size, shape and position of the emitting region, whilst variations in frequency
indicate motion of the emitting region on the neutron star surface. We examine
both long-term variability over the course of the outburst, and short-term
variability during the bursts. For most of the bursts, fractional amplitude is
consistent with that of the accretion pulsations, implying a low degree of fuel
spread. There is however a population of bursts whose fractional amplitudes are
substantially lower, implying a higher degree of fuel spread, possibly forced
by the explosive burning front of a precursor burst. For the first harmonic,
substantial differences between the burst and accretion pulsations suggest that
hotspot geometry is not the only mechanism giving rise to harmonic content in
the latter. Fractional amplitude variability during the bursts is low; we
cannot rule out the hypothesis that the fractional amplitude remains constant
for bursts that do not exhibit photospheric radius expansion (PRE). There are
no significant variations in frequency in any of the bursts except for the one
burst that exhibits PRE. This burst exhibits a highly significant but small
(Hz) drop in frequency in the burst rise. The timescale of the
frequency shift is slower than simple burning layer expansion models predict,
suggesting that other mechanisms may be at work.Comment: 20 pages, 20 figures, accepted for publication in ApJ. Uses
emulateapj.cl
A comment on Liu and Yau's positive quasi-local mass
Liu and Yau (Phys.Rev.Lett. 90, 231102, 2003) propose a definition of quasi-local mass for any space-like, topological 2-sphere with positive Gauss curvature (and subject to a second, convexity, condition). They are able to show it is positive using a result of Shi and Tam (J.Diff.Geom. 62, 79, 2002). However, as we show here, their definition can give a strictly positive mass for a sphere in flat space
Scalar--flat K\"ahler metrics with conformal Bianchi V symmetry
We provide an affirmative answer to a question posed by Tod \cite{Tod:1995b},
and construct all four-dimensional Kahler metrics with vanishing scalar
curvature which are invariant under the conformal action of Bianchi V group.
The construction is based on the combination of twistor theory and the
isomonodromic problem with two double poles. The resulting metrics are
non-diagonal in the left-invariant basis and are explicitly given in terms of
Bessel functions and their integrals. We also make a connection with the LeBrun
ansatz, and characterise the associated solutions of the SU(\infty) Toda
equation by the existence a non-abelian two-dimensional group of point
symmetries.Comment: Dedicated to Maciej Przanowski on the occasion of his 65th birthday.
Minor corrections. To appear in CQ
Isotropic cosmological singularities: other matter models
Isotropic cosmological singularities are singularities which can be removed
by rescaling the metric. In some cases already studied (gr-qc/9903008,
gr-qc/9903009, gr-qc/9903018) existence and uniqueness of cosmological models
with data at the singularity has been established. These were cosmologies with,
as source, either perfect fluids with linear equations of state or massless,
collisionless particles. In this article we consider how to extend these
results to a variety of other matter models. These are scalar fields, massive
collisionless matter, the Yang-Mills plasma of Choquet-Bruhat, or matter
satisfying the Einstein-Boltzmann equation.Comment: LaTeX, 19 pages, no figure
Design of Covalent Organic Frameworks for Methane Storage
We designed 14 new covalent organic frameworks (COFs), which are expected to adsorb large amounts of methane (CH_4) at 298 K and up to 300 bar. We have calculated their delivery uptake using grand canonical Monte Carlo (GCMC) simulations. We also report their thermodynamic stability based on 7.5 ns molecular dynamics simulations. Two new frameworks, COF-103-Eth-trans and COF-102-Ant, are found to exceed the DOE target of 180 v(STP)/v at 35 bar for methane storage. Their performance is comparable to the best previously reported materials: PCN-14 and Ni-MOF-74. Our results indicate that using thin vinyl bridging groups aid performance by minimizing the interaction methane-COF at low pressure. This is a new feature that can be used to enhance loading in addition to the common practice of adding extra fused benzene rings. Most importantly, this report shows that pure nonbonding interactions, van der Waals (vdW) and electrostatic forces in light elements (C, O, B, H, and Si), can rival the enhancement in uptake obtained for microporous materials derived from early transition metals
The 2004 Hyperflare from SGR 1806-20: Further Evidence for Global Torsional Vibrations
We report an analysis of the archival Rossi X-ray Timing Explorer (RXTE) data from the December 2004 hyperflare from SGR 1806-20. In addition to the approx. equal to 90 Hz QPO first discovered by Israel et al., we report the detection of higher frequency oscillations at approx. equal to 150, 625, and 1,835 Hz. In addition to these frequencies there are indications of oscillations at approx. equal to 720, and 2,384 Hz, but with lower significances. The 150 Hz QPO has a width (FWHM) of about 17 Hz, an average amplitude (rms) of 6.5%, and is detected in average power spectra centered on the rotational phase of the strongest peak in the pulse profile. This is approximately half a rotational cycle from the phase at which the 90 Hz QPO is strongly detected. The 625 Hz oscillation was first detected in an average power spectrum from nine successive cycles beginning approximately 180 s after the initial hard spike. It has a width (FWHM) of approx. equal to 2 Hz and an average amplitude (rms) during this interval of 9%. We find a strong detection of the 625 Hz oscillation in a pair of successive rotation cycles beginning about 230 s after the start of the flare. In these cycles we also detect the 1,835 Hz QPO with the 625 Hz oscillation. The rotational phase in which the 625 Hz &PO is detected is similar to that for the 90 Hz QPO, indeed, this feature is seen in the same average power spectrum. During the time the 625 Hz QPO is detected we also confirm the simultaneous presence of 30 and 92 Hz QPOs, first reported by Israel et al. The centroid frequency of the 625 Hz QPO detected with RXTE is within 1 Hz of the M 626 Hz oscillation recently found in RHESSI data from this hyperflare by Watts & Strohmayer, however, the two detections were made in different phase and energy intervals. Nevertheless, we argue that the two results likely represent detections of the same oscillation frequency intrinsic to the source, but we comment on some of the difficulties in making direct comparisons between the RXTE and RHESSI measurement
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