3,631 research outputs found
Testing the transition layer model of quasi-periodic oscillations in neutron star X-ray binarie
We compare the theoretical predictions of the transition layer model with
some observational features of quasi-periodic oscillations (QPOs) in neutron
star X-ray binaries. We found that the correlation between horizontal branch
oscillation (HBO) frequencies and kilohertz (kHz) QPO frequencies, the
difference between the low-frequency QPOs in atoll sources and HBOs in Z
sources, and the correlation between the frequencies of low-frequency QPOs and
break frequencies can be well explained by the transition layer model, provided
the neutron star mass is around 1.4 solar mass and the angle between
magnetosphere equator and accretion disk plane is around 6 degree. The observed
decrease of peak separation between two kHz QPO frequencies with the increase
of kHz QPO frequencies and the increase of QPO frequencies with the increase of
inferred mass accretion rate are also consistent with the theoretical
predictions of transition layer model. In addition, we derive a simple equation
that can be adopted to estimate the angle () between magnetosphere
equator and accretion disk plane by use of the simultaneously observed QPO
frequency data. We estimate these angles, in the range of 4 to 8 degrees, for
five Z sources and two atoll sources. The nearly constant value for
each source, derived from the different sets of simultaneously observed QPO
frequency data, provides a strong test of the theoretical model. Finally, we
suggest that the similar transition layer oscillations may be also responsible
for the observed QPOs in accretion-powered millisecond X-ray pulsar and
Galactic black hole candidates.Comment: 10 pages, 5 figures, to appear in ApJ, Vol. 55
Advanced sine wave modulation of continuous wave laser system for atmospheric CO2 differential absorption measurements
In this theoretical study, modulation techniques are developed to support the
Active Sensing of CO2 Emissions over Nights, Days, and Seasons (ASCENDS)
mission. A CW lidar system using sine waves modulated by ML pseudo random noise
codes is described for making simultaneous online/offline differential
absorption measurements. Amplitude and Phase Shift Keying (PSK) modulated IM
carriers, in addition to a hybrid pulse technique are investigated that exhibit
optimal autocorrelation properties. A method is presented to bandwidth limit
the ML sequence based on a filter implemented in terms of Jacobi theta
functions that does not significantly degrade the resolution or introduce side
lobes as a means of reducing aliasing and IM carrier bandwidth.Comment: Accepted for publication in Applied Optic
Chandra View of DA 530: A Sub-Energetic Supernova Remnant with a Pulsar Wind Nebula?
Based on a Chandra ACIS observation, we report the detection of an extended
X-ray feature close to the center of the remnant DA 530 with 5.3 sigma above
the background within a circle of 20'' radius. This feature, characterized by a
power-law with the photon index gamma=1.6+-0.8 and spatially coinciding with a
nonthermal radiosource, most likely represents a pulsar wind nebula. We have
further examined the spectrum of the diffuse X-ray emission from the remnant
interior with a background-subtracted count rate of ~0.06 counts s^-1 in
0.3-3.5 keV. The spectrum of the emission can be described by a thermal plasma
with a temperature of ~0.3-0.6 keV and a Si over-abundance of >~7 solar. These
spectral characteristics, together with the extremely low X-ray luminosity,
suggest that the remnant arises from a supernova with an anomalously low
mechanical energy (<10^50 ergs). The centrally-filled thermal X-ray emission of
the remnant may indicate an early thermalization of the SN ejecta by the
circum-stellar medium. Our results suggest that the remnant is likely the
product of a core-collapsed SN with a progenitor mass of 8-12 Msun. Similar
remnants are probably common in the Galaxy, but have rarely been studied.Comment: 23 pages, 7 figures, accepted for publication in ApJ; complete the
abstract on astro-ph and correct some typo
Modulated Sine Waves for Differential Absorption Measurements Using a CW Laser System
A continuous wave Light Detection and Ranging (CW LiDAR) system utilizes two or more laser frequencies and time or range shifted pseudorandom noise (PN) codes to discriminate between the laser frequencies. The performance of these codes can be improved by subtracting out the bias before processing. The CW LiDAR system may be mounted to an artificial satellite orbiting the earth, and the relative strength of the return signal for each frequency can be utilized to determine the concentration of selected gases or other substances in the atmosphere
Comparison of eta and eta' production in the pp -> pp eta(eta') reactions near threshold
The total cross section of the pp -> pp eta' reaction has been measured at
two energies near threshold by detecting the final protons in a magnetic
spectrometer. The values obtained are about a factor of 70 less than for the
corresponding eta production, in good agreement with the predictions of a
one-pion-exchange model.Comment: 10 pages, Latex with 3 eps figure
Linearly polarized X-ray flares following short gamma-ray bursts
Soft X-ray flares were detected to follow the short-duration gamma-ray burst
GRB 050724. The temporal properties of the flares suggest that they are likely
due to the late time activity of the central engine. We argue that if short
GRBs are generated through compact star mergers, as is supported by the recent
observations, the jet powering the late X-ray flares must be launched via
magnetic processes rather than via neutrino-antineutrino annihilations. As a
result, the X-ray flares following short GRBs are expected to be linearly
polarized. The argument may also apply to the X-ray flares following long GRBs.
Future observations with the upcoming X-ray polarimeters will test this
prediction.Comment: 4 pages (no figure), accepted for publication in ApJL, typos
correcte
Excesses in the Cosmic Ray Spectrum and Possible Interpretations
The data collected by ATIC, PPB-BETS, FERMI-LAT and HESS all indicate that
there is an electron/positron excess in the cosmic ray energy spectrum above
100 GeV, although different instrumental teams do not agree on the
detailed spectral shape. PAMELA also reported a clear excess feature of the
positron fraction above several GeV, but no excess in anti-protons. Here we
review the observational status and theoretical models of this interesting
observational feature. We pay special attention to various physical
interpretations proposed in the literature, including modified supernova
remnant models for the background, new astrophysical sources, and new
physics (the dark matter models). We suggest that although most models can make
a case to interpret the data, with the current observational constraints the
dark matter interpretations, especially those invoking annihilation, require
much more exotic assumptions than some astrophysical interpretations. Future
observations may present some ``smoking-gun'' observational tests to
differentiate among different models and to identify the correct interpretation
to the phenomenon.Comment: 48 pages, including 10 figures and 1 tabel. Invited review to be
published in IJMP
The Wide-Field X and Gamma-Ray Telescope ECLAIRs aboard the Gamma-Ray Burst Multi-Wavelength Space Mission SVOM
The X and Gamma-ray telescope ECLAIRs is foreseen to be launched on a low
Earth orbit (h=630 km, i=30 degrees) aboard the SVOM satellite (Space-based
multi-band astronomical Variable Objects Monitor), a French-Chinese mission
with Italian contribution. Observations are expected to start in 2013. It has
been designed to detect and localize Gamma-Ray Bursts (GRBs) or persistent
sources of the sky, thanks to its wide field of view (about 2 sr) and its
remarkable sensitivity in the 4-250 keV energy range, with enhanced imaging
sensitivity in the 4-70 keV energy band. These characteristics are well suited
to detect highly redshifted GRBs, and consequently to provide fast and accurate
triggers to other onboard or ground-based instruments able to follow-up the
detected events in a very short time from the optical wavelength bands up to
the few MeV Gamma-Ray domain.Comment: Proccedings of the "2008 Nanjing GRB Conference", June 23-27 2008,
Nanjing, Chin
An experimental and numerical investigation into damage mechanisms in tapered laminates under tensile loading
Through-thickness thickness reductions in laminated composites are essential for weight and aerodynamic efficiency, but they can also be the site of damage initiation. Tensile failure mechanisms of modestly tapered laminates, loaded via gripping their thick and thin ends, and a severely tapered laminate, loaded by contact at its tapered section, were investigated via experiments and high-fidelity finite element modelling. The primary failure mode is by delamination, initiated from a terminated ply in the tapered region, which is quite sensitive to a small delamination defect at a ply drop location. Experimental measurements and FE predictions correlate very well in all cases for both location and load. In the severely tapered case, the contact stress influences the delamination and so a fibre failure criterion also needs to be considered, including both the fibre direction tensile stress and its interaction with through-thickness direct and shear stresses imposed by the contact
Near-Threshold Production of omega Mesons in the pp -> pp omega Reaction
The total cross section for omega production in the pp -> pp omega reaction
has been measured at five c.m. excess energies from 3.8 to 30 MeV. The energy
dependence is easily understood in terms of a strong proton-proton final state
interaction combined with a smearing over the width of the state. The ratio of
near-threshold phi and omega production is consistent with the predictions of a
one-pion-exchange model and the degree of violation of the OZI rule is similar
to that found in the pi-p -> n omega/phi reactions.Comment: Report in LaTeX2e. 12 pages with 2 eps figure
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