11,137 research outputs found
Understanding the spectral and timing behavior of a newly discovered transient X-ray pulsar Swift J0243.6+6124
We present the results obtained from timing and spectral studies of the newly
discovered accreting X-ray binary pulsar Swift J0243.6+6124 using a NuSTAR
observation in 2017 October at a flux level of ~280 mCrab. Pulsations at
9.85423(5) s were detected in the X-ray light curves of the pulsar. Pulse
profiles of the pulsar were found to be strongly energy dependent. A broad
profile at lower energies was found to evolve into a double peaked profile in
30keV. The 3-79 keV continuum spectrum of the pulsar was well described
with a negative and positive exponential cutoff or high energy cutoff power law
models modified with a hot blackbody at 3 keV. An iron emission line was
also detected at 6.4 keV in the source spectrum. We did not find any signature
of cyclotron absorption line in our study. Results obtained from phase-resolved
and time-resolved spectroscopy are discussed in the paper.Comment: 7 pages, 6 figures, Accepted for publication in Monthly Notices of
the Royal Astronomical Society Journa
Finding NHIM: Identifying High Dimensional Phase Space Structures in Reaction Dynamics using Lagrangian Descriptors
Phase space structures such as dividing surfaces, normally hyperbolic
invariant manifolds, their stable and unstable manifolds have been an integral
part of computing quantitative results such as transition fraction, stability
erosion in multi-stable mechanical systems, and reaction rates in chemical
reaction dynamics. Thus, methods that can reveal their geometry in high
dimensional phase space (4 or more dimensions) need to be benchmarked by
comparing with known results. In this study, we assess the capability of one
such method called Lagrangian descriptor for revealing the types of high
dimensional phase space structures associated with index-1 saddle in
Hamiltonian systems. The Lagrangian descriptor based approach is applied to two
and three degree-of-freedom quadratic Hamiltonian systems where the high
dimensional phase space structures are known, that is as closed-form analytical
expressions. This leads to a direct comparison of features in the Lagrangian
descriptor plots and the phase space structures' intersection with an
isoenergetic two-dimensional surface and hence provides a validation of the
approach.Comment: 39 pages, 7 figures, Submitted to Communications in Nonlinear Science
and Numerical Simulatio
Towards developing robust algorithms for solving partial differential equations on MIMD machines
Methods for efficient computation of numerical algorithms on a wide variety of MIMD machines are proposed. These techniques reorganize the data dependency patterns to improve the processor utilization. The model problem finds the time-accurate solution to a parabolic partial differential equation discretized in space and implicitly marched forward in time. The algorithms are extensions of Jacobi and SOR. The extensions consist of iterating over a window of several timesteps, allowing efficient overlap of computation with communication. The methods increase the degree to which work can be performed while data are communicated between processors. The effect of the window size and of domain partitioning on the system performance is examined both by implementing the algorithm on a simulated multiprocessor system
Fracture mechanics analysis for various fiber/matrix interface loadings
Fiber/matrix (F/M) cracking was analyzed to provide better understanding and guidance in developing F/M interface fracture toughness tests. Two configurations, corresponding to F/M cracking at a broken fiber and at the free edge, were investigated. The effects of mechanical loading, thermal cooldown, and friction were investigated. Each configuration was analyzed for two loadings: longitudinal and normal to the fiber. A nonlinear finite element analysis was performed to model friction and slip at the F/M interface. A new procedure for fitting a square-root singularity to calculated stresses was developed to determine stress intensity factors (K sub I and K sub II) for a bimaterial interface crack. For the case of F/M cracking at a broken fiber with longitudinal loading, crack tip conditions were strongly influenced by interface friction. As a result, an F/M interface toughness test based on this case was not recommended because nonlinear data analysis methods would be required. For the free edge crack configuration, both mechanical and thermal loading caused crack opening, thereby avoiding frictional effects. A F/M interface toughness test based on this configuration would provide data for K(sub I)/K(sub II) ratios of about 0.7 and 1.6 for fiber and radial normal loading, respectively. However, thermal effects must be accounted for in the data analysis
Ply-level failure analysis of a graphite/epoxy laminate under bearing-bypass loading
A combined experimental and analytical study was conducted to investigate and predict the failure modes of a graphite/epoxy laminate subjected to combined bearing and bypass loading. Tests were conducted in a test machine that allowed the bearing-bypass load ratio to be controlled while a single-fastener coupon was loaded to failure in either tension or compression. Onset and ultimate failure modes and strengths were determined for each test case. The damage-onset modes were studied in detail by sectioning and micrographing the damaged specimens. A two-dimensional, finite-element analysis was conducted to determine lamina strains around the bolt hole. Damage onset consisted of matrix cracks, delamination, and fiber failures. Stiffness loss appeared to be caused by fiber failures rather than by matrix cracking and delamination. An unusual offset-compression mode was observed for compressive bearing-bypass laoding in which the specimen failed across its width along a line offset from the hole. The computed lamina strains in the fiber direction were used in a combined analytical and experimental approach to predict bearing-bypass diagrams for damage onset from a few simple tests
Combined bearing and bypass loading on a graphite/epoxy laminate
A combined experimental and analytical study was conducted to determine the behavior of a graphite/epoxy laminate subjected to combined bearing and bypass loading. Single-fastener quasi-isotropic specimens were loaded at various bearing-bypass ratios until damage was produced at the fastener hole. Damage-onset strengths and damage modes were then analyzed using local hole-boundary stresses calculated by a finite-element analysis. The tension data showed the expected linear interaction for combined bearing and bypass loading with damage developing in the net-section tension mode. However, the compression bearing-bypass strengths showed an unexpected interaction involving the bearing mode. Compressive bypass loads reduced the bearing strength by decreasing the bolt-hole contact arc and thus increasing the severity of the bearing loads. The bearing stresses at the hole boundary were not accurately estimated by superposition of the stress components for separate bearing and bypass loading. However, superposition produced reasonably accurate estimates for tangential stresses especially near the specimen net-section
Failure analysis of a graphite/epoxy laminate subjected to bolt bearing loads
Quasi-isotropic graphite/epoxy laminates (T300/5208) were tested under bolt bearing loads to study failure modes, strengths, and failure energy. Specimens had a range of configurations to produce failures by the three nominal failure modes: tension, shearout, and bearing. Radiographs were made after damage onset and after ultimate load to examine the failure modes. Also, the laminate stresses near the bolt hole calculated for each test specimen configuration, and then used with a failure criterion to analyze the test data. Failures involving extensive bearing damage were found to dissipate significantly more energy than tension dominated failures. The specimen configuration influenced the failure modes and therefore also influenced the failure energy. In the width-to-diameter ratio range of 4 to 5, which is typical of structural joints, a transition from the tension mode to the bearing mode was shown to cause a large increase in failure energy. The failure modes associated with ultimate strength were usually different from those associated with the damage onset. Typical damage sequences involved bearing damage onset at the hole boundary followed by tension damage progressing from the hole boundary
An Optically Plugged Quadrupole Trap for Bose-Einstein Condensates
We created sodium Bose-Einstein condensates in an optically plugged
quadrupole magnetic trap (OPT). A focused, 532nm laser beam repelled atoms from
the coil center where Majorana loss is significant. We produced condensates of
up to atoms, a factor of 60 improvement over previous work [1],
a number comparable to the best all-magnetic traps, and transferred up to atoms into a purely optical trap. Due to the tight axial
confinement and azimuthal symmetry of the quadrupole coils, the OPT shows
promise for creating Bose-Einstein condensates in a ring geometry
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