16,966 research outputs found
Bounds on Compactness for LMXB Neutron Stars from X-ray Burst Oscillations
We have modelled X-ray burst oscillations observed with the Rossi X-ray
Timing Explorer (RXTE) from two low mass X-ray binaries (LMXB): 4U 1636-53 with
a frequency of 580 Hz, and 4U 1728-34 at a frequency of 363 Hz. We have
computed least squares fits to the oscillations observed during the rising
phase of bursts using a model which includes emission from either a single
circular hot spot or a pair of circular antipodal hot spots on the surface of a
neutron star. We model the spreading of the thermonuclear hot spots by assuming
that the hot spot angular size grows linearly with time. We calculate the flux
as a function of rotational phase from the hot spots and take into account
photon deflection in the relativistic gravitational field of the neutron star
assuming the exterior spacetime is the Schwarzschild metric. We find acceptable
fits with our model and we use these to place constraints on the compactness of
the neutron stars in these sources. For 4U 1636-53, in which detection of a 290
Hz sub-harmonic supports the two spot model, we find that the compactness
(i.e., mass/radius ratio) is constrained to be M/R < 0.163 at 90% confidence (G
= c = 1). This requires a relatively stiff equation of state (EOS) for the
stellar interior. For example, if the neutron star has a mass of 1.4 Msun then
its radius must be > 12.8 km. Fits using a single hot spot model are not as
highly constraining. We discuss the implications of our findings for recent
efforts to calculate the EOS of dense nucleon matter and the structure of
neutron stars.Comment: 27 pages, 6 figures, AASTeX. Revised and expanded version.
Resubmitted to Astrophysical Journa
Single File Diffusion enhancement in a fluctuating modulated 1D channel
We show that the diffusion of a single file of particles moving in a
fluctuating modulated 1D channel is enhanced with respect to the one in a bald
pipe. This effect, induced by the fluctuations of the modulation, is favored by
the incommensurability between the channel potential modulation and the moving
file periodicity. This phenomenon could be of importance in order to optimize
the critical current in superconductors, in particular in the case where mobile
vortices move in 1D channels designed by adapted patterns of pinning sites.Comment: 4 pages, 4 figure
The lower Krishna Basin trajectory: relationships between basin development and downstream environmental degradation
River basin development / Lakes / Environmental degradation / Ecosystems / Mangroves / Water allocation / Groundwater / Water quality / Salinity / Irrigated farming / Institutions / Irrigation canals / Rural development
Improvements and modifications to the NASA microwave signature acquisition system
A user oriented description of the modified and upgraded Microwave Signature Acquisition System is provided. The present configuration of the sensor system and its operating characteristics are documented and a step-by-step operating procedure provides instruction for mounting the antenna truss assembly, readying the system for data acquisition, and for controlling the system during the data collection sequence. The resulting data products are also identified
Local Symmetries and Order-Disorder Transitions in Small Macroscopic Wigner Islands
The influence of local order on the disordering scenario of small Wigner
islands is discussed. A first disordering step is put in evidence by the time
correlation functions and is linked to individual excitations resulting in
configuration transitions, which are very sensitive to the local symmetries.
This is followed by two other transitions, corresponding to orthoradial and
radial diffusion, for which both individual and collective excitations play a
significant role. Finally, we show that, contrary to large systems, the focus
that is commonly made on collective excitations for such small systems through
the Lindemann criterion has to be made carefully in order to clearly identify
the relative contributions in the whole disordering process.Comment: 14 pages, 10 figure
Measuring Topological Chaos
The orbits of fluid particles in two dimensions effectively act as
topological obstacles to material lines. A spacetime plot of the orbits of such
particles can be regarded as a braid whose properties reflect the underlying
dynamics. For a chaotic flow, the braid generated by the motion of three or
more fluid particles is computed. A ``braiding exponent'' is then defined to
characterize the complexity of the braid. This exponent is proportional to the
usual Lyapunov exponent of the flow, associated with separation of nearby
trajectories. Measuring chaos in this manner has several advantages, especially
from the experimental viewpoint, since neither nearby trajectories nor
derivatives of the velocity field are needed.Comment: 4 pages, 6 figures. RevTeX 4 with PSFrag macro
Gapped Heisenberg spin chains in a field
We consider the fully anisotropic Heisenberg spin-1/2 antiferromagnet in a
uniform magnetic field, whose ground-state is characterized by broken spin
rotation symmetry and gapped spinon excitations. We expand on a recent
mean-field approach to the problem by incorporating fluctuations in a loop
expansion. Quantitative results for the magnetization, excitation gap and
specific heat are obtained. We compare our predictions with new DMRG and exact
diagonalization data and, for zero field, with the exact solution of the
spin chain from the Bethe Ansatz.Comment: 11 pages, 14 figure
Evolution of AQL X-1 During the Rising Phase of its 1998 Outburst
We present results from 16 snapshots of Aql X-1 with RXTE during the rising
phase of its recent outburst. The observations were carried out at a typical
rate of once or twice per day. The source shows interesting spectral evolution
during this period. Phenomenologically, it bears remarkable similarities to
``atoll'' sources. Shortly after the onset of the outburst, the source is seen
to be in an ``island'' state, but with little X-ray variability. It then
appears to have made a rapid spectral transition (on a time scale less than
half a day) to another ``island'' state, where it evolves slightly and stays
for 4 days. In this state, the observed X-ray flux becomes increasingly
variable as the source brightens. Quasi-period oscillation (QPO) in the X-ray
intensity is detected in the frequency range 670--870 Hz. The QPO frequency
increases with the X-ray flux while its fractional rms decreases. The QPO
becomes undetectable following a transition to a ``banana'' state, where the
source continues its evolution by moving up and down the ``banana'' branch in
the color-color diagram as the flux (presumably, the mass accretion rate)
fluctuates around the peak of the outburst. Throughout the entire period, the
power density spectrum is dominated by very-low frequency noises. Little power
can be seen above ~1 Hz, which is different from typical ``atoll'' sources. In
the ``banana'' state, the overall X-ray variability remains low (with
fractional rms ~3--4%) but roughly constant. The observed X-ray spectrum is
soft with few photons from above 25 keV, implying the thermal origin of
the emission. The evolution of both spectral and temporal X-ray properties is
discussed in the context of disk-instability models.Comment: 13 pages, including one table and five figures. To appear in ApJ
Letters (July 20
Infinite dimensional integrals beyond Monte Carlo methods: yet another approach to normalized infinite dimensional integrals
An approach to (normalized) infinite dimensional integrals, including
normalized oscillatory integrals, through a sequence of evaluations in the
spirit of the Monte Carlo method for probability measures is proposed. in this
approach the normalization through the partition function is included in the
definition. For suitable sequences of evaluations, the ("classical")
expectation values of cylinder functions are recoveredComment: Submitted as a communication in the ICMSQUARE conference, september
201
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