4,939 research outputs found
Modeling the non-recycled Fermi gamma-ray pulsar population
We use Fermi Gamma-ray Space Telescope detections and upper limits on
non-recycled pulsars obtained from the Large Area Telescope (LAT) to constrain
how the gamma-ray luminosity L depends on the period P and the period
derivative \dot{P}. We use a Bayesian analysis to calculate a best-fit
luminosity law, or dependence of L on P and \dot{P}, including different
methods for modeling the beaming factor. An outer gap (OG) magnetosphere
geometry provides the best-fit model, which is L \propto P^{-a} \dot{P}^{b}
where a=1.36\pm0.03 and b=0.44\pm0.02, similar to but not identical to the
commonly assumed L \propto \sqrt{\dot{E}} \propto P^{-1.5} \dot{P}^{0.5}. Given
upper limits on gamma-ray fluxes of currently known radio pulsars and using the
OG model, we find that about 92% of the radio-detected pulsars have gamma-ray
beams that intersect our line of sight. By modeling the misalignment of radio
and gamma-ray beams of these pulsars, we find an average gamma-ray beaming
solid angle of about 3.7{\pi} for the OG model, assuming a uniform beam. Using
LAT-measured diffuse fluxes, we place a 2{\sigma} upper limit on the average
braking index and a 2{\sigma} lower limit on the average surface magnetic field
strength of the pulsar population of 3.8 and 3.2 X 10^{10} G, respectively. We
then predict the number of non-recycled pulsars detectable by the LAT based on
our population model. Using the two-year sensitivity, we find that the LAT is
capable of detecting emission from about 380 non-recycled pulsars, including
150 currently identified radio pulsars. Using the expected five-year
sensitivity, about 620 non-recycled pulsars are detectable, including about 220
currently identified radio pulsars. We note that these predictions
significantly depend on our model assumptions.Comment: 26 pages, 10 figures, Accepted by ApJ on 8 September 201
Theory of Bose-Einstein condensation for trapped atoms
We outline the general features of the conventional mean-field theory for the
description of Bose-Einstein condensates at near zero temperatures. This
approach, based on a phenomenological model, appears to give excellent
agreement with experimental data. We argue, however, that such an approach is
not rigorous and cannot contain the full effect of collisional dynamics due to
the presence of the mean-field. We thus discuss an alternative microscopic
approach and explain, within our new formalism, the physical origin of these
effects. Furthermore, we discuss the potential formulation of a consistent
finite-temperature mean-field theory, which we claim necessiates an analysis
beyond the conventional treatment.Comment: 12 pages. To appear in Phil. Trans. R. Soc. Lond. A 355 (1997
Just how long can you live in a black hole and what can be done about it?
We study the problem of how long a journey within a black hole can last.
Based on our observations, we make two conjectures. First, for observers that
have entered a black hole from an asymptotic region, we conjecture that the
length of their journey within is bounded by a multiple of the future
asymptotic ``size'' of the black hole, provided the spacetime is globally
hyperbolic and satisfies the dominant-energy and non-negative-pressures
conditions. Second, for spacetimes with Cauchy surfaces (or an
appropriate generalization thereof) and satisfying the dominant energy and
non-negative-pressures conditions, we conjecture that the length of a journey
anywhere within a black hole is again bounded, although here the bound requires
a knowledge of the initial data for the gravitational field on a Cauchy
surface. We prove these conjectures in the spherically symmetric case. We also
prove that there is an upper bound on the lifetimes of observers lying ``deep
within'' a black hole, provided the spacetime satisfies the
timelike-convergence condition and possesses a maximal Cauchy surface. Further,
we investigate whether one can increase the lifetime of an observer that has
entered a black hole, e.g., by throwing additional matter into the hole.
Lastly, in an appendix, we prove that the surface area of the event horizon
of a black hole in a spherically symmetric spacetime with ADM mass
is always bounded by , provided
that future null infinity is complete and the spacetime is globally hyperbolic
and satisfies the dominant-energy condition.Comment: 20 pages, REVTeX 3.0, 6 figures included, self-unpackin
Recommended from our members
In Search of the Solar Wind Nitrogen Isotope Composition: Analysis of a Gold Plate from the Genesis Spacecraft Concentrator
We report N isotope analysis of a gold plate from the Genesis spacecraft concentrator. We did not find evidence for a light N component in the solar wind
Fermi Large Area Telescope Fourth Source Catalog Data Release 4 (4FGL-DR4)
We present an incremental version (4FGL-DR4, for Data Release 4) of the
fourth Fermi-LAT catalog of gamma-ray sources. Based on the first 14 years of
science data in the energy range from 50 MeV to 1 TeV, it uses the same
analysis methods as the 4FGL-DR3 catalog did for 12 years of data, with only a
few improvements. The spectral parameters, spectral energy distributions, light
curves and associations are updated for all sources.
We add four new extended sources and modify two existing ones. Among the 6658
4FGL-DR3 sources, we delete 14 and change the localization of 10, while 26 are
newly associated and two associations were changed. We add 546 point sources,
among which 8 are considered identified and 228 have a plausible counterpart at
other wavelengths. Most are just above the detection threshold, and 14 are
transient sources below the detection threshold that can affect the light
curves of nearby sources.Comment: Data files at
https://fermi.gsfc.nasa.gov/ssc/data/access/lat/14yr_catalog/. Refereed paper
is DOI 10.3847/1538-4365/ac675
Temporal coherence, anomalous moments, and pairing correlations in the classical-field description of a degenerate Bose gas
The coherence properties of degenerate Bose gases have usually been expressed
in terms of spatial correlation functions, neglecting the rich information
encoded in their temporal behavior. In this paper we show, using a Hamiltonian
classical-field formalism, that temporal correlations can be used to
characterize familiar properties of a finite-temperature degenerate Bose gas.
The temporal coherence of a Bose-Einstein condensate is limited only by the
slow diffusion of its phase, and thus the presence of a condensate is indicated
by a sharp feature in the temporal power spectrum of the field. We show that
the condensate mode can be obtained by averaging the field for a short time in
an appropriate phase-rotating frame, and that for a wide range of temperatures,
the condensate obtained in this approach agrees well with that defined by the
Penrose-Onsager criterion based on one-body (spatial) correlations. For time
periods long compared to the phase diffusion time, the field will average to
zero, as we would expect from the overall U(1) symmetry of the Hamiltonian. We
identify the emergence of the first moment on short time scales with the
concept of U(1) symmetry breaking that is central to traditional mean-field
theories of Bose condensation. We demonstrate that the short-time averaging
procedure constitutes a general analog of the 'anomalous' averaging operation
of symmetry-broken theories by calculating the anomalous thermal density of the
field, which we find to have form and temperature dependence consistent with
the results of mean-field theories.Comment: 11 pages, 6 figures. v3: Final version. Typos fixed, and other minor
change
The cutting edge - Micro-CT for quantitative toolmark analysis of sharp force trauma to bone
Toolmark analysis involves examining marks created on an object to identify the likely tool responsible for creating those marks (e.g., a knife). Although a potentially powerful forensic tool, knife mark analysis is still in its infancy and the validation of imaging techniques as well as quantitative approaches is ongoing. This study builds on previous work by simulating real-world stabbings experimentally and statistically exploring quantitative toolmark properties, such as cut mark angle captured by micro-CT imaging, to predict the knife responsible. In Experiment 1 a mechanical stab rig and two knives were used to create 14 knife cut marks on dry pig ribs. The toolmarks were laser and micro-CT scanned to allow for quantitative measurements of numerous toolmark properties. The findings from Experiment 1 demonstrated that both knives produced statistically different cut mark widths, wall angle and shapes. Experiment 2 examined knife marks created on fleshed pig torsos with conditions designed to better simulate real-world stabbings. Eight knives were used to generate 64 incision cut marks that were also micro-CT scanned. Statistical exploration of these cut marks suggested that knife type, serrated or plain, can be predicted from cut mark width and wall angle. Preliminary results suggest that knives type can be predicted from cut mark width, and that knife edge thickness correlates with cut mark width. An additional 16 cut marks walls were imaged for striation marks using scanning electron microscopy with results suggesting that this approach might not be useful for knife mark analysis. Results also indicated that observer judgements of cut mark shape were more consistent when rated from micro-CT images than light microscopy images. The potential to combine micro-CT data, medical grade CT data and photographs to develop highly realistic virtual models for visualisation and 3D printing is also demonstrated. This is the first study to statistically explore simulated real-world knife marks imaged by micro-CT to demonstrate the potential of quantitative approaches in knife mark analysis. Findings and methods presented in this study are relevant to both forensic toolmark researchers as well as practitioners. Limitations of the experimental methodologies and imaging techniques are discussed, and further work is recommended
Interference of Bose-Einstein condensates in momentum space
We suggest an experiment to investigate the linear superposition of two
spatially separated Bose-Einstein condensates. Due to the coherent combination
of the two wave functions, the dynamic structure factor, measurable through
inelastic photon scattering at high momentum transfer , is predicted to
exhibit interference fringes with frequency period where
is the distance between the condensates. We show that the coherent
configuration corresponds to an eigenstate of the physical observable measured
in the experiment and that the relative phase of the condensates is hence
created through the measurement process.Comment: 4 pages and 2 eps figure
Reexamining evidence-based practice in community corrections: beyond 'a confined view' of what works
This article aims to reexamine the development and scope of evidence-based practice (EBP) in community corrections by exploring three sets of issues. Firstly, we examine the relationships between the contested purposes of community supervision and their relationships to questions of evidence. Secondly, we explore the range of forms of evidence that might inform the pursuit of one purpose of supervision—the rehabilitation of offenders—making the case for a fuller engagement with “desistance” research in supporting this process. Thirdly, we examine who can and should be involved in conversations about EBP, arguing that both ex/offenders’ and practitioners’ voices need to be respected and heard in this debate
- …