164,167 research outputs found

### The Angular Momenta of Neutron Stars and Black Holes as a Window on Supernovae

It is now clear that a subset of supernovae display evidence for jets and are
observed as gamma-ray bursts. The angular momentum distribution of massive
stellar endpoints provides a rare means of constraining the nature of the
central engine in core-collapse explosions. Unlike supermassive black holes,
the spin of stellar-mass black holes in X-ray binary systems is little affected
by accretion, and accurately reflects the spin set at birth. A modest number of
stellar-mass black hole angular momenta have now been measured using two
independent X-ray spectroscopic techniques. In contrast, rotation-powered
pulsars spin-down over time, via magnetic braking, but a modest number of natal
spin periods have now been estimated. For both canonical and extreme neutron
star parameters, statistical tests strongly suggest that the angular momentum
distributions of black holes and neutron stars are markedly different. Within
the context of prevalent models for core-collapse supernovae, the angular
momentum distributions are consistent with black holes typically being produced
in GRB-like supernovae with jets, and with neutron stars typically being
produced in supernovae with too little angular momentum to produce jets via
magnetohydrodynamic processes. It is possible that neutron stars are imbued
with high spin initially, and rapidly spun-down shortly after the supernova
event, but the available mechanisms may be inconsistent with some observed
pulsar properties.Comment: ApJ Letters, accepte

### Quantum wires from coupled InAs/GaAs strained quantum dots

The electronic structure of an infinite 1D array of vertically coupled
InAs/GaAs strained quantum dots is calculated using an eight-band
strain-dependent k-dot-p Hamiltonian. The coupled dots form a unique quantum
wire structure in which the miniband widths and effective masses are controlled
by the distance between the islands, d. The miniband structure is calculated as
a function of d, and it is shown that for d>4 nm the miniband is narrower than
the optical phonon energy, while the gap between the first and second minibands
is greater than the optical phonon energy. This leads to decreased optical
phonon scattering, providing improved quantum wire behavior at high
temperatures. These miniband properties are also ideal for Bloch oscillation.Comment: 5 pages revtex, epsf, 8 postscript figure

### A high-speed spectrograph shutter

Device can operate in close-open-close mode. Beam splitter placed behind static-slit assembly allows use of more than one camera. Each frame in particular series may be conveniently varied in exposure time and spacing. This can be done independent of other frames in the series. In ''open'' position, shutter transmits light over wide wavelength range

### Giant Antiferromagnetically Coupled Moments in a Molecule-Based Magnet with Interpenetrating Lattices

The molecule-based magnet [Ru$_2$(O$_2$CMe)$_4$]$_3$[Cr(CN)$_6$] contains two
weakly-coupled, interpenetrating sublattices in a body-centered cubic
structure. Although the field-dependent magnetization indicates a metamagnetic
transition from an antiferromagnet to a paramagnet, the hysteresis loop also
exhibits a substantial magnetic remanance and coercive field uncharacteristic
of a typical metamagnet. We demonstrate that this material behaves like two
giant moments with a weak antiferromagnetic coupling and a large energy barrier
between the orientations of each moment. Because the sublattice moments only
weakly depend on field in the transition region, the magnetic correlation
length can be directly estimated from the magnetization.Comment: 3 figure

### Second order parameter-uniform convergence for a finite difference method for a singularly perturbed linear reaction-diffusion system

A singularly perturbed linear system of second order ordinary differential
equations of reaction-diffusion type with given boundary conditions is
considered. The leading term of each equation is multiplied by a small positive
parameter. These singular perturbation parameters are assumed to be distinct.
The components of the solution exhibit overlapping layers. Shishkin
piecewise-uniform meshes are introduced, which are used in conjunction with a
classical finite difference discretisation, to construct a numerical method for
solving this problem. It is proved that the numerical approximations obtained
with this method is essentially second order convergent uniformly with respect
to all of the parameters

### Experimental investigation of leading-edge thrust at supersonic speeds

Wings, designed for leading edge thrust at supersonic speeds, were investigated in the Unitary Plan Wind Tunnel at Mach numbers of 1.60, 1.80, 2.00, 2.16, and 2.36. Experimental data were obtained on a uncambered wing which had three interchangeable leading edges that varied from sharp to blunt. The leading edge thrust concept was evaluated. Results from the investigation showed that leading edge flow separation characteristics of all wings tested agree well with theoretical predictions. The experimental data showed that significant changes in wing leading edge bluntness did not affect the zero lift drag of the uncambered wings

### Identifying the Higgs Spin and Parity in Decays to Z Pairs

Higgs decays to Z boson pairs may be exploited to determine spin and parity
of the Higgs boson, a method complementary to spin-parity measurements in
Higgs-strahlung. For a Higgs mass above the on-shell ZZ decay threshold, a
model-independent analysis can be performed, but only by making use of
additional angular correlation effects in gluon-gluon fusion at the LHC and
gamma-gamma fusion at linear colliders. In the intermediate mass range, in
which the Higgs boson decays into pairs of real and virtual Z bosons, threshold
effects and angular correlations, parallel to Higgs-strahlung, may be adopted
to determine spin and parity, though high event rates will be required for the
analysis in practice.Comment: 14 pages, 2 postscript figure

### A Comparison of Intermediate Mass Black Hole Candidate ULXs and Stellar-Mass Black Holes

Cool thermal emission components have recently been revealed in the X-ray
spectra of a small number of ultra-luminous X-ray (ULX) sources with L_X > 1
E+40 erg/s in nearby galaxies. These components can be well fitted with
accretion disk models, with temperatures approximately 5-10 times lower than
disk temperatures measured in stellar-mass Galactic black holes when observed
in their brightest states. Because disk temperature is expected to fall with
increasing black hole mass, and because the X-ray luminosity of these sources
exceeds the Eddington limit for 10 Msun black holes (L_Edd = 1.3 E+39 erg/s),
these sources are extremely promising intermediate-mass black hole candidates
(IMBHCs). In this Letter, we directly compare the inferred disk temperatures
and luminosities of these ULXs, with the disk temperatures and luminosities of
a number of Galactic black holes. The sample of stellar-mass black holes was
selected to include different orbital periods, companion types, inclinations,
and column densities. These ULXs and stellar-mass black holes occupy distinct
regions of a L_X -- kT diagram, suggesting these ULXs may harbor IMBHs. We
briefly discuss the important strengths and weaknesses of this interpretation.Comment: 4 pages, 2 color figures, uses emulateapj.sty and apjfonts.sty, subm.
to ApJ

### Gravitational Radiation from Black Hole Binaries in Globular Clusters

A populations of stellar mass black hole binaries may exist in globular
clusters. The dynamics of globular cluster evolution imply that there may be at
most one black hole binary is a globular cluster. The population of binaries
are expected to have orbital periods greater than a few hours and to have a
thermal distribution of eccentricities. In the LISA band, the gravitational
wave signal from these binaries will consist of several of the higher harmonics
of the orbital frequency. A Monte Carlo simulation of the galactic globular
cluster system indicates that LISA will detect binaries in 10 % of the clusters
with an angular resolution sufficient to identify the host cluster of the
binary.Comment: 7 pages, 2 eps figures, uses iopart styl

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