142,844 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
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
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
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
Transmission Oscillator Ultrasonic Spectrometer (TOUS): A new research instrument
TOUS is capable for measuring very small changes in acoustic attenuation and phase velocity. Its high sensitivity to small changes in ultrasonic absorption results in part from operation under marginal conditions. In spite of high sensitivity, TOUS system is relatively simple, inexpensive, and compact
X-ray emission from the Ultramassive Black Hole candidate NGC1277: implications and speculation on its origin
We study the X-ray emission from NGC1277, a galaxy in the core of the Perseus
cluster, for which van den Bosch et al. have recently claimed the presence of
an UltraMassive Black Hole (UMBH) of mass 1.7 times 10^10 Msun, unless the IMF
of the stars in the stellar bulge is extremely bottom heavy. The X-rays
originate in a power-law component of luminosity 1.3 times 10^40 erg/s embedded
in a 1 keV thermal minicorona which has a half-light radius of about 360 pc,
typical of many early-type galaxies in rich clusters of galaxies. If Bondi
accretion operated onto the UMBH from the minicorona with a radiative
efficiency of 10 per cent, then the object would appear as a quasar with
luminosity 10^46 erg/s, a factor of almost 10^6 times higher than observed. The
accretion flow must be highly radiatively inefficient, similar to past results
on M87 and NGC3115. The UMBH in NGC1277 is definitely not undergoing any
significant growth at the present epoch. We note that there are 3 UMBH
candidates in the Perseus cluster and that the inferred present mean mass
density in UMBH could be 10^5 Msun/Mpc^3, which is 20 to 30 per cent of the
estimated mean mass density of all black holes. We speculate on the implied
growth of UMBH and their hosts, and discuss the possibiity that extreme AGN
feedback could make all UMBH host galaxies have low stellar masses at redshifts
around 3. Only those which end up at the centres of groups and clusters later
accrete large stellar envelopes and become Brightest Cluster Galaxies. NGC1277
and the other Perseus core UMBH, NGC1270, have not however been able to gather
more stars or gas owing to their rapid orbital motion in the cluster core.Comment: 5 pages, 4 figures, MNRAS in pres
Giant Antiferromagnetically Coupled Moments in a Molecule-Based Magnet with Interpenetrating Lattices
The molecule-based magnet [Ru(OCMe)][Cr(CN)] 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
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
Automatic-repeat-request error control schemes
Error detection incorporated with automatic-repeat-request (ARQ) is widely used for error control in data communication systems. This method of error control is simple and provides high system reliability. If a properly chosen code is used for error detection, virtually error-free data transmission can be attained. Various types of ARQ and hybrid ARQ schemes, and error detection using linear block codes are surveyed
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