137,445 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
Trends and issues in the Medicaid 1915(c) waiver program.
Over the past 15 years, Medicaid 1915(c) home and community-based waivers have made a substantial contribution to States' efforts to transform their long-term care (LTC) systems from largely institutional to community-based systems. By 1997, every State had implemented a waiver program for at least some subgroups of individuals with disabilities, and expenditures increased from 8.1 billion in 1997. Emerging, as well as long-standing, policy issues related to the waiver program include concerns with access, variation in availability by disability group, State decisions related to the provision of community-based LTC, and evidence on effectiveness
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
Rayleigh lidar observations of gravity wave activity in the stratosphere and lower mesosphere
Forty-two monochromatic gravity wave events were observed in the 25 to 55 km altitude region during 16 nights of Rayleigh lidar measurements at Poker Flat, Alaska and Urbana, Illinois. The measured wave parameters were compared to previous radar and lidar measurements of gravity wave activity. Vertical wavelengths, lambda(z), between 2 and 11.5 km with vertical phase velocities, c(z), between 0.1 and 1 m/s were observed. Measured values of lambda(z) and c(z) were used to infer observed wave periods, T(ob), between 50 and 1000 minutes and horizontal wavelengths, lambda(x), from 25 to 2000 km. Dominant wave activity was found at vertical wavelengths between 2 to 4 km and 7 to 10 km. No seasonal variations were evident in the observed wave parameters. Vertical and horizontal wavelengths showed a clear tendency to increase with T(ob), which is consistent with recent sodium lidar studies of monochromatic wave events near the mesopause. Measured power law relationships between the wave parameters were lambda(z) varies as T(ob) sup 0.96, lambda(x) varies as T(ob) sup 1.8, and c(z) varies as T(ob) sup -0.85. The kinetic energy calculated for the monochromatic wave events varied as k(z) sup -2, k(x) sup -1, and f(ob) sup -1.7. The atmospheric scale heights calculated for each observation date range from 6.5 to 7.6 km with a mean value of 7 km. The increase of rms wind perturbations with altitude indicated an amplitude growth length of 20.9 km. The altitude profile of kinetic energy density decreased with height, suggesting that waves in this altitude region were subject to dissipation or saturation effects
Emergence of highly-designable protein-backbone conformations in an off-lattice model
Despite the variety of protein sizes, shapes, and backbone configurations
found in nature, the design of novel protein folds remains an open problem.
Within simple lattice models it has been shown that all structures are not
equally suitable for design. Rather, certain structures are distinguished by
unusually high designability: the number of amino-acid sequences for which they
represent the unique ground state; sequences associated with such structures
possess both robustness to mutation and thermodynamic stability. Here we report
that highly designable backbone conformations also emerge in a realistic
off-lattice model. The highly designable conformations of a chain of 23 amino
acids are identified, and found to be remarkably insensitive to model
parameters. While some of these conformations correspond closely to known
natural protein folds, such as the zinc finger and the helix-turn-helix motifs,
others do not resemble known folds and may be candidates for novel fold design.Comment: 7 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
Rayleigh lidar observations of gravity wave activity in the upper stratosphere at Urbana, Ill.
During 13 nights of Rayleigh lidar measurements at Urbana, Ill. in 1984 to 1986, thirty-six quasi-monochromatic gravity waves were observed in the 35 to 50 km altitude region of the stratosphere. The characteristics of the waves are compared with other lidar and radar measurements of gravity waves and the theoretical models of wave saturation and dissipation phenomena. The measured vertical wavelengths ranged from 2 to 11.5 km and the measured vertical phase velocities ranged from 10 to 85 cm/s. The vertical wavelengths and vertical phase velocities were used to infer observed wave periods which ranged from 100 to 1000 min and horizontal wavelengths which ranged from 70 to 2000 km. Dominant wave activity was found at vertical wavelengths between 2 to 4 km and 7 to 10 km. No significant seasonal variations were evident in the observed parameters. Vertical and horizontal wavelengths showed a clear tendency to increase with wave periods, which is consistent with recent sodium lidar studies of quasi-monochromatic waves near the mesopause. An average amplitude growth length of 20.9 km for the rms wind perturbations was estimated from the data. Kinetic energy density associated with the waves decreased with height, suggesting that waves in this altitude region were subject to dissipation or saturation effects
<theoretical studies on the relationships be- tween the thermionic work function of refrac- tory intermetallic compounds and their electronic and crystal structures< progress report, 1 jan. - 30 jun. 1965
Thermionic work function of refractory intermetallic compounds and their electronic and crystal structure
A search for radiative neutrino decay from supernovae
This document presents the data analysis procedures proposed for use with the COMPTEL instrument aboard the Compton Gamma Ray Observatory (GRO) in the search for radiative neutrino decay from supernovae. The proposed analysis methodology is an extension of a standard procedure used by the COMPTEL team in searching for a variety of source types. We have applied the procedures to a set of simulated data to demonstrate the feasibility of the method to this project
Noise predictions and economic effects of Boeing nacelle modifications
Noise level predictions and economics of Boeing nacelle modification
- …