1,591 research outputs found
Stochastic backgrounds of gravitational waves from extragalactic sources
Astrophysical sources emit gravitational waves in a large variety of
processes occurred since the beginning of star and galaxy formation. These
waves permeate our high redshift Universe, and form a background which is the
result of the superposition of different components, each associated to a
specific astrophysical process. Each component has different spectral
properties and features that it is important to investigate in view of a
possible, future detection. In this contribution, we will review recent
theoretical predictions for backgrounds produced by extragalactic sources and
discuss their detectability with current and future gravitational wave
observatories.Comment: 10 pages, 9 figures, proceedings of the GWDAW 10 Conference,
submitted to Class. & Quantum Gra
Neutrino Transport in Strongly Magnetized Proto-Neutron Stars and the Origin of Pulsar Kicks: The Effect of Asymmetric Magnetic Field Topology
In proto-neutron stars with strong magnetic fields, the cross section for
() absorption on neutrons (protons) depends on the local
magnetic field strength due to the quantization of energy levels for the
() produced in the final state. If the neutron star possesses an
asymmetric magnetic field topology in the sense that the magnitude of magnetic
field in the north pole is different from that in the south pole, then
asymmetric neutrino emission may be generated. We calculate the absorption
cross sections of \nue and \bnue in strong magnetic fields as a function of
the neutrino energy. These cross sections exhibit oscillatory behaviors which
occur because new Landau levels for the () become accessible as the
neutrino energy increases. By evaluating the appropriately averaged neutrino
opacities, we demonstrate that the change in the local neutrino flux due to the
modified opacities is rather small. To generate appreciable kick velocity
( km~s) to the newly-formed neutron star, the difference in
the field strengths at the two opposite poles of the star must be at least
~G. We also consider the magnetic field effect on the spectral
neutrino energy fluxes. The oscillatory features in the absorption opacities
give rise to modulations in the emergent spectra of and .Comment: AASTeX, 25 pages. Expanded introduction and references. This revised
version was accepted by ApJ in April 1998 (to appear in the Oct 1 issue
Forearm muscle oxidative capacity index predicts sport rock-climbing performance
Abstract: Rock-climbing performance is largely dependent on the endurance of the forearm flexors. Recently, it was reported that forearm flexor endurance in elite climbers is independent of the ability to regulate conduit artery (brachial) blood flow, suggesting that endurance is not primarily dependent on the ability of the brachial artery to deliver oxygen, but rather the ability of the muscle to perfuse and use oxygen, i.e., skeletal muscle oxidative capacity. Purpose: The aim of the study was to determine whether an index of oxidative capacity in the flexor digitorum profundus (FDP) predicts the best sport climbing red-point grade within the last 6 months. Participants consisted of 46 sport climbers with a range of abilities. Methods: Using near-infrared spectroscopy, the oxidative capacity index of the FDP was assessed by calculating the half-time for tissue oxygen resaturation (O2HTR) following 3â5 min of ischemia. Results: Linear regression, adjusted for age, sex, BMI, and training experience, revealed a 1-s decrease in O2HTR was associated with an increase in red-point grade by 0.65 (95 % CI 0.35â0.94, Adj R2 = 0.53). Conclusions: Considering a grade of 0.4 separated the top four competitors in the 2015 International Federation Sport Climbing World Cup, this finding suggests that forearm flexor oxidative capacity index is an important determinant of rock-climbing performance
Core-Collapse Simulations of Rotating Stars
We present the results from a series of two-dimensional core-collapse
simulations using a rotating progenitor star. We find that the convection in
these simulations is less vigorous because a) rotation weakens the core bounce
which seeds the neutrino-driven convection and b) the angular momentum profile
in the rotating core stabilizes against convection. The limited convection
leads to explosions which occur later and are weaker than the explosions
produced from the collapse of non-rotating cores. However, because the
convection is constrained to the polar regions, when the explosion occurs, it
is stronger along the polar axis. This asymmetric explosion can explain the
polarization measurements of core-collapse supernovae. These asymmetries also
provide a natural mechanism to mix the products of nucleosynthesis out into the
helium and hydrogen layers of the star. We also discuss the role the collapse
of these rotating stars play on the generation of magnetic fields and neutron
star kicks. Given a range of progenitor rotation periods, we predict a range of
supernova energies for the same progenitor mass. The critical mass for black
hole formation also depends upon the rotation speed of the progenitor.Comment: 16 pages text + 13 figures, submitted to Ap
Gravitational Waves from Gravitational Collapse
Gravitational wave emission from the gravitational collapse of massive stars
has been studied for more than three decades. Current state of the art
numerical investigations of collapse include those that use progenitors with
realistic angular momentum profiles, properly treat microphysics issues,
account for general relativity, and examine non--axisymmetric effects in three
dimensions. Such simulations predict that gravitational waves from various
phenomena associated with gravitational collapse could be detectable with
advanced ground--based and future space--based interferometric observatories.Comment: 68 pages including 13 figures; revised version accepted for
publication in Living Reviews in Relativity (http://www.livingreviews.org
Understanding Compact Object Formation and Natal Kicks: II. The case of XTE J1118+480
In recent years, an increasing number of proper motions have been measured
for Galactic X-ray binaries. When supplemented with accurate determinations of
the component masses, orbital period, and donor effective temperature, these
kinematical constraints harbor a wealth of information on the system's past
evolution. Here, we consider all this available information to reconstruct the
full evolutionary history of the black hole X-ray binary XTE J1118+480,
assuming that the system originated in the Galactic disk and the donor has
solar metallicity. This analysis accounts for four evolutionary phases: mass
transfer through the ongoing X-ray phase, tidal evolution before the onset of
Roche-lobe overflow, motion through the Galactic potential after the formation
of the black hole, and binary orbital dynamics due to explosive mass loss and
possibly a black hole natal kick at the time of core collapse. We find that
right after black hole formation, the system consists of a ~6.0-10.0 solar
masses black hole and a ~1.0-1.6 solar masses main-sequence star. We also find
that that an asymmetric natal kick is not only plausible but required for the
formation of this system, and derive a lower and upper limit on the black hole
natal kick velocity magnitude of 80 km/s and 310 km/s, respectively.Comment: 32 pages, 5 figures, submitted to Ap
The limits of social class in explaining ethnic gaps in educational attainment
This paper reports an analysis of the educational attainment and progress between age 11 and age 14 of over 14,500 students from the nationally representative Longitudinal Study of Young People in England (LSYPE). The mean attainment gap in national tests at age 14 between White British and several ethnic minority groups were large, more than three times the size of the gender gap, but at the same time only about one-third of the size of the social class gap. Socio-economic variables could account for the attainment gaps for Black African, Pakistani and Bangladeshi students, but not for Black Caribbean students. Further controls for parental and student attitudes, expectations and behaviours indicated minority ethnic groups were on average more advantaged on these measures than White British students, but this was not reflected proportionately in their levels of attainment. Black Caribbean students were distinctive as the only group making less progress than White British students between age 11 and 14 and this could not be accounted for by any of the measured contextual variables. Possible explanations for the White British-Black Caribbean gap are considered
How Massive Single Stars End their Life
How massive stars die -- what sort of explosion and remnant each produces --
depends chiefly on the masses of their helium cores and hydrogen envelopes at
death. For single stars, stellar winds are the only means of mass loss, and
these are chiefly a function of the metallicity of the star. We discuss how
metallicity, and a simplified prescription for its effect on mass loss, affects
the evolution and final fate of massive stars. We map, as a function of mass
and metallicity, where black holes and neutron stars are likely to form and
where different types of supernovae are produced. Integrating over an initial
mass function, we derive the relative populations as a function of metallicity.
Provided single stars rotate rapidly enough at death, we speculate upon stellar
populations that might produce gamma-ray bursts and jet-driven supernovae.Comment: 24 pages, 9 figues, submitted to Ap
Time-frequency detection algorithm for gravitational wave bursts
An efficient algorithm is presented for the identification of short bursts of
gravitational radiation in the data from broad-band interferometric detectors.
The algorithm consists of three steps: pixels of the time-frequency
representation of the data that have power above a fixed threshold are first
identified. Clusters of such pixels that conform to a set of rules on their
size and their proximity to other clusters are formed, and a final threshold is
applied on the power integrated over all pixels in such clusters. Formal
arguments are given to support the conjecture that this algorithm is very
efficient for a wide class of signals. A precise model for the false alarm rate
of this algorithm is presented, and it is shown using a number of
representative numerical simulations to be accurate at the 1% level for most
values of the parameters, with maximal error around 10%.Comment: 26 pages, 15 figures, to appear in PR
Formation of millisecond pulsars with CO white dwarf companions - I. PSR J1614-2230: Evidence for a neutron star born massive
The recent discovery of a 2 M_sun binary millisecond pulsar (Demorest et al.
2010) has not only important consequences for the equation-of-state of nuclear
matter at high densities but also raises the interesting question if the
neutron star PSR J1614-2230 was born massive. The answer is vital for
understanding neutron star formation in core collapse supernovae. Furthermore,
this system raises interesting issues about the nature of the progenitor binary
and how it evolved during its mass exchanging X-ray phase. In this paper we
discuss the progenitor evolution of PSR J1614-2230. We have performed detailed
stellar evolution modelling of intermediate-mass X-ray binaries undergoing Case
A Roche-lobe overflow (RLO) and applied an analytic parameterization for
calculating the outcome of either a common envelope evolution or the highly
super-Eddington isotropic re-emission mode. We find two viable possibilities
for the formation of the PSR J1614-2230 system: either it contained a 2.2-2.6
M_sun giant donor star and evolved through a common envelope and spiral-in
phase or, more likely, it descended from a close binary system with a 4.0-5.0
M_sun main sequence donor star via Case A RLO. We conclude that the neutron
star must have been born with a mass of ~1.95 M_sun or 1.7+-0.15 M_sun,
respectively - which significantly exceeds neutron star birth masses in
previously discovered radio pulsar systems. Based on the expected neutron star
birth masses from considerations of stellar evolution and explosion models, we
find that the progenitor star of PSR J1614-2230 is likely to have been more
massive than 20 M_sun.Comment: 15 pages, 10 figures, MNRAS in press. 3 pages added (mainly extended
discussion on neutron star birth masses
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