2,398 research outputs found
The Nucleosynthetic Yields of Core-collapse Supernovae: Prospects for the Next Generation of Gamma-Ray Astronomy
Though the neutrino-driven convection model for the core-collapse explosion mechanism has received strong support in recent years, there are still many uncertainties in the explosion parametersâsuch as explosion energy, remnant mass, and end-of-life stellar abundances as initial conditions. Using a broad set of spherically symmetric core-collapse simulations we examine the effects of these key parameters on explosive nucleosynthesis and final explosion yields. The post-bounce temperature and density evolution of zero-age main-sequence 15, 20, and 25 solar mass progenitors are post-processed through the Nucleosynthesis Grid nuclear network to obtain detailed explosive yields. In particular, this study focuses on radio isotopes that are of particular interest to the next generation of gamma-ray astronomical observations: 43K, 47Ca, 44Sc, 47Sc, 48V, 48Cr, 51Cr, 52Mn, 59Fe, 56Co, 57Co, and 57Ni. These nuclides may be key in advancing our understanding of the inner workings of core-collapse supernovae by probing the parameters of the explosion engine. We find that the isotopes that are strong indicators of explosion energy are 43K, 47Ca, 44Sc, 47Sc, and 59Fe, those that are dependent on the progenitor structure are 48V, 51Cr, and 57Co, and those that probe neither are 48Cr, 52Mn, 57Ni, and 56Co. We discuss the prospects of observing these radionuclides in supernova remnants
High-Energy X-ray Imaging of the Pulsar Wind Nebula MSH~15-52: Constraints on Particle Acceleration and Transport
We present the first images of the pulsar wind nebula (PWN) MSH 15-52 in the
hard X-ray band (>8 keV), as measured with the Nuclear Spectroscopic Telescope
Array (NuSTAR). Overall, the morphology of the PWN as measured by NuSTAR in the
3-7 keV band is similar to that seen in Chandra high-resolution imaging.
However, the spatial extent decreases with energy, which we attribute to
synchrotron energy losses as the particles move away from the shock. The
hard-band maps show a relative deficit of counts in the northern region towards
the RCW 89 thermal remnant, with significant asymmetry. We find that the
integrated PWN spectra measured with NuSTAR and Chandra suggest that there is a
spectral break at 6 keV which may be explained by a break in the
synchrotron-emitting electron distribution at ~200 TeV and/or imperfect cross
calibration. We also measure spatially resolved spectra, showing that the
spectrum of the PWN softens away from the central pulsar B1509-58, and that
there exists a roughly sinusoidal variation of spectral hardness in the
azimuthal direction. We discuss the results using particle flow models. We find
non-monotonic structure in the variation with distance of spectral hardness
within 50" of the pulsar moving in the jet direction, which may imply particle
and magnetic-field compression by magnetic hoop stress as previously suggested
for this source. We also present 2-D maps of spectral parameters and find an
interesting shell-like structure in the NH map. We discuss possible origins of
the shell-like structure and their implications.Comment: 15 pages, 9 figures, accepted for publication in Ap
A luminosity distribution for kilonovae based on short gamma-ray burst afterglows
The combined detection of a gravitational-wave signal, kilonova, and short
gamma-ray burst (sGRB) from GW170817 marked a scientific breakthrough in the
field of multi-messenger astronomy. But even before GW170817, there have been a
number of sGRBs with possible associated kilonova detections. In this work, we
re-examine these "historical" sGRB afterglows with a combination of
state-of-the-art afterglow and kilonova models. This allows us to include
optical/near-infrared synchrotron emission produced by the sGRB as well as
ultraviolet/optical/near-infrared emission powered by the radioactive decay of
-process elements (i.e., the kilonova). Fitting the lightcurves, we derive
the velocity and the mass distribution as well as the composition of the
ejected material. The posteriors on kilonova parameters obtained from the fit
were turned into distributions for the peak magnitude of the kilonova emission
in different bands and the time at which this peak occurs. From the sGRB with
an associated kilonova, we found that the peak magnitude in H bands falls in
the range [-16.2, -13.1] ( of confidence) and occurs within after the sGRB prompt emission. In g band instead we obtain a peak
magnitude in range [-16.8, -12.3] occurring within the first after
the sGRB prompt. From the luminosity distributions of GW170817/AT2017gfo,
kilonova candidates GRB130603B, GRB050709 and GRB060614 (with the possible
inclusion of GRB150101B) and the upper limits from all the other sGRBs not
associated with any kilonova detection we obtain for the first time a kilonova
luminosity function in different bands.Comment: Published in MNRAS, 24 pages, 14 figure
Parallelization of Kinetic Theory Simulations
Numerical studies of shock waves in large scale systems via kinetic
simulations with millions of particles are too computationally demanding to be
processed in serial. In this work we focus on optimizing the parallel
performance of a kinetic Monte Carlo code for astrophysical simulations such as
core-collapse supernovae. Our goal is to attain a flexible program that scales
well with the architecture of modern supercomputers. This approach requires a
hybrid model of programming that combines a message passing interface (MPI)
with a multithreading model (OpenMP) in C++. We report on our approach to
implement the hybrid design into the kinetic code and show first results which
demonstrate a significant gain in performance when many processors are applied.Comment: 10 pages, 3 figures, conference proceeding
Hyper-Accreting Black Holes and Gamma-Ray Bursts
A variety of current models for gamma-ray bursts (GRBs) suggest a common
engine - a black hole of several solar masses accreting matter from a disk at a
rate 0.01 to 10 solar masses per second. Using a numerical model for
relativistic disk accretion, we have studied steady-state accretion at these
high rates. Inside a radius ~ 10**8 cm, for accretion rates greater than about
0.01 solar masses per second, a global state of balanced power comes to exist
between neutrino losses, chiefly pair capture on nucleons, and dissipation.
Energy emitted in neutrinos is less, and in the case of low accretion rates,
very much less, than the maximum efficiency factor for black hole accretion
(0.057 for no rotation; 0.42 for extreme Kerr rotation) times Mdot c**2. The
efficiency for producing a pair fireball along the rotational axis by neutrino
annihilation is calculated and found to be highly variable and very sensitive
to the accretion rate. For some of the higher accretion rates studied, it can
be several per cent or more; for accretion rates less than 0.05 solar masses
per second, it is essentially zero. The efficiency of the Blandford-Znajek
mechanism in extracting rotational energy from the black hole is also
estimated. In light of these results, the viability of various gamma-ray burst
models is discussed and the sensitivity of the results to disk viscosity, black
hole rotation rate, and black hole mass explored. A diverse range of GRB
energies seems unavoidable and neutrino annihilation in hyper-accreting black
hole systems can explain bursts up to 10**52 erg. Larger energies may be
inferred for beaming systems.Comment: 46 pages, includes 9 figures, LaTeX (uses aaspp4.sty), accepted by
The Astrophysical Journal. Additional solutions in Tables and Figs. 4 and 5,
minor revisions to text, references adde
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
Crater lake cichlids individually specialize along the benthic-limnetic axis
A common pattern of adaptive diversification in freshwater fishes is the repeated evolution of elongated open water (limnetic) species and high-bodied shore (benthic) species from generalist ancestors. Studies on phenotype-diet correlations have suggested that population-wide individual specialization occurs at an early evolutionary and ecological stage of divergence and niche partitioning. This variable restricted niche use across individuals can provide the raw material for earliest stages of sympatric divergence. We investigated variation in morphology and diet as well as their correlations along the benthic-limnetic axis in an extremely young Midas cichlid species, Amphilophus tolteca, endemic to the Nicaraguan crater lake Asososca Managua. We found that A. tolteca varied continuously in ecologically relevant traits such as body shape and lower pharyngeal jaw morphology. The correlation of these phenotypes with niche suggested that individuals are specialized along the benthic-limnetic axis. No genetic differentiation within the crater lake was detected based on genotypes from 13 microsatellite loci. Overall, we found that individual specialization in this young crater lake species encompasses the limnetic- as well as the benthic macro-habitat. Yet there is no evidence for any diversification within the species, making this a candidate system for studying what might be the early stages preceding sympatric divergence
The consequences of nuclear electron capture in core collapse supernovae
The most important weak nuclear interaction to the dynamics of stellar core
collapse is electron capture, primarily on nuclei with masses larger than 60.
In prior simulations of core collapse, electron capture on these nuclei has
been treated in a highly parameterized fashion, if not ignored. With realistic
treatment of electron capture on heavy nuclei come significant changes in the
hydrodynamics of core collapse and bounce. We discuss these as well as the
ramifications for the post-bounce evolution in core collapse supernovae.Comment: Accepted by PRL, 5 pages, 2 figure
Prostate cancer risk: associations with ultraviolet radiation, tyrosinase and melanocortin-1 receptor genotypes
Exposure to ultraviolet radiation may reduce prostate cancer risk, suggesting that polymorphism in genes that mediate host pigmentation will be associated with susceptibility to this cancer. We studied 210 prostate cancer cases and 155 controls to determine whether vitamin D receptor (VDR, Taql and Fokl variants), tyrosinase (TYR, codon 192 variant) and melanocortin-1 receptor (MC1R, Arg151Cys, Arg160Trp, Val92Met, Asp294His and Asp84Glu variants) genotypes are associated with risk. UV exposure was determined using a questionnaire. MC1R Arg160/Arg160 homozygotes were at increased risk (P = 0.027, odds ratio = 1.94) while TYR A2/A2 homozygotes were at reduced risk of prostate cancer (P = 0.033, odds ratio = 0.48). These associations remained significant after correction for UV-exposure. Stratification of cases and controls by quartiles of exposure, showed that the protective effect of TYR A1A2 (P = 0.006, odds ratio 0.075) and A2A2 (P = 0.003, odds ratio 0.055) was particularly strong in subjects who had received the greatest exposure. Our data show for the first time, that allelism in genes linked with skin pigment synthesis is associated with prostate cancer risk possibly because it mediates the protective effects of UV. Importantly, susceptibility is associated with an interaction between host predisposition and exposure. © 2001 Cancer Research Campaignââhttp://www.bjcancer.co
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