2,274 research outputs found
Formation Rates of Black Hole Accretion Disk Gamma-Ray Bursts
While many models have been proposed for GRBs, those currently favored are
all based upon the formation of and/or rapid accretion into stellar mass black
holes. We present population synthesis calculations of these models using a
Monte Carlo approach in which the many uncertain parameters intrinsic to such
calculations are varied. We estimate the event rate for each class of model as
well as the propagation distance for those having significant delay between
formation and burst production, i.e., double neutron star (DNS) mergers and
black hole-neutron star (BH/NS) mergers. For reasonable assumptions regarding
the many uncertainties in population synthesis, we calculate a daily event rate
in the universe for i) merging neutron stars: ~100/day; ii) neutron-star black
hole mergers: ~450/day; iii) collapsars: ~10,000/day; iv) helium star black
hole mergers: ~1000/day; and v) white dwarf black hole mergers: ~20/day. The
range of uncertainty in these numbers however, is very large, typically two to
three orders of magnitude. These rates must additionally be multiplied by any
relevant beaming factor and sampling fraction (if the entire universal set of
models is not being observed). Depending upon the mass of the host galaxy, half
of the DNS and BH/NS mergers will happen within 60kpc (for a Milky-Way massed
galaxy) to 5Mpc (for a galaxy with negligible mass) from the galactic center.
Because of the delay time, neutron star and black hole mergers will happen at a
redshift 0.5 to 0.8 times that of the other classes of models. Information is
still lacking regarding the hosts of short hard bursts, but we suggest that
they are due to DNS and BH/NS mergers and thus will ultimately be determined to
lie outside of galaxies and at a closer mean distance than long complex bursts
(which we attribute to collapsars).Comment: 57 pages total, 23 figures, submitted by Ap
Merger of white dwarf-neutron star binaries: Prelude to hydrodynamic simulations in general relativity
White dwarf-neutron star binaries generate detectable gravitational
radiation. We construct Newtonian equilibrium models of corotational white
dwarf-neutron star (WDNS) binaries in circular orbit and find that these models
terminate at the Roche limit. At this point the binary will undergo either
stable mass transfer (SMT) and evolve on a secular time scale, or unstable mass
transfer (UMT), which results in the tidal disruption of the WD. The path a
given binary will follow depends primarily on its mass ratio. We analyze the
fate of known WDNS binaries and use population synthesis results to estimate
the number of LISA-resolved galactic binaries that will undergo either SMT or
UMT. We model the quasistationary SMT epoch by solving a set of simple ordinary
differential equations and compute the corresponding gravitational waveforms.
Finally, we discuss in general terms the possible fate of binaries that undergo
UMT and construct approximate Newtonian equilibrium configurations of merged
WDNS remnants. We use these configurations to assess plausible outcomes of our
future, fully relativistic simulations of these systems. If sufficient WD
debris lands on the NS, the remnant may collapse, whereby the gravitational
waves from the inspiral, merger, and collapse phases will sweep from LISA
through LIGO frequency bands. If the debris forms a disk about the NS, it may
fragment and form planets.Comment: 28 pages, 25 figures, 6 table
Black Hole - Neutron Star Mergers as Central Engines of Gamma-Ray Bursts
Hydrodynamic simulations of the merger of stellar mass black hole - neutron
star binaries (BH/NS) are compared with mergers of binary neutron stars
(NS/NS). The simulations are Newtonian, but take into account the emission and
backreaction of gravitational waves. The use of a physical nuclear equation of
state allows us to include the effects of neutrino emission. For low neutron
star to black hole mass ratios the neutron star transfers mass to the black
hole during a few cycles of orbital decay and subsequent widening before
finally being disrupted, whereas for ratios near unity the neutron star is
already distroyed during its first approach. A gas mass between about 0.3 and
about 0.7 solar masses is left in an accretion torus around the black hole and
radiates neutrinos at a luminosity of several 10^{53} erg/s during an estimated
accretion time scale of about 0.1 s. The emitted neutrinos and antineutrinos
annihilate into electron-positron pairs with efficiencies of 1-3% percent and
rates of up to 2*10^{52} erg/s, thus depositing an energy of up to 10^{51} erg
above the poles of the black hole in a region which contains less than 10^{-5}
solar masses of baryonic matter. This could allow for relativistic expansion
with Lorentz factors around 100 and is sufficient to explain apparent burst
luminosities of up to several 10^{53} erg/s for burst durations of
approximately 0.1-1 s, if the gamma emission is collimated in two moderately
focussed jets in a fraction of about 1/100-1/10 of the sky.Comment: 8 pages, LaTex, 4 postscript figures, 2 tables. ApJ Letters,
accepted; revised and shortened version, Fig. 2 change
Deficiency of Capicua disrupts bile acid homeostasis
Capicua (CIC) has been implicated in pathogenesis of spinocerebellar ataxia type 1 and cancer in mammals; however, the in vivo physiological functions of CIC remain largely unknown. Here we show that Cic hypomorphic (Cic-L-/-) mice have impaired bile acid (BA) homeostasis associated with induction of proinflammatory cytokines. We discovered that several drug metabolism and BA transporter genes were down-regulated in Cic-L-/- liver, and that BA was increased in the liver and serum whereas bile was decreased within the gallbladder of Cic-L-/- mice. We also found that levels of proinflammatory cytokine genes were up-regulated in Cic-L-/- liver. Consistent with this finding, levels of hepatic transcriptional regulators, such as hepatic nuclear factor 1 alpha (HNF1 alpha), CCAAT/enhancer-binding protein beta (C/EBP beta), forkhead box protein A2 (FOXA2), and retinoid X receptor alpha (RXR alpha), were markedly decreased in Cic-L-/- mice. Moreover, induction of tumor necrosis factor alpha (Tnf alpha) expression and decrease in the levels of FOXA2, C/EBP beta, and RXRa were found in Cic-L-/- liver before BA was accumulated, suggesting that inflammation might be the cause for the cholestasis in Cic-L-/- mice. Our findings indicate that CIC is a critical regulator of BA homeostasis, and that its dysfunction might be associated with chronic liver disease and metabolic disorders.open11810Ysciescopu
Effects of neutrino-driven kicks on the supernova explosion mechanism
We show that neutrino-driven pulsar kicks can increase the energy of the
supernova shock. The observed large velocities of pulsars are believed to
originate in the supernova explosion, either from asymmetries in the ejecta or
from an anisotropic emission of neutrinos (or other light particles) from the
cooling neutron star. In this paper we assume the velocities are caused by
anisotropic neutrino emission and study the effects of these neutrino-driven
kicks on the supernova explosion. We find that if the collapsed star is
marginally unable to produce an explosion, the neutrino-driven mechanisms can
drive the convection to make a successful explosion. The resultant explosion is
asymmetric, with the strongest ejecta motion roughly in the direction of the
neutron star kick. This is in sharp contrast with the ejecta-driven mechanisms,
which predict the motion of the ejecta in the opposite direction. This
difference can be used to distinguish between the two mechanisms based on the
observations of the supernova remnants.Comment: 22 pages including 8 figures, submitted to ApJ, version with high
resolution figures can be found at http://qso.lanl.gov/~cl
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
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
Inferring Core-Collapse Supernova Physics with Gravitational Waves
Stellar collapse and the subsequent development of a core-collapse supernova
explosion emit bursts of gravitational waves (GWs) that might be detected by
the advanced generation of laser interferometer gravitational-wave
observatories such as Advanced LIGO, Advanced Virgo, and LCGT. GW bursts from
core-collapse supernovae encode information on the intricate multi-dimensional
dynamics at work at the core of a dying massive star and may provide direct
evidence for the yet uncertain mechanism driving supernovae in massive stars.
Recent multi-dimensional simulations of core-collapse supernovae exploding via
the neutrino, magnetorotational, and acoustic explosion mechanisms have
predicted GW signals which have distinct structure in both the time and
frequency domains. Motivated by this, we describe a promising method for
determining the most likely explosion mechanism underlying a hypothetical GW
signal, based on Principal Component Analysis and Bayesian model selection.
Using simulated Advanced LIGO noise and assuming a single detector and linear
waveform polarization for simplicity, we demonstrate that our method can
distinguish magnetorotational explosions throughout the Milky Way (D <~ 10kpc)
and explosions driven by the neutrino and acoustic mechanisms to D <~ 2kpc.
Furthermore, we show that we can differentiate between models for rotating
accretion-induced collapse of massive white dwarfs and models of rotating iron
core collapse with high reliability out to several kpc.Comment: 22 pages, 9 figure
Physics of Neutron Star Kicks
It is no longer necessary to `sell' the idea of pulsar kicks, the notion that
neutron stars receive a large velocity (a few hundred to a thousand km
s) at birth. However, the origin of the kicks remains mysterious. We
review the physics of different kick mechanisms, including hydrodynamically
driven, neutrino and magnetically driven kicks.Comment: 8 pages including 1 figure. To be published in "Stellar Astrophysics"
(Pacific Rim Conference Proceedings), (Kluwer Pub.
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