811 research outputs found
MHD simulations of the collapsar model for GRBs
We present results from axisymmetric, time-dependent magnetohydrodynamic
(MHD) simulations of the collapsar model for gamma-ray bursts. Our main
conclusion is that, within the collapsar model, MHD effects alone are able to
launch, accelerate and sustain a strong polar outflow. We also find that the
outflow is Poynting flux-dominated, and note that this provides favorable
initial conditions for the subsequent production of a baryon-poor fireball.Comment: 4 pages, to appear in proceedings of "2003 GRB Conference" (Santa Fe,
NM, September 8-12, 2003), needs aipprocs LaTeX class, movies are available
at http://rocinante.colorado.edu/~proga
Tools for Dissecting Supernova Remnants Observed with Chandra: Methods and Application to the Galactic Remnant W49B
We introduce methods to quantify the X-ray morphologies of supernova remnants
observed with the Chandra X-ray Telescope. These include a power-ratio
technique to measure morphological asymmetries, correlation-length analysis to
probe chemical segregation and distribution, and wavelet-transform analysis to
quantify X-ray substructure. We demonstrate the utility and accuracy of these
techniques on relevant synthetic data. Additionally, we show the methods'
capabilities by applying them to the 55-ks Chandra ACIS observation of the
galactic supernova remnant W49B. We analyze the images of prominent emission
lines in W49B and use the results to discern physical properties. We find that
the iron morphology is very distinct from the other elements: it is
statistically more asymmetric, more segregated, and has 25% larger emitting
substructures than the lighter ions. Comparatively, the silicon, sulfur, argon,
and calcium are well-mixed, more isotropic, and have smaller, equally-sized
emitting substructures. Based on fits of XMM-Newton spectra in regions
identified as iron rich and iron poor, we determine that the iron in W49B must
have been anisotropically ejected. We measure the abundance ratios in many
regions, and we find that large, local variations are persistent throughout the
remnant. We compare the mean, global abundance ratios to those predicted by
spherical and bipolar core-collapse explosions; the results are consistent with
a bipolar origin from a 25 solar mass progenitor. We calculate the filling
factor of iron from the volume of its emitting substructures, enabling more
precise mass estimates than previous studies. Overall, this work is a first
step toward rigorously describing the physical properties of supernova remnants
for comparison within and between sources.Comment: 51 pages, 24 figures, accepted by ApJ. For full resolution figures,
see http://www.astro.ucsc.edu/~lopez/paper.html Fixed typo in URL; no other
change
Stellar Explosions by Magnetic Towers
We propose a magnetic mechanism for the collimated explosion of a massive
star relevant for GRBs, XRFs and asymmetric supernovae. We apply Lynden-Bell's
magnetic tower scenario to the interior of a massive rotating star after the
core has collapsed to form a black hole with an accretion disk or a millisecond
magnetar acting as a central engine. We solve the force-free Grad-Shafranov
equation to calculate the magnetic structure and growth of a tower embedded in
a stellar environment. The pressure of the toroidal magnetic field,
continuously generated by differential rotation of the central engine, drives a
rapid expansion which becomes vertically collimated after lateral force balance
with the surrounding gas pressure is reached. The collimation naturally occurs
because hoop stress concentrates magnetic field toward the rotation axis and
inhibits lateral expansion. This leads to the growth of a self-collimated
magnetic tower. When embedded in a massive star, the supersonic expansion of
the tower drives a strong bow shock behind which an over-pressured cocoon
forms. The cocoon confines the tower by supplying collimating pressure and
provides stabilization against disruption due to MHD instabilities. Because the
tower consists of closed field lines starting and ending on the central engine,
mixing of baryons from the cocoon into the tower is suppressed. The channel
cleared by the growing tower is thus plausibly free of baryons and allows the
escape of magnetic energy from the central engine through the star. While
propagating down the stellar density gradient, the tower accelerates and
becomes relativistic. During the expansion, fast collisionless reconnection
becomes possible resulting in dissipation of magnetic energy which may be
responsible for GRB prompt emission.Comment: 19 pages, 8 figures, accepted to ApJ, updated references and
additional discussion adde
A Decline in the X-ray through Radio Emission from GW170817 Continues to Support an Off-Axis Structured Jet
We present new observations of the binary neutron star merger GW170817 at
days post-merger, at radio (Karl G. Jansky Very Large
Array; VLA), X-ray (Chandra X-ray Observatory) and optical (Hubble Space
Telescope; HST) wavelengths. These observations provide the first evidence for
a turnover in the X-ray light curve, mirroring a decline in the radio emission
at significance. The radio-to-X-ray spectral energy
distribution exhibits no evolution into the declining phase. Our full
multi-wavelength dataset is consistent with the predicted behavior of our
previously published models of a successful structured jet expanding into a
low-density circumbinary medium, but pure cocoon models with a choked jet
cannot be ruled out. If future observations continue to track our predictions,
we expect that the radio and X-ray emission will remain detectable until days post-merger.Comment: Accepted to ApJL. Updated version includes new VLA observations
extending through 2018 June
The Binary Neutron Star event LIGO/VIRGO GW170817 a hundred and sixty days after merger: synchrotron emission across the electromagnetic spectrum
We report deep Chandra, HST and VLA observations of the binary neutron star
event GW170817 at d after merger. These observations show that GW170817
has been steadily brightening with time and might have now reached its peak,
and constrain the emission process as non-thermal synchrotron emission where
the cooling frequency is above the X-ray band and the synchrotron
frequency is below the radio band. The very simple power-law spectrum
extending for eight orders of magnitude in frequency enables the most precise
measurement of the index of the distribution of non-thermal relativistic
electrons accelerated by a shock launched by a
NS-NS merger to date. We find , which indicates that radiation
from ejecta with dominates the observed emission. While
constraining the nature of the emission process, these observations do
\emph{not} constrain the nature of the relativistic ejecta. We employ
simulations of explosive outflows launched in NS ejecta clouds to show that the
spectral and temporal evolution of the non-thermal emission from GW170817 is
consistent with both emission from radially stratified quasi-spherical ejecta
traveling at mildly relativistic speeds, \emph{and} emission from off-axis
collimated ejecta characterized by a narrow cone of ultra-relativistic material
with slower wings extending to larger angles. In the latter scenario, GW170817
harbored a normal SGRB directed away from our line of sight. Observations at
days are unlikely to settle the debate as in both scenarios the
observed emission is effectively dominated by radiation from mildly
relativistic material.Comment: Updated with the latest VLA and Chandra dat
Collapsars - Gamma-Ray Bursts and Explosions in "Failed Supernovae"
Using a two-dimensional hydrodynamics code (PROMETHEUS), we study the
continued evolution of rotating massive helium stars whose iron core collapse
does not produce a successful outgoing shock, but instead forms a black hole.
We study the formation of a disk, the associated flow patterns, and the
accretion rate for disk viscosity parameter, alpha ~ 0.001 and 0.1. For the
standard 14 solar mass model the average accretion rate for 15 s is 0.07 solar
masses per second and the total energy deposited along the rotational axes by
neutrino annihilation is (1 - 14) x 10**51 erg, depending upon the evolution of
the Kerr parameter and uncertain neutrino efficiencies. Simulated deposition of
this energy in the polar regions results in strong relativistic outflow - jets
beamed to about 1.5% of the sky. The jets remain highly focused, and are
capable of penetrating the star in 5 - 10 s. After the jet breaks through the
surface of the star, highly relativistic flow can commence. Because of the
sensitivity of the mass ejection and jets to accretion rate, angular momentum,
and disk viscosity, and the variation of observational consequences with
viewing angle, a large range of outcomes is possible ranging from bright GRBs
like GRB 971214 to faint GRB-supernovae like SN 1998bw. X-ray precursors are
also possible as the jet first breaks out of the star. While only a small
fraction of supernovae make GRBs, we predict that all GRBs longer than a few
seconds will make supernovae similar to SN 1998bw. However, hard, energetic
GRBs shorter than a few seconds will be difficult to make in this model.Comment: Latex, 66 pages including 27 figures (9 color), Submitted to The
Astrophysical Journal, latex uses aaspp4.sty. Figures also available at
http://www.ucolick.org/~andre
Relativistic Jets in Collapsars
We examine the propagation of 2-dimensional relativistic jets through the
stellar progenitor in the collapsar model for gamma-ray bursts. In agreement
with previous studies, we find that relativistic jets are collimated by their
passage through the stellar mantle. Interaction of these jets with the star and
their own cocoons also causes mixing that sporadically decelerates the flow. We
speculate that this mixing instability is chiefly responsible for the variable
Lorentz factor needed in the internal shock model and for the complex light
curves seen in many GRBs. In all cases studied, the jet is shocked deep inside
the star following a brief period of adiabatic expansion. The jet that finally
emerges from the star thus has a moderate Lorentz factor, modulated by mixing,
and a very large internal energy. In a second series of calculations, we follow
the escape of that sort of jet. Because of the large ratio of internal to
kinetic energy in both the jet and its cocoon, the opening angle of the final
jet is significantly greater than at breakout. A small amount of material
emerges at large angles, but with a Lorentz factor still sufficiently large to
make a weak GRB. This leads us to propose a "unified model" in which a variety
of high energy transients, ranging from x-ray flashes to "classic" GRBs, may be
seen depending upon the angle at which a standard collapsar is observed. We
also speculate that the breakout of a relativistic jet and its collision with
the stellar wind will produce a brief transient with properties similar to the
class of "short-hard" GRBs. Implications of our calculations for GRB light
curves, the luminosity-variability relation, and the GRB-supernova association
are also discussed. (Abridged)Comment: 40 pages, 16 figures, To appear in vol. 586, ApJ, March 20, 200
INHIBITION OF TRANSPLANTABLE MOUSE TUMOR GROWTH BY TISSUE EXTRACTS AND THEIR PROTEIN FRACTIONS
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