804 research outputs found
Relativistic Jets from Collapsars
We have studied the relativistic beamed outflow proposed to occur in the
collapsar model of gamma-ray bursts. A jet forms as a consequence of an assumed
energy deposition of erg/s within a cone
around the rotation axis of the progenitor star. The generated jet flow is
strongly beamed (\la few degrees) and reaches the surface of the stellar
progenitor (r cm) intact. At break-out the maximum Lorentz
factor of the jet flow is about 33. Simulations have been performed with the
GENESIS multi-dimensional relativistic hydrodynamic code.Comment: 6 pages, 2 figures, to appear in the proceedings of the conference
"Godunov methods: theory and applications", Oxford, October 199
Temporal properties of the short gamma-ray bursts
A temporal analysis has been performed on a sample of 100 bright gamma-ray
bursts (GRBs) with T90<2s from the BATSE current catalog. The GRBs were
denoised using a median filter and subjected to an automated pulse selection
algorithm as an objective way of idenitifing the effects of neighbouring
pulses. The rise times, fall times, FWHM, pulse amplitudes and areas were
measured and the frequency distributions are presented here. All are consistent
with lognormal distributions. The distribution of the time intervals between
pulses is not random but consistent with a lognormal distribution. The time
intervals between pulses and pulse amplitudes are highly correlated with each
other. These results are in excellent agreement with a similar analysis that
revealed lognormal distributions for pulse properties and correlated time
intervals between pulses in bright GRBs with T90>2s. The two sub-classes of
GRBs appear to have the same emission mechanism which is probably caused by
internal shocks. They may not have the same progenitors because of the generic
nature of the fireball model.Comment: 4 pages, 7 figure
Identification of biomarker volatile organic compounds released by three stored-grain insect pests in wheat
Monitoring and early detection of stored-grain insect infestation is essential to implement timely and effective pest management decisions to protect stored grains. We report a reliable analytical procedure based on headspace solid-phase microextraction coupled with gas chromatographyâmass spectrometry (HS-SPME-GC-MS) to assess stored-grain infestation through the detection of volatile compounds emitted by insects. Four different fibre coatings were assessed; 85 ”m CAR/PDMS had optimal efficiency in the extraction of analytes from wheat. The headspace profiles of volatile compounds produced by Tribolium castaneum (Herbst), Rhyzopertha dominica (Fabricius), and Sitophilus granarius (Linnaeus), either alone or with wheat, were compared with those of non-infested wheat grains. Qualitative analysis of chromatograms showed the presence of different volatile compound profiles in wheat with pest infestation compared with the wheat controls. Wheat-specific and insect-specific volatile compounds were identified, including the aggregation pheromones, dominicalure-1 and dominicalure-2, from R. dominica, and benzoquinones homologs from T. castaneum. For the first time, the presence of 3-hydroxy-2-butanone was reported from S. granarius, which might function as an alarm pheromone. These identified candidate biomarker compounds can be utilized in insect surveillance and monitoring in stored grain to safeguard our grain products in future
HI Observations of the Supermassive Binary Black Hole System in 0402+379
We have recently discovered a supermassive binary black hole system with a
projected separation between the two black holes of 7.3 parsecs in the radio
galaxy 0402+379. This is the most compact supermassive binary black hole pair
yet imaged by more than two orders of magnitude. We present Global VLBI
observations at 1.3464 GHz of this radio galaxy, taken to improve the quality
of the HI data. Two absorption lines are found toward the southern jet of the
source, one redshifted by 370 +/- 10 km/s and the other blueshifted by 700 +/-
10 km/s with respect to the systemic velocity of the source, which, along with
the results obtained for the opacity distribution over the source, suggests the
presence of two mass clumps rotating around the central region of the source.
We propose a model consisting of a geometrically thick disk, of which we only
see a couple of clumps, that reproduces the velocities measured from the HI
absorption profiles. These clumps rotate in circular Keplerian orbits around an
axis that crosses one of the supermassive black holes of the binary system in
0402+379. We find an upper limit for the inclination angle of the twin jets of
the source to the line of sight of 66 degrees, which, according to the proposed
model, implies a lower limit on the central mass of ~7 x 10^8 Msun and a lower
limit for the scale height of the thick disk of ~12 pc .Comment: 20 pages, 7 figures. Accepted on the Astrophysical Journa
Infrared afterglow of GRB041219 as a result of reradiation on dust in a circumstellar cloud
Observations of gamma ray bursts (GRB) afterglows in different spectral bands
provide a most valuable information about their nature, as well as about
properties of surrounding medium. Powerful infrared afterglow was observed from
the strong GRB041219. Here we explain the observed IR afterglow in the model of
a dust reradiation of the main GRB signal in the envelope surrounding the GRB
source. In this model we do not expect appearance of the prompt optical
emission which should be absorbed in the dust envelope. We estimate the
collimation angle of the gamma ray emission, and obtain restrictions on the
redshift (distance to GRB source), by fitting the model parameters to the
observational data.Comment: 6 pages, 2 figures, Submited to Astrofizik
Axisymmetric MHD simulations of the collapsar model for gamma-ray bursts
We present results from axisymmetric, time-dependent magnetohydrodynamic
(MHD) simulations of the collapsar model for gamma-ray bursts. We begin the
simulations after the 1.7 MSUN iron core of a 25 MSUN presupernova star has
collapsed and study the ensuing accretion of the 7 MSUN helium envelope onto
the central black hole formed by the collapsed iron core. We consider a
spherically symmetric progenitor model, but with spherical symmetry broken by
the introduction of a small, latitude-dependent angular momentum and a weak
radial magnetic field. Our MHD simulations include a realistic equation of
state, neutrino cooling, photodisintegration of helium, and resistive heating.
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: submitted in ApJ Letter
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
INHIBITION OF TRANSPLANTABLE MOUSE TUMOR GROWTH BY TISSUE EXTRACTS AND THEIR PROTEIN FRACTIONS
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