132 research outputs found
Discovery of a tight correlation between pulse lag/luminosity and jet-break times: a connection between gamma-ray burst and afterglow properties
A correlation is presented between the pulse lag and the jet-break time for
seven BATSE gamma-ray bursts with known redshifts. This is, to our best
knowledge, the first known direct tight correlation between a property of the
gamma-ray burst phase (the pulse lag) and the afterglow phase (the jet-break
time). As pulse lag and luminosity have been found to be correlated this also
represents a correlation between peak luminosity and jet-break time. Observed
timescales (variability or spectral lags) as well as peak luminosity naturally
have a strong dependence on the Lorentz factor of the outflow and so we propose
that much of the variety among GRBs has a purely kinematic origin (the speed or
direction of the outflow).
We explore a model in which the variation among GRBs is due to a variation in
jet-opening angles, and find that the narrowest jets have the fastest outflows.
We also explore models in which the jets have similar morphology and size, and
the variation among bursts is caused by variation in viewing angle and/or due
to a velocity profile. The relations between luminosity, variability, spectral
lag and jet-break time can be qualitatively understood from models in which the
Lorentz factor decreases as a function of angle from the jet axis. One expects
to see high luminosities, short pulse lags and high variability as well as an
early jet-break time for bursts viewed on axis, while higher viewing
inclinations will yield lower luminosities, longer pulse lags, smoother bursts
and later jet-break times.Comment: 10 pages, 3 figures, accepted to ApJ (new version contains minor
changes
Gamma-Ray Bursts via Pair Plasma Fireballs from Heated Neutron Stars
In this paper we model the emission from a relativistically expanding
electron-positron pair plasma fireball originating near the surface of a heated
neutron star. This pair fireball is deposited via the annihilation of neutrino
pairs emanating from the surface of the hot neutron star. The heating of
neutron stars may occur in close neutron star binary systems near their last
stable orbit. We model the relativistic expansion and subsequent emission of
the plasma and find 10^51 to 10^52 ergs in gamma-rays are produced with
spectral and temporal properties consistent with observed gamma-ray bursts.Comment: 5 pages, 3 figures. Submitted to the Conference Proceedings of the
5th Huntsville Gamma-Ray Burst Symposiu
A Model for Short Gamma-Ray Bursts: Heated Neutron Stars in Close Binary Systems
In this paper we present a model for the short (< second) population of
gamma-ray bursts (GRBs). In this model heated neutron stars in a close binary
system near their last stable orbit emit neutrinos at large luminosities (~
10^53 ergs/sec). A fraction of these neutrinos will annihilate to form an
electron-positron pair plasma wind which will, in turn, expand and recombine to
photons which make the gamma-ray burst. We study neutrino annihilation and show
that a substantial fraction (~ 50%) of energy deposited comes from inter-star
neutrinos, where each member of the neutrino pair originates from each neutron
star. Thus, in addition to the annihilation of neutrinos blowing off of a
single star, we have a new source of baryon free energy that is deposited
between the stars. To model the pair plasma wind between stars, we do
three-dimensional relativistic numerical hydrodynamic calculations.
Preliminary results are also presented of new, fully general relativistic
calculations of gravitationally attracting stars falling from infinity with no
angular momentum. These simulations exhibit a compression effect.Comment: 3 pages, 3 postscript figs (2 color), to appear in "Gamma-Ray Burst
and Afterglow Astronomy 2001", Woods Hole; 5-9 Nov, 200
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Numerical Modeling of the Radio Nebula from the 2004 December 27 Giant Flare of SGR 1806-20
We use the relativistic hydrodynamics code Cosmos++ to model the evolution of the radio nebula triggered by the Dec. 27, 2004 giant flare event of soft gamma repeater 1806-20. We primarily focus on the rebrightening and centroid motion occurring subsequent to day 20 following the flare event. We model this period as a mildly relativistic (gamma ~ 1.07 - 1.67) jetted outflow expanding into the interstellar medium (ISM). We demonstrate that a jet with total energy ~ 10^46 ergs confined to a half opening angle ~ 20 degrees fits the key observables of this event, e.g. the flux lightcurve, emission map centroid position, and aspect ratio. In particular, we find excellent agreement with observations if the rebrightening is due to the jet, moving at 0.5c and inclined ~ 0 - 40 degrees toward the observer, colliding with a density discontinuity in the ISM at a radius of several 10^16 cm. We also find that a jet with a higher velocity, >~ 0.7c, and larger inclination, >~ 70 degrees, moving into a uniform ISM can fit the observations in general, but tends to miss the details of rebrightening. The latter, uniform ISM model predicts an ISM density more than 100 times lower than that of the former model, and thus suggests an independent test which might discriminate between the two. One of the strongest constraints of both models is that the data seems to require a non-uniform jet in order to be well fit
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