10,558 research outputs found
Experimental performance of a conical pressure probe at Mach numbers of 3.0, 4.5, and 6.0
Wind tunnel investigation of performance of conical pressure probe at hypersonic speed
The development of word recognition: The use of the possible-word constraint by 12-month-olds
Heterogeneity in Short Gamma-ray Bursts
We analyze the Swift/BAT sample of short gamma-ray bursts, using an objective
Bayesian Block procedure to extract temporal descriptors of the bursts' initial
pulse complexes (IPCs). The sample comprises 12 and 41 bursts with and without
extended emission (EE) components, respectively. IPCs of non-EE bursts are
dominated by single pulse structures, while EE bursts tend to have two or more
pulse structures. The medians of characteristic timescales - durations, pulse
structure widths, and peak intervals - for EE bursts are factors of ~ 2-3
longer than for non-EE bursts. A trend previously reported by Hakkila and
colleagues unifying long and short bursts - the anti-correlation of pulse
intensity and width - continues in the two short burst groups, with non-EE
bursts extending to more intense, narrower pulses. In addition we find that
preceding and succeeding pulse intensities are anti-correlated with pulse
interval. We also examine the short burst X-ray afterglows as observed by the
Swift/XRT. The median flux of the initial XRT detections for EE bursts (~ 6 x
10^-10 erg cm^-2 s^-1) is ~> 20 x brighter than for non-EE bursts, and the
median X-ray afterglow duration for EE bursts (~ 60,000 s) is ~ 30 x longer
than for non-EE bursts.
The tendency for EE bursts toward longer prompt-emission timescales and
higher initial X-ray afterglow fluxes implies larger energy injections powering
the afterglows. The longer-lasting X-ray afterglows of EE bursts may suggest
that a significant fraction explode into more dense environments than non-EE
bursts, or that the sometimes-dominant EE component efficiently powers the
afterglow. Combined, these results favor different progenitors for EE and
non-EE short bursts.Comment: 30 pages, 11 figures, 3 tables; accepted to The Astrophysical Journa
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
Calibration of Tests for Time Dilation in GRB Pulse Structures
Two tests for cosmological time dilation in -ray bursts -- the peak
alignment and auto-correlation statistics -- involve averaging information near
the times of peak intensity. Both tests require width corrections, assuming
cosmological origin for bursts, since narrower temporal structure from higher
energy would be redshifted into the band of observation, and since intervals
between pulse structures are included in the averaging procedures. We analyze
long ( 2 s) BATSE bursts and estimate total width corrections for trial
time-dilation factors (TDF = [1+]/[1+]) by
time-dilating and redshifting bright bursts. Both tests reveal significant
trends of increasing TDF with decreasing peak flux, but neither provides
sufficient discriminatory power to distinguish between actual TDFs in the range
2--3.Comment: 5 pages in LATeX, REVTEX style, 2 embedded figures. To appear in
Third Huntsville GRB Workshop Proceeding
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