11,391 research outputs found
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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
Language-universal constraints on the segmentation of English
Two word-spotting experiments are reported that examine whether the Possible-Word Constraint (PWC) [1] is a language-specific or language-universal strategy for the segmentation of continuous speech. The PWC disfavours parses which leave an impossible residue between the end of a candidate word and a known boundary. The experiments examined cases where the residue was either a CV syllable with a lax vowel, or a CVC syllable with a schwa. Although neither syllable context is a possible word in English, word-spotting in both contexts was easier than with a context consisting of a single consonant. The PWC appears to be language-universal rather than language-specific
Identification and control of structures in space
The derivation of the equations of motion for the Spacecraft Control Laboratory Experiment (SCOLE) is reported and the equations of motion of a similar structure orbiting the earth are also derived. The structure is assumed to undergo large rigid-body maneuvers and small elastic deformations. A perturbation approach is proposed whereby the quantities defining the rigid-body maneuver are assumed to be relatively large, with the elastic deformations and deviations from the rigid-body maneuver being relatively small. The perturbation equations have the form of linear equations with time-dependent coefficients. An active control technique can then be formulated to permit maneuvering of the spacecraft and simultaneously suppressing the elastic vibration
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
Test for Time Dilation of Intervals Between Pulse Structures in GRBs
If -ray bursts are at cosmological distances, then not only their
constituent pulses but also the intervals between pulses should be
time-dilated. Unlike time-dilation measures of pulse emission, intervals would
appear to require negligible correction for redshift of narrower temporal
structure from higher energy into the band of observation. However, stretching
of pulse intervals is inherently difficult to measure without incurring a
timescale-dependent bias since, as time profiles are stretched, more structure
can appear near the limit of resolution. This problem is compounded in dimmer
bursts because identification of significant structures becomes more
problematic. We attempt to minimize brightness bias by equalizing
signal-to-noise (s/n) level of all bursts. We analyze wavelet-denoised burst
profiles binned to several resolutions, identifying significant fluctuations
between pulse structures and interjacent valleys. When bursts are ranked by
peak flux, an interval time-dilation signature is evident, but its magnitude
and significance are dependent upon temporal resolution and s/n level.Comment: 5 pages in LATeX, REVTEX style, 2 embedded figures. To appear in
Third Huntsville GRB Workshop Proceeding
The ATLAS-SPT Radio Survey of Cluster Galaxies
Using a high-performance computing cluster to mosaic 4,787 pointings, we have
imaged the 100 sq. deg. South Pole Telescope (SPT) deep-field at 2.1 GHz using
the Australian Telescope Compact Array to an rms of 80 Jy and a resolution
of 8". Our goal is to generate an independent sample of radio-selected galaxy
clusters to study how the radio properties compare with cluster properties at
other wavelengths, over a wide range of redshifts in order to construct a
timeline of their evolution out to . A preliminary analysis of the
source catalogue suggests there is no spatial correlation between the clusters
identified in the SPT-SZ catalogue and our wide-angle tail galaxies.Comment: 9 pages, 4 figures. Submitted to Proceedings of Science for "The many
facets of extragalactic radio surveys: towards new scientific challenges",
Bologna, Italy 20-23 October 2015 (EXTRA-RADSUR2015
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