100 research outputs found
On the nature of gamma-ray burst time dilations
The recent discovery that faint gamma-ray bursts are stretched in time
relative to bright ones has been interpreted as support for cosmological
distances: faint bursts have their durations redshifted relative to bright
ones. It was pointed out, however, that the relative time stretching can also
be produced by an intrinsic correlation between duration and luminosity of
gamma-ray bursts in a nearby, bounded distribution. While both models can
explain the average amount of time stretching, we find a generic difference
between them in the way the duration distribution of faint bursts deviates from
that of bright ones. This allows us to distinguish between these two broad
classes of model on the basis of the duration distributions of gamma-ray
bursts, leading perhaps to an unambiguous determination of the distance scale
of gamma-ray bursts. We apply our proposed test to the second BATSE catalog and
conclude, with some caution, that the data favor a cosmological interpretation
of the time dilation.Comment: 9 pages uuencoded compressed postscript including 2 figures,
Princeton University Observatory preprint POP-567. Submitted to Astrophysical
Journal Letters, 2 June 199
The Vela pulsar `jet': a companion-punctured bubble of fallback material
Markwardt and Oegelman (1995) used ROSAT to reveal a 12 by 45 arcmin
structure in 1 keV X rays around the Vela pulsar, which they interpret as a jet
emanating from the pulsar. We here present an alternative view of the nature of
this feature, namely that it consists of material from very deep inside the
exploding star, close to the mass cut between material that became part of the
neutron star and ejected material. The initial radial velocity of the inner
material was lower than the bulk of the ejecta, and formed a bubble of slow
material that started expanding again due to heating by the young pulsar's
spindown energy. The expansion is mainly in one direction, and to explain this
we speculate that the pre-supernova system was a binary. The explosion caused
the binary to unbind, and the pulsar's former companion carved a lower-density
channel into the main ejecta. The resulting puncture of the bubble's edge
greatly facilitated expansion along its path relative to other directions. If
this is the case, we can estimate the current speed of the former binary
companion and from this reconstruct the presupernova binary orbit. It follows
that the exploding star was a helium star, hence that the supernova was of type
Ib. Since the most likely binary companion is another neutron star, the
evolution of the Vela remnant and its surroundings has been rather more
complicated than the simple expansion of one supernova blast wave into
unperturbed interstellar material.Comment: submitted to MNRAS; 6 pages laTeX, 3 figures (1 postscript, 2 gif
files of images
Evidence against field decay proportional to accreted mass in neutron stars
A specific class of pulsar recycling model, in which magnetic-field decrease
is a function only of the amount of mass accreted onto the neutron star, is
examined in detail. It is shown that no model in this class is consistent with
all available data on X-ray binaries and recycled pulsars. Only if all
constraints are stretched to their limit and a few objects (PSR B1831-00 and 4U
1626-67) are assumed to have formed in a non-standard manner is there still an
acceptable model of this kind left. Improved measurements of the parameters of
a few of the oldest known radio pulsars will soon test and probably rule out
that one as well. Evidence for the origin of PSR B1831-00 via accretion-induced
collapse of a white dwarf is called into question as a result.Comment: 8 pages LaTeX with 5 in-text postscript figures. Improvements from
previous version include extended and improved discussion and one extra
figure. MNRAS, accepted 02-Jan-9
Shocked by GRB 970228: the afterglow of a cosmological fireball
The location accuracy of the BeppoSAX Wide Field Cameras and acute
ground-based followup have led to the detection of a decaying afterglow in X
rays and optical light following the classical gamma-ray burst GRB 970228. The
afterglow in X rays and optical light fades as a power law at all wavelengths.
This behaviour was predicted for a relativistic blast wave that radiates its
energy when it decelerates by ploughing into the surrounding medium. Because
the afterglow has continued with unchanged behaviour for more than a month, its
total energy must be of order 10**51 erg, placing it firmly at a redshift of
order 1. Further tests of the model are discussed, some of which can be done
with available data, and implications for future observing strategies are
pointed out. We discuss how the afterglow can provide a probe for the nature of
the burst sources.Comment: 6 pages LaTeX, 1 postscript figure; minor edits, slightly more data
on light curve, MNRAS, IN PRESS (mid June/early July
Constraints on the Gamma-ray Burst Luminosity Function from PVO and BATSE
We examine the width of the gamma-ray burst luminosity function through the
distribution of GRB peak fluxes as detected by the Pioneer Venus Orbiter (PVO)
and the Burst and Transient Source Experiment (BATSE). The strength of the
analysis is greatly enhanced by using a merged catalog of peak fluxes from both
instruments with good cross-calibration of their sensitivities. The range of
peak fluxes is increased by approximately a factor of 20 relative to the BATSE
catalog. Thus, more sensitive investigations of the
distribution are possible. We place constraints on the width of the luminosity
function of gamma-ray bursts brighter than the BATSE completeness limit by
comparing the intensity distribution in the merged catalog with those produced
by a variety of spatial density and luminosity functions. For the models
examined, of the {\em detectable\/} bursts have peak luminosities within
a range of 10, indicating that the peak luminosities of gamma-ray bursts span a
markedly less wide range of values than many other of their measurable
properties. We also discuss for which slopes of a power-law luminosity function
the observed width is at the upper end of the constrained range. This is
important in determining the power-law slopes for which luminosity-duration
correlations could be important.Comment: 10 pages latex + 2 uuencoded figures; APJL accepte
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