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 $\log N-\log P$ 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, $90\%$ 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