1,610 research outputs found
Physical parameters of GRB 970508 and GRB 971214 from their afterglow synchrotron emission
We have calculated synchrotron spectra of relativistic blast waves, and find
predicted characteristic frequencies that are more than an order of magnitude
different from previous calculations. For the case of an adiabatically
expanding blast wave, which is applicable to observed gamma-ray burst (GRB)
afterglows at late times, we give expressions to infer the physical properties
of the afterglow from the measured spectral features.
We show that enough data exist for GRB970508 to compute unambiguously the
ambient density, n=0.03/cm**3, and the blast wave energy per unit solid angle,
E=3E52 erg/4pi sr. We also compute the energy density in electrons and magnetic
field. We find that they are 12% and 9%, respectively, of the nucleon energy
density and thus confirm for the first time that both are close to but below
equipartition.
For GRB971214, we discuss the break found in its spectrum by Ramaprakash et
al. (1998). It can be interpreted either as the peak frequency or as the
cooling frequency; both interpretations have some problems, but on balance the
break is more likely to be the cooling frequency. Even when we assume this, our
ignorance of the self-absorption frequency and presence or absence of beaming
make it impossible to constrain the physical parameters of GRB971214 very well.Comment: very strongly revised analysis of GRB971214 and discussion, submitted
to ApJ, 11 pages LaTeX, 4 figures, uses emulateapj.sty (included
Angular size and emission time scales of relativistic fireballs
The detection of delayed X-ray, optical and radio emission, ``afterglow,''
associated with gamma-ray bursts (GRBs) is consistent with models, where the
bursts are produced by relativistic expanding blast waves, driven by expanding
fireballs at cosmological distances. In particular, the time scales over which
radiation is observed at different wave bands agree with model predictions. It
had recently been claimed that the commonly used relation between observation
time t and blast wave radius r, t=r/2\gamma^{2}c where \gamma(r) is the fluid
Lorentz factor, should be replaced with t=r/16\gamma^{2}c due to blast wave
deceleration. Applying the suggested deceleration modification would make it
difficult to reconcile observed time scales with model predictions. It would
also imply an apparent source size which is too large to allow attributing
observed radio variability to diffractive scintillation. We present a detailed
analysis of the implications of the relativistic hydrodynamics of expanding
blast waves to the observed afterglow. We find that modifications due to shock
deceleration are small, therefore allowing for both the observed afterglow time
scales and for diffractive scintillation. We show that at time t the fireball
appears on the sky as a narrow ring of radius h=r/\gamma and width 0.1h, where
r and t are related by t=r/2\gamma^{2}c.Comment: Submitted to ApJL (11 pages, LaTeX
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
The Dynamics and Light Curves of Beamed Gamma Ray Burst Afterglows
The energy requirements of gamma ray bursts have in past been poorly
constrained because of three major uncertainties: The distances to bursts, the
degree of burst beaming, and the efficiency of gamma ray production. The first
of these has been resolved, with both indirect evidence (the distribution of
bursts in flux and position) and direct evidence (redshifted absorption
features in the afterglow spectrum of GRB 970508) pointing to cosmological
distances. We now wish to address the second uncertainty. Afterglows allow a
statistical test of beaming, described in an earlier paper. In this paper, we
modify a standard fireball afterglow model to explore the effects of beaming on
burst remnant dynamics and afterglow emission. If the burst ejecta are beamed
into angle zeta, the burst remnant's evolution changes qualitatively once its
bulk Lorentz factor Gamma < 1/zeta: Before this, Gamma declines as a power law
of radius, while afterwards, it declines exponentially. This change results in
a broken power law light curve whose late-time decay is faster than expected
for a purely spherical geometry. These predictions disagree with afterglow
observations of GRB 970508. We explored several variations on our model, but
none seems able to change this result. We therefore suggest that this burst is
unlikely to have been highly beamed, and that its energy requirements were near
those of isotropic models. More recent afterglows may offer the first practical
applications for our beamed models.Comment: 18 pages, uses emulateapj.sty, four embedded postscript figures.
Submitted to The Astrophysical Journal, 199
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