1,506 research outputs found
GRB 990123: Reverse and Internal Shock Flashes and Late Afterglow
The prompt (t \siml 0.16 days) light curve and initial 9-th magnitude
optical flash from GRB 990123 can be attributed to a reverse external shock, or
possibly to internal shocks. We discuss the time decay laws and spectral slopes
expected under various dynamical regimes, and discuss the constraints imposed
on the model by the observations, arguing that they provide strongly suggestive
evidence for features beyond those in the simple standard model. The longer
term afterglow behavior is discussed in the context of the forward shock, and
it is argued that, if the steepening after three days is due to a jet geometry,
this is likely to be due to jet-edge effects, rather than sideways expansion.Comment: M.N.R.A.S., subm. 2/26/99; (preprint uses aaspp4.sty), 9 page
Collapsar Jets, Bubbles and Fe Lines
In the collapsar scenario, gamma ray bursts are caused by relativistic jets
expelled along the rotation axis of a collapsing stellar core. We discuss how
the structure and time-dependence of such jets depends on the stellar envelope
and central engine properties, assuming a steady jet injection. It takes a few
seconds for the jet to bore its way through the stellar core; most of the
energy output during that period goes into a cocoon of relativistic plasma
surrounding the jet. This material subsequently forms a bubble of magnetized
plasma that takes several hours to expand, subrelativistically, through the
envelope of a high-mass supergiant. Jet break-through and a conventional burst
would be expected not only in He stars but possibly also in blue supergiants.
Shock waves and magnetic dissipation in the escaping bubble can contribute a
non thermal UV/X-ray afterglow, and also excite Fe line emission from thermal
gas, in addition to the standard jet deceleration power-law afterglow.Comment: Ap.J. Letters, accepted 6/20/01, first subm 4/24/01; aaspp4, 9 pages,
no figures; minor revision
Population III Gamma Ray Bursts
We discuss a model of Poynting-dominated gamma-ray bursts from the collapse
of very massive first generation (pop. III) stars. From redshifts of order 20,
the resulting relativistic jets would radiate in the hard X-ray range around 50
keV and above, followed after roughly a day by an external shock component
peaking around a few keV. On the same timescales an inverse Compton component
around 75 GeV may be expected, as well as a possible infra-red flash. The
fluences of these components would be above the threshold for detectors such as
Swift and Fermi, providing potentially valuable information on the formation
and properties of what may be the first luminous objects and their black holes
in the high redshift Universe.Comment: 12 pages; Apj, subm. 12/10/2009; accepted 04/12/201
'On the difference between the short and long gamma-ray bursts'
We argue that the distributions of both the intrinsic fluence and the
intrinsic duration of the gamma-ray emission in gamma-ray bursts from the BATSE
sample are well represented by log-normal distributions, in which the intrinsic
dispersion is much larger than the cosmological time dilatation and redshift
effects. We perform separate bivariate log-normal distribution fits to the
BATSE short and long burst samples. The bivariate log-normal behaviour results
in an ellipsoidal distribution, whose major axis determines an overall
statistical relation between the fluence and the duration. We show that this
fit provides evidence for a power-law dependence between the fluence and the
duration, with a statistically significant different index for the long and
short groups. We discuss possible biases, which might affect this result, and
argue that the effect is probably real. This may provide a potentially useful
constraint for models of long and short bursts.Comment: A.A. in press ; significantly revised version of astro-ph/0007438; 16
pages 5 PS figure
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