930 research outputs found
Pure and loaded fireballs in SGR giant flares
On December 27, 2004, a giant flare from SGR 180620 was detected on earth.
Its thermal spectrum and temperature suggest that the flare resulted from an
energy release of about erg/sec close to the surface of a neutron
star in the form of radiation and/or pairs. This plasma expanded under its own
pressure producing a fireball and the observed gamma-rays escaped once the
fireball became optically thin. The giant flare was followed by a bright radio
afterglow, with an observable extended size, implying an energetic relativistic
outflow. We revisit here the evolution of relativistic fireballs and we
calculate the Lorentz factor and energy remaining in relativistic outflow once
the radiation escapes. We show that pairs that arise naturally in a pure
pairs-radiation fireball do not carry enough energy to account for the observed
afterglow. We consider various alternatives and we show that if the
relativistic outflow that causes the afterglow is related directly to the
prompt flare, then the initial fireball must be loaded by baryons or Poynting
flux. While we focus on parameters applicable to the giant flare and the radio
afterglow of SGR 180620 the calculations presented here might be also
applicable to GRBs
GRB990123, The Optical Flash and The Fireball Model
We compare the ongoing observations of the remarkable burst GRB990123, the
mother of all bursts, with the predictions of the afterglow theory. We show
that the observations agree with the recent prediction that a reverse shock
propagating into the ejecta would produce a very strong prompt optical flash.
This reverse shock has also produced the 8.46GHz radio signal, observed after
one day. The forward shock, which propagates into the ISM is the origin of the
classical afterglow. It has produced the prompt X-ray signal as well as the
late optical and IR emission. It would most likely produce a radio emission
within the next few weeks. The observations suggest that the initial Lorentz
factor of the ejecta was . Within factors of order unity, this crude
model explains all current observations of GRB990123.Comment: 14 pages including 2 figure
The Expected Duration of Gamma-Ray Bursts in the Impulsive Hydrodynamic Models
Depending upon the various models and assumptions, the existing literature on
Gamma Ray Bursts (GRBs) mentions that the gross theoretical value of the
duration of the burst in the hydrodynamical models is tau~r^2/(eta^2 c), where
r is the radius at which the blastwave associated with the fireball (FB)
becomes radiative and sufficiently strong. Here eta = E/Mc^2, c is the speed of
light, E is initial lab frame energy of the FB, and M is the baryonic mass of
the same (Rees and Meszaros 1992). However, within the same basic framework,
some authors (like Katz and Piran) have given tau ~ r^2 /(eta c). We intend to
remove this confusion by considering this problem at a level deeper than what
has been considered so far. Our analysis shows that none of the previously
quoted expressions are exactly correct and in case the FB is produced
impulsively and the radiative processes responsible for the generation of the
GRB are sufficiently fast, its expected duration would be tau ~ar^2/(eta^2 c),
where a~O(10^1). We further discuss the probable change, if any, of this
expression, in case the FB propagates in an anisotropic fashion. We also
discuss some associated points in the context of the Meszaros and Rees
scenario.Comment: 21 pages, LATEX (AAMS4.STY -enclosed), 1 ps. Fig. Accepted in
Astrophysical Journa
Radiative Efficiencies of Continuously Powered Blast Waves
We use general arguments to show that a continuously powered radiative blast
wave can behave self similarly if the energy injection and radiation mechanisms
are self similar. In that case, the power-law indices of the blast wave
evolution are set by only one of the two constituent physical mechanisms. If
the luminosity of the energy source drops fast enough, the radiation mechanisms
set the power-law indices, otherwise, they are set by the behavior of the
energy source itself. We obtain self similar solutions for the Newtonian and
the ultra-relativistic limits. Both limits behave self similarly if we assume
that the central source supplies energy in the form of a hot wind, and that the
radiative mechanism is the semi-radiative mechanism of Cohen, Piran & Sari
(1998). We calculate the instantaneous radiative efficiencies for both limits
and find that a relativistic blast wave has a higher efficiency than a
Newtonian one. The instantaneous radiative efficiency depends strongly on the
hydrodynamics and cannot be approximated by an estimate of local microscopic
radiative efficiencies, since a fraction of the injected energy is deposited in
shocked matter. These solutions can be used to calculate Gamma Ray Bursts
afterglows, for cases in which the energy is not supplied instantaneously.Comment: 28 LaTeX pages, including 9 figures and 3 table
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