1,449 research outputs found
GRB 090227B: the missing link between the genuine short and long GRBs
The time-resolved spectral analysis of GRB090227B, made possible by the
Fermi-GBM data, allows to identify in this source the missing link between the
genuine short and long GRBs. Within the Fireshell model [...] we predict
genuine short GRBs: bursts with the same inner engine of the long bursts but
endowed with a severely low value of the Baryon load, B<~5x10^{-5}. A first
energetically predominant emission occurs at the transparency of the e+e-
plasma, the Proper-GRB (P-GRB), followed by a softer emission, the extended
afterglow. The typical separation between the two emissions is expected to be
[...] 10^{-3}-10^{-2}s. We identify the P-GRB [...] in the first 96ms of
emission, where a thermal component with [...] kT=(517+/-28)keV and a flux
comparable with the non thermal part of the spectrum is observed. This non
thermal component as well as the subsequent emission, where there is no
evidence for a thermal spectrum, is identified with the extended afterglow. We
deduce a theoretical cosmological redshift z=1.61+/-0.14. We then derive the
total energy E^{tot}_{e+e-}=(2.83+/-0.15)x10^{53}erg, [...]
B=(4.13+/-0.05)x10^{-5}, the Lorentz factor at transparency
\Gamma_tr=(1.44+/-0.01)x10^4, and the intrinsic duration \Delta t'~0.35s. We
also determine the average density of the CircumBurst Medium (CBM),
=(1.90+/-0.20)x10^{-5} #/cm^3. There is no evidence of beaming in the
system. In view of the energetics and of the Baryon load of the source, as well
as of the low interstellar medium and of the intrinsic time scale of the
signal, we identify the GRB progenitor as a binary neutron star. From the
recent progress in the theory of neutron stars, we obtain masses of the stars
m_1=m_2=1.34M_Sun and their corresponding radii R_1=R_2=12.24km and thickness
of their crusts ~0.47km, consistent with the above values of the Baryon load,
of the energetics and of the time duration of the event.Comment: 14 pages, 14 figures, new version with some updated references,
matching the one actually appeared on Ap
Pair plasma relaxation time scales
By numerically solving the relativistic Boltzmann equations, we compute the
time scale for relaxation to thermal equilibrium for an optically thick
electron-positron plasma with baryon loading. We focus on the time scales of
electromagnetic interactions. The collisional integrals are obtained directly
from the corresponding QED matrix elements. Thermalization time scales are
computed for a wide range of values of both the total energy density (over 10
orders of magnitude) and of the baryonic loading parameter (over 6 orders of
magnitude). This also allows us to study such interesting limiting cases as the
almost purely electron-positron plasma or electron-proton plasma as well as
intermediate cases. These results appear to be important both for laboratory
experiments aimed at generating optically thick pair plasmas as well as for
astrophysical models in which electron-positron pair plasmas play a relevant
role.Comment: Phys. Rev. E, in pres
Theory of photospheric emission from relativistic outflows
In this paper we reexamine the optical depth of ultrarelativistic spherically
symmetric outflows and reevaluate the photospheric radius for each model during
both the acceleration and coasting phases. It is shown that for both the wind
and the shell models there are two asymptotic solutions for the optical depth
during the coasting phase of the outflow. In particular we show that quite
counterintuitively a geometrically thin shell may appear as a thick wind for
photons propagating inside it. For this reason we introduce notions of photon
thick and photon thin outflows, which appear more general and better physically
motivated with respect to winds and shells. Photosphere of relativistic outflow
is a dynamic surface. We study its geometry and find that the photosphere of
photon thin outflow has always a convex shape, while in the photon thick one it
is initially convex (there is always a photon thin layer in any outflow) and
then it becomes concave asymptotically approaching the photosphere of an
infinitely long wind. We find that both instantaneous and time integrated
observed spectra are very close to the thermal one for photon thick outflows,
in line with existing studies. It is our main finding that the photospheric
emission from the photon thin outflow produces non thermal time integrated
spectra, which may be described by the Band function well known in the GRB
literature. We find that energetic GRBs should produce photon thin outflows
with photospheric emission lasting less than one second for the total energy
erg and baryonic loading parameter . It means
that only time integrated spectra may be observed from such GRBs.Comment: Revision of the previous version, new effect is discussed.
Conclusions remain unchange
Evidence Against Astrophysical Dyadospheres
It is shown how pair production itself would almost certainly prevent the
astrophysical formation of macroscopic dyadospheres, hypothetical regions,
extending many electron Compton wavelengths in all directions, where the
electric field exceeds the critical value for microscopically rapid Schwinger
pair production. Pair production is a self-regulating process that would
discharge a growing electric field, in the example of a hypothetical collapsing
charged stellar core, before it reached 6% of the minimum dyadosphere value,
keeping the pair production rate more than 26 orders of magnitude below the
dyadosphere value, and keeping the efficiency below 0.0002 (M/M_sun)^{1/2}.Comment: 27 pages, LaTeX, shortened version of astro-ph/0605432 accepted 2006
Aug. 17 by The Astrophysical Journal, but also with some significant new
materia
Electron-positron plasma in GRBs and in cosmology
Electron-positron plasma is believed to play an imporant role both in the early Universe and in sources of Gamma-Ray Bursts (GRBs). We focus on analogies and differences between physical conditions of electron-positron plasma in the early Universe and in sources of GRBs. We discuss: a) dynamical differences, namely thermal acceleration of the outflow in GRB sources vs. cosmological deceleration; b) nuclear composition differences as synthesis of light elements in the early Universe and possible destruction of heavy elements in GRB plasma; c) different physical conditions during last scattering of photons by electrons. Only during the acceleration phase of the optically thick electron-positron plasma the comoving observer may find it similar to the early Universe. This similarity breaks down during the coasting phase. Reprocessing of nuclear abundances may likely take place in GRB sources. Heavy nuclear elements are then destroyed, resulting mainly in
protons with small admixture of helium. Unlike the primordial plasma which recombines to form neutral hydrogen, and emits the Cosmic Microwave Background
Radiation, GRB plasma does not cool down enough to recombine
Maximum Mass-Radius Ratios for Charged Compact General Relativistic Objects
Upper limits for the mass-radius ratio and total charge are derived for
stable charged general relativistic matter distributions. For charged compact
objects the mass-radius ratio exceeds the value 4/9 corresponding to neutral
stars. General restrictions for the redshift and total energy (including the
gravitational contribution) are also obtained.Comment: 6 pages, 2 figures, RevTex. To appear in Europhys. Let
On the structure of the burst and afterglow of Gamma-Ray Bursts I: the radial approximation
We have proposed three paradigms for the theoretical interpretation of
gamma-ray bursts (GRBs). (1) The relative space-time transformation (RSTT)
paradigm emphasizes how the knowledge of the entire world-line of the source
from the moment of gravitational collapse is a necessary condition to interpret
GRB data. (2) The interpretation of the burst structure (IBS) paradigm
differentiates in all GRBs between an injector phase and a beam-target phase.
(3) The GRB-supernova time sequence (GSTS) paradigm introduces the concept of
induced supernova explosion in the supernovae-GRB association. These three
paradigms are illustrated using our theory based on the vacuum polarization
process occurring around an electromagnetic black hole (EMBH theory) and using
GRB 991216 as a prototype. We illustrate the five fundamental eras of the EMBH
theory: the self acceleration of the pair-electromagnetic plasma (PEM
pulse), its interaction with the baryonic remnant of the progenitor star (PEMB
pulse). We then study the approach of the PEMB pulse to transparency, the
emission of the proper GRB (P-GRB) and its relation to the ``short GRBs''.
Finally the three different regimes of the afterglow are described within the
fully radiative and radial approximations. The best fit of the theory leads to
an unequivocal identification of the ``long GRBs'' as extended emission
occurring at the afterglow peak (E-APE). The relative intensities, the time
separation and the hardness ratio of the P-GRB and the E-APE are used as
distinctive observational test of the EMBH theory and the excellent agreement
between our theoretical predictions and the observations are documented. The
afterglow power-law indexes in the EMBH theory are compared and contrasted with
the ones in the literature, and no beaming process is found for GRB 991216.Comment: 96 pages, 40 figures, to appear on Int. Journ. Mod. Phys.
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