2,239 research outputs found

### 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

### System of self-gravitating bosons

Self gravitating boson system equilibrium configuration

### GRB970228 and a class of GRBs with an initial spikelike emission

(Shortened) The Swift and HETE-2 discovery of an afterglow associated
possibly with short GRBs opened the new problematic of their nature and
classification. This has been further enhanced by the GRB060614 observation and
by a re-analysis of the BATSE catalog leading to the identification of a new
GRB class with "an occasional softer extended emission lasting tenths of
seconds after an initial spikelike emission". We plan: a) to fit this new class
of "hybrid" sources within our "canonical GRB" scenario, where all GRBs are
generated by a "common engine" (i.e. the gravitational collapse to a black
hole); b) to propose GRB970228 as the prototype of the such a class. We analyze
BeppoSAX data on GRB970228 in the 40-700 keV and 2-26 keV energy bands within
the "fireshell" model. We find that GRB970228 is a "canonical GRB", like e.g.
GRB050315, with the main peculiarity of a particularly low CircumBurst Medium
(CBM) average density n_{cbm}~10^{-3} #/cm^3. We also simulate the light curve
corresponding to a rescaled CBM density profile with n_{cbm}=1 #/cm^3. From
such a comparison it follows that the total time-integrated luminosity is a
faithful indicator of the GRB nature, contrary to the peak luminosity which is
merely a function of the CBM density. We call attention on discriminating the
short GRBs between the "genuine" and the "fake" ones. The "genuine" ones are
intrinsically short, with baryon loading B \la 10^{-5}, as stated in our
original classification. The "fake" ones, characterized by an initial spikelike
emission followed by an extended emission lasting tenths of seconds, have a
baryon loading 10^{-4} \la B \leq 10^{-2}. They are observed as such only due
to an underdense CBM consistent with a galactic halo environment which deflates
the afterglow intensity.Comment: 4 pages, 4 figures, to appear on A&A Letter

### GRB060218 and GRBs associated with Supernovae Ib/c

We plan to fit the complete gamma- and X-ray light curves of the long
duration GRB060218, including the prompt emission, in order to clarify the
nature of the progenitors and the astrophysical scenario of the class of GRBs
associated to SNe Ib/c. The initial total energy of the electron-positron
plasma E_{e^\pm}^{tot}=2.32\times 10^{50} erg has a particularly low value
similarly to the other GRBs associated with SNe. For the first time we observe
a baryon loading B=10^{-2} which coincides with the upper limit for the
dynamical stability of the fireshell. The effective CircumBurst Medium (CBM)
density shows a radial dependence n_{cbm} \propto r^{-\alpha} with
1.0<\alpha<1.7 and monotonically decreases from 1 to 10^{-6} particles/cm^3.
Such a behavior is interpreted as due to a fragmentation in the fireshell.
Analogies with the fragmented density and filling factor characterizing Novae
are outlined. The fit presented is particularly significant in view of the
complete data set available for GRB060218 and of the fact that it fulfills the
Amati relation. We fit GRB060218, usually considered as an X-Ray Flash (XRF),
as a "canonical GRB" within our theoretical model. The smallest possible black
hole, formed by the gravitational collapse of a neutron star in a binary
system, is consistent with the especially low energetics of the class of GRBs
associated with SNe Ib/c. We give the first evidence for a fragmentation in the
fireshell. Such a fragmentation is crucial in explaining both the unusually
large T_{90} and the consequently inferred abnormal low value of the CBM
effective density.Comment: 4 pages, 3 figures, to appear in A&A Letter

### The Amati relation in the "fireshell" model

(Shortened) CONTEXT: [...] AIMS: Motivated by the relation proposed by Amati
and collaborators, we look within the ``fireshell'' model for a relation
between the peak energy E_p of the \nu F_\nu total time-integrated spectrum of
the afterglow and the total energy of the afterglow E_{aft}, which in our model
encompasses and extends the prompt emission. METODS: [...] Within the fireshell
model [...] We can then build two sets of ``gedanken'' GRBs varying the total
energy of the electron-positron plasma E^{e^\pm}_{tot} and keeping the same
baryon loading B of GRB050315. The first set assumes for the effective CBM
density the one obtained in the fit of GRB050315. The second set assumes
instead a constant CBM density equal to the average value of the GRB050315
prompt phase. RESULTS: For the first set of ``gedanken'' GRBs we find a
relation E_p\propto (E_{aft})^a, with a = 0.45 \pm 0.01, whose slope strictly
agrees with the Amati one. Such a relation, in the limit B \to 10^{-2},
coincides with the Amati one. Instead, in the second set of ``gedanken'' GRBs
no correlation is found. CONCLUSIONS: Our analysis excludes the Proper-GRB
(P-GRB) from the prompt emission, extends all the way to the latest afterglow
phases and is independent on the assumed cosmological model, since all
``gedanken'' GRBs are at the same redshift. The Amati relation, on the other
hand, includes also the P-GRB, focuses on the prompt emission only, and is
therefore influenced by the instrumental threshold which fixes the end of the
prompt emission, and depends on the assumed cosmology. This may well explain
the intrinsic scatter observed in the Amati relation.Comment: 4 pages, 5 figures, to appear on A&A Letter

### Energy Extraction From Gravitational Collapse to Static Black Holes

The mass--energy formula of black holes implies that up to 50% of the energy
can be extracted from a static black hole. Such a result is reexamined using
the recently established analytic formulas for the collapse of a shell and
expression for the irreducible mass of a static black hole. It is shown that
the efficiency of energy extraction process during the formation of the black
hole is linked in an essential way to the gravitational binding energy, the
formation of the horizon and the reduction of the kinetic energy of implosion.
Here a maximum efficiency of 50% in the extraction of the mass energy is shown
to be generally attainable in the collapse of a spherically symmetric shell:
surprisingly this result holds as well in the two limiting cases of the
Schwarzschild and extreme Reissner-Nordstr\"{o}m space-times. Moreover, the
analytic expression recently found for the implosion of a spherical shell onto
an already formed black hole leads to a new exact analytic expression for the
energy extraction which results in an efficiency strictly less than 100% for
any physical implementable process. There appears to be no incompatibility
between General Relativity and Thermodynamics at this classical level.Comment: 7 pages, 2 figures, to appear on Int. Journ. Mod. Phys.

### 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

### GRB060614: a "fake" short GRB from a merging binary system

(Shortened) CONTEXT: [...] GRB060614 is the first nearby long duration GRB
clearly not associated to a bright Ib/c supernova. Moreover, its duration
(T_{90} ~ 100s) makes it hardly classifiable as a short GRB. It presents strong
similarities with GRB970228, the prototype of the new class of "fake" short
GRBs that appear to originate from the coalescence of binary neutron stars or
white dwarfs spiraled out into the galactic halo. AIMS: Within the "canonical"
GRB scenario based on the "fireshell" model, we test if GRB060614 can be a
"fake" or "disguised" short GRB. [...] METHODS: We fit GRB060614 light curves
in Swift's BAT (15-150keV) and XRT (0.2-10keV) energy bands. Within the
fireshell model, light curves are formed by two well defined and different
components: the Proper-GRB (P-GRB), emitted at the fireshell transparency, and
the extended afterglow, due to the interaction between the leftover accelerated
baryonic and leptonic shell and the CBM. RESULTS: We determine the two free
parameters describing the GRB source within the fireshell model. [...] A small
average CBM density [...] is inferred, typical of galactic halos. The first
spikelike emission is identified with the P-GRB and the following prolonged
emission with the extended afterglow peak.[...] CONCLUSIONS: The anomalous
GRB060614 finds a natural interpretation within our canonical GRB scenario: it
is a "disguised" short GRB. [...] This result points to an old binary system,
likely formed by a white dwarf and a neutron star, as the progenitor of
GRB060614 and well justify the absence of an associated SN Ib/c. Particularly
important for further studies of the final merging process are the temporal
structures in the P-GRB down to 0.1s.Comment: 7 pages, 5 figures, to appear on Astronomy & Astrophysics. This new
version fixes a typo in one label of Fig.

### Relativistic Thomas-Fermi Model at Finite Temperatures

We briefly review the Thomas-Fermi statistical model of atoms in the
classical non-relativistic formulation and in the generalised finite-nucleus
relativistic formulation. We then discuss the classical generalisation of the
model to finite temperatures in the non-relativistic approximation and present
a new relativistic model at finite temperatures, investigating how to recover
the existing theory in the limit of low temperatures. This work is intended to
be a propedeutical study for the evaluation of equilibrium configurations of
relativistic ``hot'' white dwarfs.Comment: 8 pages, Latex wsp9.cls. Proceedings Marcel Grossmann IX, Rome
(Italy), 2-9 July 2000 (World Scientific

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