1,906 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
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
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
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.
On the core-halo distribution of dark matter in galaxies
We investigate the distribution of dark matter in galaxies by solving the
equations of equilibrium of a self-gravitating system of massive fermions
(`inos') at selected temperatures and degeneracy parameters within general
relativity. Our most general solutions show, as a function of the radius, a
segregation of three physical regimes: 1) an inner core of almost constant
density governed by degenerate quantum statistics; 2) an intermediate region
with a sharply decreasing density distribution followed by an extended plateau,
implying quantum corrections; 3) an asymptotic, classical
Boltzmann regime fulfilling, as an eigenvalue problem, a fixed value of the
flat rotation curves. This eigenvalue problem determines, for each value of the
central degeneracy parameter, the mass of the ino as well as the radius and
mass of the inner quantum core. Consequences of this alternative approach to
the central and halo regions of galaxies, ranging from dwarf to big spirals,
for SgrA*, as well as for the existing estimates of the ino mass, are outlined.Comment: 8 pages, 5 figures. Accepted for publication by MNRA
Complex networks in brain electrical activity
We analyze the complex networks associated with brain electrical activity.
Multichannel EEG measurements are first processed to obtain 3D voxel
activations using the tomographic algorithm LORETA. Then, the correlation of
the current intensity activation between voxel pairs is computed to produce a
voxel cross-correlation coefficient matrix. Using several correlation
thresholds, the cross-correlation matrix is then transformed into a network
connectivity matrix and analyzed. To study a specific example, we selected data
from an earlier experiment focusing on the MMN brain wave. The resulting
analysis highlights significant differences between the spatial activations
associated with Standard and Deviant tones, with interesting physiological
implications. When compared to random data networks, physiological networks are
more connected, with longer links and shorter path lengths. Furthermore, as
compared to the Deviant case, Standard data networks are more connected, with
longer links and shorter path lengths--i.e., with a stronger ``small worlds''
character. The comparison between both networks shows that areas known to be
activated in the MMN wave are connected. In particular, the analysis supports
the idea that supra-temporal and inferior frontal data work together in the
processing of the differences between sounds by highlighting an increased
connectivity in the response to a novel sound.Comment: 22 pages, 5 figures. Starlab preprint. This version is an attempt to
include better figures (no content change
Circular motion of neutral test particles in Reissner-Nordstr\"om spacetime
We investigate the motion of neutral test particles in the gravitational
field of a mass with charge described by the Reissner-Nordstr\"om (RN)
spacetime. We focus on the study of circular stable and unstable orbits around
configurations describing either black holes or naked singularities. We show
that at the classical radius, defined as , there exist orbits with zero
angular momentum due to the presence of repulsive gravity. The analysis of the
stability of circular orbits indicates that black holes are characterized by a
continuous region of stability. In the case of naked singularities, the region
of stability can split into two non-connected regions inside which test
particles move along stable circular orbits.Comment: 23 pages, 22 figures. To be published Phys. Rev.
- …