16,501 research outputs found
GRB970228 and the class of GRBs with an initial spikelike emission: do they follow the Amati relation?
On the basis of the recent understanding of GRB050315 and GRB060218, we
return to GRB970228, the first Gamma-Ray Burst (GRB) with detected afterglow.
We proposed it as the prototype for a new class of GRBs with "an occasional
softer extended emission lasting tenths of seconds after an initial spikelike
emission". Detailed theoretical computation of the GRB970228 light curves in
selected energy bands for the prompt emission are presented and compared with
observational BeppoSAX data. From our analysis we conclude that GRB970228 and
likely the ones of the above mentioned new class of GRBs are "canonical GRBs"
have only one peculiarity: they exploded in a galactic environment, possibly
the halo, with a very low value of CBM density. Here we investigate how
GRB970228 unveils another peculiarity of this class of GRBs: they do not
fulfill the "Amati relation". We provide a theoretical explanation within the
fireshell model for the apparent absence of such correlation for the GRBs
belonging to this new class.Comment: 5 pages, 3 figures, in the Proceedings of the "4th Italian-Sino
Workshop on Relativistic Astrophysics", held in Pescara, Italy, July 20-28,
2007, C.L. Bianco, S.-S. Xue, Editor
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
Web User Session Characterization via Clustering Techniques
We focus on the identification and definition of "Web user-sessions", an aggregation of several TCP connections generated by the same source host on the basis of TCP connection opening time. The identification of a user session is non trivial; traditional approaches rely on threshold based mechanisms, which are very sensitive to the value assumed for the threshold and may be difficult to correctly set. By applying clustering techniques, we define a novel methodology to identify Web user-sessions without requiring an a priori definition of threshold values. We analyze the characteristics of user sessions extracted from real traces, studying the statistical properties of the identified sessions. From the study it emerges that Web user-sessions tend to be Poisson, but correlation may arise during periods of network/hosts anomalous functioning
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
- …