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),
<n_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.
