* Astronomy

Title: GRB 090227B: the missing link between the genuine short and disguised short GRBs
Authors: Marco Muccino, Remo Ruffini, Carlo Luciano Bianco, Luca Izzo, Ana Virginia Penacchioni

The observational progress obtained by Fermi-GBM and Konus-Wind satellites is used to identify the new class of genuine short GRBs: short bursts with the same inner engine of the long GRBs but endowed with a severely low value of the Baryon load, B<~5x10^{-5}. The emission from these GRBs mainly consists in a first emission, the P-GRB, followed by a softer emission "squeezed" on the first one. The typical separation between the two components is expected to be smaller than 10^{-3}-10^{-2}s. Attention is given to the time-resolved spectral analysis of GRB090227B. From the 16ms time-binned light curves we find a significant thermal emission in the first 96ms, which we identify with the P-GRB. The subsequent emission is identified with the extended afterglow. We find a P-GRB with the highest temperature ever observed, kT=517keV. We estimate from our theoretical model the cosmological redshift z=1.61 and, consequently, we derive the total energy E^{tot}_{e^+e^-}=2.83x10^{53}ergs, the Baryon load B=4.13x10^{-5}, the Lorentz \Gamma factor at transparency \Gamma_{tr}=14365, and the intrinsic duration \Delta t'~0.35. We also determine the average density of the CircumBurst Medium (CBM), <n_{CBM}>=1.9x10^{-5}particles/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 developed theory of the neutron stars configuration, we estimate the masses of the stars, m_1=m_2=1.34 solar masses, their radii, R_1=R_2=12.24km, and the 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.

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