GRB 090227b: The Missing Link Between the Genuine Short and Long GRBS

Abstract

The time-resolved spectral analysis of GRB 090227B, 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 of the Gamma-Ray Bursts (GRBs) 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 5 × 10 −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 of the order of 10 −3 -10 −2 s. We identify the P-GRB of GRB 090227B in the first 96 ms of emission, where a thermal component with the temperature 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) × 10 53 ergs, the Baryon load B = (4.13 ± 0.05) × 10 −5, the Lorentz Γ factor at transparency Γ tr = (1.44 ± 0.01) × 10 4 , and the intrinsic duration ∆t ′ ∼ 0.35 s. We also determine the average density of the CircumBurst Medium (CBM), n CBM = (1.90 ± 0.20) × 10 −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 progress in the theory of neutron stars, we obtain masses of the stars m 1 = m 2 = 1.34M ⊙ and their corresponding radii R 1 = R 2 = 12.24 km and thickness of their crusts ∼ 0.47 km, consistent with the above values of the Baryon load, of the energetics and of the time duration of the event.