## GRB 090227B, a genuine short burst, and GRB 090510, a disguised short burst explosion in thehighest CircumBurst Medium ever inferred

Author: Marco Muccino

Abstract: Thanks to the observation by the Fermi satellite, we compare and contrast two bursts classically classified as short, GRB 090227B and GRB 090510.

Within the Fireshell model, GRB 090227B is identified as the first example of genuine short GRBs with an estimated redshift z = 1.61 ± 0.15 and a total energy Ee+e- = (2.83±0.15)×1053 erg. The Baryon load is B=(4.13±0.05)×10-5 and the average density of the CircumBurst Medium (CBM) =(1.90±0.20)×10-5 particles/cm3. In view of the above values of the Baryon load and of the energetics as well as of the low CBM, we assume the progenitor of this burst to be a symmetric binary neutron stars with masses m=m1=m2=1.34 Msun, radii, R=R1=R2=12.24 km, and the thickness of their crusts, 0.47 km.

For GRB 090510, assuming a total plasma energy Ee+e- = (1.10±0.06)×1053 erg, we derive the Baryon load B=(1.45±0.28)\times10^{-3} and the Lorentz factor at transpareny Γ=(6.7±1.7)×102, which are characteristic of the long GRB class. From our simulation we evaluate the highest CBM average density ever measured, (1.85±0.14)×103 particles/cm3. The joint effect of the high Lorentz factor and the high density compresses in time and "inflates" in intensity the extended afterglow, making appear GRB 090510 as a short burst, which we here define as "disguised short GRB by excess" occurring an overdense galactic region with 103 particles/cm3.