GRB 090510 , observed both by Fermi and AGILE satellites , is the first bright short-hard Gamma-Ray Burst ( GRB ) with an emission from the keV up to the GeV energy range .
Within the Fireshell model , we interpret the faint precursor in the light curve as the emission at the transparency of the expanding e ^ { + } e ^ { - } plasma : the Proper-GRB ( P-GRB ) .
From the observed isotropic energy we assume a total plasma energy E ^ { tot } _ { e ^ { + } e ^ { - } } = ( 1.10 \pm 0.06 ) \times 10 ^ { 53 } erg and derive a Baryon load B = ( 1.45 \pm 0.28 ) \times 10 ^ { -3 } and a Lorentz factor at transparency \Gamma _ { tr } = ( 6.7 \pm 1.6 ) \times 10 ^ { 2 } .
The main emission \sim 0.4 s after the initial spike is interpreted as the extended afterglow , due to the interaction of the ultrarelativistic baryons with the CircumBurst Medium ( CBM ) .
Using the condition of fully radiative regime , we infer a CBM average spherically symmetric density of \langle n _ { CBM } \rangle = ( 1.85 \pm 0.14 ) \times 10 ^ { 3 } particles/cm ^ { 3 } , one of the highest found in the Fireshell model .
The value of the filling factor , 1.5 \times 10 ^ { -10 } \leq \mathcal { R } \leq 3.8 \times 10 ^ { -8 } , leads to the estimate of filaments with densities n _ { fil } = n _ { CBM } / \mathcal { R } \approx ( 10 ^ { 6 } -10 ^ { 14 } ) particles/cm ^ { 3 } .
The sub-MeV and the MeV emissions are well reproduced .
When compared to the canonical GRBs with \langle n _ { CBM } \rangle \approx 1 particles/cm ^ { 3 } and to the disguised short GRBs with \langle n _ { CBM } \rangle \approx 10 ^ { -3 } particles/cm ^ { 3 } , the case of GRB 090510 leads to the existence of a new family of bursts exploding in an over-dense galactic region with \langle n _ { CBM } \rangle \approx 10 ^ { 3 } particles/cm ^ { 3 } .
The joint effect of the high \Gamma _ { tr } and the high density compresses in time and ‘ ‘ inflates ’ ’ in intensity the extended afterglow , making it appear as a short burst , which we here define as ‘ ‘ disguised short GRB by excess ’ ’ .
The determination of the above parameters values may represent an important step towards the explanation of the GeV emission .