GRB 090510: A DISGUISED SHORT GAMMA-RAY BURST WITH THE HIGHEST LORENTZ FACTOR AND CIRCUMBURST MEDIUM

GRB 090510, observed by both 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. From the observed isotropic energy, we assume a total plasma energy Etote+e-=(1.10±0.06)×1053 erg and derive a Baryon load B = (1.45 ± 0.28) × 10–3 and a Lorentz factor at transparency Γtr = (6.7 ± 1.6) × 102. The main emission ∼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 (nCBM) = (1.85 ± 0.14) × 103 particles cm–3, one of the highest found in the Fireshell model. The value of the filling factor, 1.5×10-10≤R≤3.8×10-8, leads to the estimate of filaments with densities nfil = nCBM/R approx. (106-1014) particles cm–3. The sub-MeV and the MeV emissions are well reproduced. When compared to the canonical GRBs with (nCBM) ≈ 1 particles cm–3 and to the disguised short GRBs with (nCBM) ≈ 10–3 particles cm–3, the case of GRB 090510 leads to the existence of a new family of bursts exploding in an overdense galactic region with (nCBM) ≈ 103 particles cm–3. The joint effect of the high Γtr 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 a 'disguised short GRB by excess'. The determination of the above parameter values may represent an important step toward the explanation of the GeV emission