The genuine short GRB 090227B and the disguised by excess GRB 090510

GRB 090227B and GRB 090510, traditionally classified as short gamma-ray Bursts (GRBs), indeed originate from different systems. For GRB 090227B we inferred a total energy of the e + e − plasma = (2.83 ± 0.15) × 10 53 erg, a baryon load of B = (4.1 ± 0.05) × 10 −5 , and a CircumBurstMedium (CBM) average density 〈 n CBM 〉 = (1.90 ± 0.20) × 10 −5 cm −3 . From these results we have assumed the progenitor of this burst to be a symmetric neutron stars (NSs) merger with masses m = 1.34 M ⊙ , radii R = 12.24 km. GRB 090510, instead, has = (1.10 ± 0.06) × 10 53 erg, B = (1.45 ± 0.28) × 10 −3 , implying a Lorentz factor at transparency of Γ = (6.7 ± 1.7) × 10 2 , which are characteristic of the long GRB class, and a very high CBM density, 〈 n CBM 〉 = (1.85 ± 0.14) × 10 3 cm −3 . The joint effect of the high values of Γ and of 〈 n CBM 〉 compresses in time and “inflates” in intensity in an extended afterglow, making appear GRB 090510 as a short burst, which we here define as “disguised short GRB by excess” occurring an overdense region with 10 3 cm −3 .