Two short bursts originating from different astrophysical systems: The genuine short GRB 090227B and the disguised short GRB 090510 by excess

Abstract

GRB 090227B and GRB 090510 are two gamma-ray bursts (GRBs) traditionally classified as short bursts. The major outcome of our analysis is that they indeed originate from different systems. In the case of GRB 090227B, from the inferred values of the total energy of the e + e − plasma, $$E_{e^ + e^ - }^{tot} = (2.83 \pm 0.15) \times 10^{53}$$ erg, the engulfed baryonic mass M B , expressed as $$B = M_B c^2 /E_{e^ + e^ - }^{tot} = (4.1 \pm 0.05) \times 10^{ - 5}$$ , and the circumburst medium (CBM) average density, 〈n CBM 〉 = (1.90±0.20)×10−5 cm−3, we have assumed the progenitor of this burst to be a symmetric neutron star (NS) merger with masses m = 1.34 M⊙, radii R = 12.24 km, and crustal thicknesses of ~ 0.47 km. In the case of GRB 090510, we have derived the total plasma energy, $$E_{e + e^ - }^{tot} = \left( {1.10 \pm 0.06} \right) \times 10^{53} $$ erg, the Baryon load, B = (1.45 ± 0.28) × 10−3, and the Lorentz factor at transparency, Γ = (6.7 ± 1.7) × 102, which are characteristic of the long GRB class, as well as a very high CBM density, 〈n CBM 〉 = (1.85 ± 0.14) × 103 cm−3. The joint effect of the high values of G and 〈n CBM 〉 compresses in time and “inflates” in intensity the extended afterglow, making GRB 090510 appear to be a short burst, which we here define as a “disguised short GRB by excess”, occurring in an overdense region with 103 cm−3.