GRB 140619B: a short GRB from a binary neutron star merger leading to black hole formation

Abstract

We show the existence of two families of short GRBs, both originating from the merger of binary neutron stars (NSs): family-1 with $E_{iso}<10^{52}$ erg, leading to a massive NS as the merged core, and family-2 with $E_{iso}>10^{52}$ erg, leading to a black hole (BH). Following the identification of the prototype GRB 090227B, we present the details of a new example of family-2 short burst: GRB 140619B. From the spectral analysis of the early $\sim0.2$ s, we infer an observed temperature $kT =(324\pm33)$ keV of the $e^+e^-$-plasma at transparency (P-GRB), a theoretically derived redshift $z=2.67\pm0.37$, a total burst energy $E^{tot}_{e^+e^-}=(6.03\pm0.79)\times10^{52}$ erg, a rest-frame peak energy $E_{p,i}=4.7$ MeV, and a baryon load $B=(5.52\pm0.73)\times10^{-5}$. We also estimate the corresponding emission of gravitational waves. Two additional examples of family-2 short bursts are identified: GRB 081024B and GRB 090510, remarkable for its well determined cosmological distance. We show that marked differences exist in the nature of the afterglows of these two families of short bursts: family-2 bursts, leading to BH formation, consistently exhibit high energy emission following the P-GRB emission; family-1 bursts, leading to the formation of a massive NS, should never exhibit high energy emission. We also show that both the families fulfill an $E_{p,i}$--$E_{iso}$ relation with slope $\gamma=0.59\pm0.07$ and a normalization constant incompatible with the one for long GRBs. The observed rate of such family-2 events is $\rho_0=\left(2.1^{+2.8}_{-1.4}\right)\times10^{-4}$Gpc$^{-3}$yr$^{-1}$.