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Ruffini et al. (2015) have introduced a sub-classification of short GRBs into two families based on the observational properties of the final fate of a neutron star (NS) binary merger. Family-1burts, with isotropic energies lower than 1E52 erg, are emitted by the formation of a massive NS after during the merger, while family-2 bursts, with isotropic energies higher than 1E52 erg, originate from a black hole (BH) formation. Clearly the relevant parameter which defines the fate of the post-merger object is the NS critical mass: the merger might form either a BH or a massive NS if the mass of the post-merger object exceed or not, respectively, the critical mass value. We compute the structure of the post-merger object from the numerical solution of the axially symmetric Einstein equations in full rotation, and assuming that baryon-mass and angular momentum are conserved during the merger. We assess the stability of the post-merger NS with respect to the mass-shedding limit and the secular axisymmetric instability. Thus, we establish the value of the mass of the NS binary components over which BH is formed after the merger leading to Family-2 short GRBs. For binary components less massive that this value, Family-1 short GRBs are expected. We compare our results with the masses of the galactic binary NS and show that, in principle, none of these sources would lead to BH formation after the final merger, and therefore the local rate of high energetic Family-2 short GRBs must be extremely low, much lower than the local rate of binary NSs. The consequences on the detection of gravitational wave emission from these systems are outlined. The sensitivity of the results on the nuclear equation of state of NSs is also discussed.

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Accepted

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Talk

Title

Abstract

A neutron star (NS) binary merger might give origin either to a new massive NS or to a prompt collapse to a black hole (BH) depending if the mass of the post-merger object is lower or higher than the NS critical mass. Both cases might lead to short GRBs and, as pointed out by Ruffini et al. (2015), they have distinct observational features which can be gathered in a sub-classification of short GRBs into two families: family-1burts, with isotropic energies lower than 1E52 erg, are emitted by the formation of a massive NS after during the merger, while family-2 bursts, with isotropic energies higher than 1E52 erg, originate from a black hole (BH) formation. Assuming baryon-mass and angular momentum conservation during the merger, we compute the structure of the post-merger object from the numerical solution of the axially symmetric Einstein equations in full rotation. We evaluate the stability of the post-merger NS with respect to the mass-shedding limit and the secular axisymmetric instability, establishing the value of the mass of the NS binary components over which a BH is formed after the merger. Our results indicate that none of the galactic binary NSs will lead to BH formation after the final merger, and therefore the local rate of high energetic Family-2 short GRBs must be extremely low, much lower than the local rate of binary NSs. The sensitivity of the results on the nuclear equation of state of NSs is discussed.

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