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Astrophysics, High Energy Astrophysical Phenomena, astro-ph.HE,General Relativity and Quantum Cosmology, gr-qc
Abstract:
In recent years, detailed observations and accurate numerical simulations
have provided support to the idea that mergers of compact binaries containing
either two neutron stars (NSs) or an NS and a black hole (BH) may constitute
the central engine of short gamma-ray bursts (SGRBs). The merger of such
compact binaries is expected to lead to the production of a spinning BH
surrounded by an accreting torus. Several mechanisms can extract energy from
this system and power the SGRBs. Here we connect observations and numerical
simulations of compact binary mergers, and use the current sample of SGRBs with
measured energies to constrain the mass of their powering tori. By comparing
the masses of the tori with the results of fully general-relativistic
simulations, we are able to infer the properties of the binary progenitors
which yield SGRBs. By assuming a constant efficiency in converting torus mass
into jet energy, epsilon_{jet}=10%, we find that most of the tori have masses
smaller than 0.01M_{sun}, favoring "high-mass" binary NSs mergers, i.e.,
binaries with total masses >~1.5 the maximum mass of an isolated NS. This has
important consequences for the gravitational-wave signals that may be detected
in association with SGRBs, since "high-mass" systems do not form a long-lived
hypermassive NS after the merger. While NS-BH systems cannot be excluded to be
the engine of at least some of the SGRBs, the BH would need to have an initial
spin of ~0.9, or higher.
