Two GRB Progenitor Models Collapsars

The generic nature of the fireball model is based on the fact that the details of the primary energy generation mechanism is largely undetermined, i.e., it can be reconciled with different scenarios, such as a binary compact object merger [67], a failed supernova [101], a young highly magnetic pulsar [92], or a hypernova/collapsar [54,68].

The presently favored scenario for long-duration GRBs is the hypernova scenario, i.e., the core collapse of a single, massive star. A supernova explosion is naturally expected to be associated with this collapse and observationally proven (see Sect. 24.2.2).

A major concern with the hypernova explanation is whether baryons can be efficiently excluded from the flow, so that the expanding flow can achieve the high Lorentz factor (T ~ 300) needed. Mergers

Proposed as the first cosmological scenario, the merging of a NS-NS or NS-black hole binary happens on the timescale of milliseconds, and thus is now the leading conjecture for the subclass of short/hard GRBs [25,67]. Compact binaries provide a huge reservoir of gravitational binding energy, and due to the centrifugal force also a baryon-free fireball can easily be formed along the rotation axis of the binary system due to the compressional heating and dissipation associated with the accretion. Numerical modeling shows that due to the high densities and temperatures reached in the merger, the dominant emission is through neutrinos [77,79].

Population analysis of double neutron star binaries suggests that the majority of mergers should happen outside the host galaxy, because they happen so late in the binary evolution that even small systemic kick velocities (induced during the collapse of one of the two constituents to a compact object) would allow most binaries to travel large distances. After the first handful of afterglows detected from short GRBs, future Swift observations are expected to verify the validity of this scenario.

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