Close Binary Progenitors of Long Gamma Ray Bursts

TL;DR

This paper explores how close binary systems, especially WR star mergers, can produce long gamma-ray bursts with extended activity phases, emphasizing magnetic mechanisms over neutrino annihilation.

Contribution

It re-examines the binary progenitor scenario allowing for late accretion disk formation, expanding potential LGRB progenitors beyond rapidly rotating single stars.

Findings

Long-lived accretion disks can form in binary mergers, explaining extended gamma-ray burst activity.

Magnetic mechanisms may be responsible for prolonged central engine activity.

Binary progenitors broaden the range of possible LGRB origins.

Abstract

The strong dependence of the neutrino annihilation mechanism on the mass accretion rate makes it difficult to explain the LGRBs with duration in excess of 100 seconds as well as the precursors separated from the main gamma-ray pulse by few hundreds of seconds. Even more difficult is to explain the Swift observations of the shallow decay phase and X-ray flares, if they indeed indicate activity of the central engine for as long as 10,000 seconds. These data suggest that some other, most likely magnetic mechanisms have to be considered. The magnetic models do not require the development of accretion disk within the first few seconds of the stellar collapse and hence do not require very rapidly rotating stellar cores at the pre-supernova state. This widens the range of potential LGRB progenitors. In this paper, we re-examine the close binary scenario allowing for the possibility of late development of accretion disks in the collapsar model and investigate the available range of mass accretion rates, black hole masses, and spins. A particularly interesting version of the binary progenitor involves merger of a WR star with an ultra-compact companion, neutron star or black hole. In this case we expect the formation of very long-lived accretion disks, that may explain the phase of shallow decay and X-ray flares observed by Swift. Similarly long-lived magnetic central engines are expected in the current single star models of LGRB progenitors due to their assumed exceptionally fast rotation.