Long Gamma-Ray Transients from Collapsars

TL;DR

This paper explores long gamma-ray transients resulting from collapsar scenarios involving stars with specific angular momentum conditions, proposing they are common in the universe and useful for studying early stellar populations.

Contribution

It introduces a new class of long gamma-ray transients from stars with marginal angular momentum, expanding the understanding of gamma-ray burst progenitors beyond the traditional collapsar model.

Findings

Long gamma-ray transients can result from stars with minimal angular momentum forming accretion disks.

These events could be more frequent in the early universe due to lower mass loss rates.

Such transients may be observable as multiple active events in the sky at once.

Abstract

In the collapsar model for common gamma-ray bursts, the formation of a centrifugally supported disk occurs during the first $\sim$10 seconds following the collapse of the iron core in a massive star. This only occurs in a small fraction of massive stellar deaths, however, and requires unusual conditions. A much more frequent occurrence could be the death of a star that makes a black hole and a weak or absent outgoing shock, but in a progenitor that only has enough angular momentum in its outermost layers to make a disk. We consider several cases where this is likely to occur - blue supergiants with low mass loss rates, tidally-interacting binaries involving either helium stars or giant stars, and the collapse to a black hole of very massive pair-instability supernovae. These events have in common the accretion of a solar mass or so of material through a disk over a period much longer than the duration of a common gamma-ray burst. A broad range of powers is possible, $10^{47}$ to $10^{50}\,$erg s$^{-1}$, and this brightness could be enhanced by beaming. Such events were probably more frequent in the early universe where mass loss rates were lower. Indeed this could be one of the most common forms of gamma-ray transients in the universe and could be used to study first generation stars. Several events could be active in the sky at any one time. A recent example of this sort of event may have been the SWIFT transient Sw-1644+57.