Low angular momentum accretion in the collapsar: how long can a long GRB be?

TL;DR

This paper investigates how the angular momentum of progenitor stars affects the duration and energy of long gamma-ray bursts within the collapsar model, revealing that realistic angular momentum limits significantly reduce expected GRB durations.

Contribution

It introduces a revised estimate of torus formation potential by accounting for the increasing angular momentum threshold during accretion, which reduces predicted GRB durations compared to previous models.

Findings

Long GRB durations are limited by the progenitor's angular momentum.

Revised estimates show a potential order of magnitude reduction in GRB energy and duration.

High progenitor rotation rates are necessary but constrained by observations.

Abstract

The collapsar model is the most promising scenario to explain the huge release of energy associated with long duration gamma-ray-bursts (GRBs). Within this scenario GRBs are believed to be powered by accretion through a rotationally support torus or by fast rotation of a compact object. In both cases then, rotation of the progenitor star is one of the key properties because it must be high enough for the torus to form, the compact object to rotate very fast, or both. Here, we check what rotational properties a progenitor star must have in order to sustain torus accretion over relatively long activity periods as observed in most GRBs. We show that simple, often cited, estimates of the total mass available for torus formation and consequently the duration of a GRB are only upper limits. We revise these estimates by taking into account the long term effect that as the compact object accretes the minimum specific angular momentum needed for torus formation increases. This in turn leads to a smaller fraction of the stellar envelope that can form a torus. We demostrate that this effect can lead to a significant, an order of magnidute, reduction of the total energy and overall duration of a GRB event. This of course can be mitigated by assuming that the progenitor star rotates faster then we assumed. However, our assumed rotation is already high compared to observational and theoretical constraints. We also discuss implications of our result.