On the Duration of Gamma-Ray Bursts

TL;DR

This paper explores the complex factors influencing gamma-ray burst durations, proposing a multi-factor framework that accounts for progenitors, engines, emitters, and geometry, challenging the traditional duration-progenitor link.

Contribution

It introduces a comprehensive model for GRB duration that incorporates multiple physical factors beyond progenitors alone, including engine type and emission geometry.

Findings

GRB durations are shaped by progenitor, engine, emitter, and geometry factors.

Some GRBs may be powered by magnetars rather than accretion.

Complex lightcurves indicate multiple contributing factors to GRB duration.

Abstract

Recently, a short-duration GRB with supernova association (GRB 200826A) and two long-duration GRBs with kilonova associations (GRB 211211A and GRB 230307A) have been detected, which demolished the hope for a tidy connection between GRB duration and their progenitor systems. Here I summarize various physical factors that can shape the duration of a GRB and propose that the duration of a GRB can be defined by four factors: progentor, central engine, emitter, and geometry. The progenitor-defined duration is only relevant when the central engine is powered by accretion and when the modifications by other factors are not important. The untidy situation of duration - progenitor mismatches suggests that other factors likely play important roles in defining GRB duration at least in some GRBs. In particular, a GRB may not be powered by accretion but rather by a millisecond magnetar at least for some GRBs. The complicated lightcurve of GRB 211211A suggests both progenitor- and engine-defined durations, which may require a new type of progenitor system involving a white dwarf - neutron star merger with a magnetar merger product. The single broad pulse lightcurve with well-behaved energy-dependent behavior of GRB 230307A suggests an emitter-defined long duration. The central engine timescale may be short enough to be accommodated within the framework of a standard binary neutron star merger. Its spiky lightcurve with fast variability as well as extended X-ray emission suggest the existence of mini-jets in the global dissipation region, powered by an underlying magnetar.