On the observed duration distribution of gamma-ray bursts from collapsars

TL;DR

This study explores how the observed durations of long gamma-ray bursts relate to the actual engine activity, highlighting the effects of viewing angle and redshift, and suggesting most bursts have engines lasting around 20 seconds.

Contribution

It combines simulations and observations to clarify the relationship between engine activity duration and observed burst duration, accounting for viewing and redshift effects.

Findings

Observed burst duration closely proxies engine duration after jet breakout.

Viewing angle significantly affects observed burst length.

Most long GRBs have engine durations around 20 seconds.

Abstract

The duration of the prompt emission of long gamma-ray bursts is generally considered to be fairly similar to the duration of the activity of the engine in the center of the progenitor star. Here, we investigate the relation between the duration of the engine activity and that of the observed light curve, using inputs from both numerical simulations and observations. We find that the observed burst duration is a good proxy for the engine duration after the time necessary for the jet to break out the star's surface is subtracted. However, the observed duration is a function of the viewing angle and can be significantly shorter than the duration of the engine activity. We also show that the observed, redshift-corrected burst duration evolves only moderately with redshift for both observations and synthetic light curves. We conclude that the broad distribution of the observed duration of long BATSE gamma-ray bursts is mostly accounted for by an engine lasting ~20 s, the dispersion being due to viewing and redshift effects. Our results do not rule out the existence of engines with very long duration. However, we find that they are constrained to be a small minority of the BATSE detected bursts.