The Dark Bursts population in a complete sample of bright Swift Long Gamma-Ray Bursts

TL;DR

This study analyzes a complete sample of bright Swift long gamma-ray bursts to identify and characterize optically dark events, concluding that dust extinction in dense environments is the main cause of darkness.

Contribution

It provides the first robust estimate of the dark burst fraction (~30%) in a complete, bright GRB sample and links darkness to dust extinction rather than high redshift or low density.

Findings

Dark burst fraction is approximately 30%.

Dark bursts have slightly higher X-ray luminosity than non-dark bursts.

Optically dark events are likely caused by dust extinction in dense environments.

Abstract

We study the properties of the population of optically dark events present in a carefully selected complete sample of bright Swift long gamma-ray bursts. The high level of completeness in redshift of our sample (52 objects out of 58) allow us to establish the existence of a genuine dark population and we are able to estimate the maximum fraction of dark burst events (~30%) expected for the whole class of long gamma-ray burst. The redshift distribution of this population of dark bursts is similar to the one of the whole sample. Interestingly, the rest-frame X-ray luminosity (and the de-absorbed X-ray flux) of the sub-class of dark bursts is slightly higher than the average luminosity of the non-dark events. At the same time the prompt properties do not differ and the optical flux of dark events is at the lower tail of the optical flux distribution, corrected for Galactic absorption. All these properties suggest that dark bursts events generate in much denser environments with respect to normal bright events. We can therefore exclude the high-z and the low-density scenarios and conclude that the major cause of the origin of optically dark events is the dust extinction.