Optical afterglow luminosities in the Swift epoch: confirming clustering and bimodality

TL;DR

This study confirms that the optical afterglow luminosities of GRBs observed by Swift are bimodal and narrow, suggesting intrinsic bimodality or significant dust absorption, with dark bursts likely being underluminous.

Contribution

It provides the first confirmation of bimodal optical afterglow luminosity distribution in Swift GRBs and explores the role of dust absorption and selection effects.

Findings

Optical afterglow luminosities are bimodal and narrow at 12 hours post-trigger.

Swift GRBs have higher average redshift than pre-Swift, yet show similar luminosity distribution.

Most dark bursts are likely underluminous due to dust absorption.

Abstract

We show that Gamma Ray Bursts (GRBs) of known redshift and rest frame optical extinction detected by the Swift satellite fully confirm earlier results concerning the distribution of the optical afterglow luminosity at 12 hours after trigger (rest frame time). This distribution is bimodal and relatively narrow, especially for the high luminosity branch. This is intriguing, given that Swift GRBs have, on average, a redshift larger than pre-Swift ones, and is unexpected in the common scenario explaining the GRB afterglow. We investigate if the observed distribution can be the result of selection effects affecting a unimodal parent luminosity distribution, and find that either the distribution is intrinsically bimodal, or most (60 per cent) of the bursts are absorbed by a substantial amount of grey dust. In both cases we suggest that most dark bursts should belong to the underluminous optical family.