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

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.

5 pages 3 figures, minor revision, added reference, accepted for publication in MNRAS Letters

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• GRB 060605: multi-wavelength analysis of the first GRB observed using integral field spectroscopy
• Optical and X-ray Rest-frame Light Curves of the BAT6 sample
• Diverse Properties of Interstellar Medium Embedding Gamma-Ray Bursts at the Epoch of Reionization
• XIAO: a soft X-ray telescope for the SVOM Mission