Optical afterglows of gamma-ray bursts: a bimodal distribution?"

TL;DR

This study investigates the apparent bimodal distribution of optical afterglow luminosities in gamma-ray bursts, suggesting it may be due to intrinsic bimodality or grey absorption, impacting understanding of dark bursts.

Contribution

It provides evidence that the bimodality in optical afterglow luminosities is real and explores possible causes like intrinsic properties or grey absorption effects.

Findings

Evidence supports a genuine bimodal distribution of optical luminosities.

A significant population of grey-absorbed or intrinsically weak bursts may explain dark bursts.

Simulations match observed distributions, indicating possible causes of bimodality.

Abstract

The luminosities of the optical afterglows of Gamma Ray Bursts, 12 hours (rest frame time) after the trigger, show a surprising clustering, with a minority of events being at a significant smaller luminosity. If real, this dichotomy would be a crucial clue to understand the nature of optically dark afterglows, i.e. bursts that are detected in the X-ray band, but not in the optical. We investigate this issue by studying bursts of the pre-Swift era, both detected and undetected in the optical. The limiting magnitudes of the undetected ones are used to construct the probability that a generic bursts is observed down to a given magnitude limit. Then, by simulating a large number of bursts with pre-assigned characteristics, we can compare the properties of the observed optical luminosity distribution with the simulated one. Our results suggest that the hints of bimodality present in the observed distribution reflects a real bimodality: either the optical luminosity distributions of bursts is intrinsically bimodal, or there exists a population of bursts with a quite significant grey absorption, i.e. wavelength independent extinction. This population of intrinsically weak or grey-absorbed events can be associated to dark bursts.