Early optical observations of GRBs by the TAROT telescopes: period 2001-2008

TL;DR

This paper presents early optical observations of gamma-ray bursts (GRBs) by TAROT telescopes from 2001 to 2008, revealing that a small percentage exhibit bright optical flashes during prompt emission, and emphasizing the importance of rapid multi-wavelength studies.

Contribution

First comprehensive analysis of early optical emission of GRBs using TAROT data, highlighting the brightness distribution and confirming previous findings with new observations.

Findings

5% to 20% of GRBs show bright optical flashes during prompt emission

More than 50% of GRBs have optical emission fainter than R=15.5 during gamma-ray activity

Observations confirm the optical brightness distribution derived by other groups

Abstract

The TAROT telescopes (Telescopes a Action Rapide pour les Objets Transitoires) are two robotic observatories designed to observe the prompt optical emission counterpart and the early afterglow of gamma ray bursts (GRBs). We present data acquired between 2001 and 2008 and discuss the properties of the optical emission of GRBs, noting various interesting results. The optical emission observed during the prompt GRB phase is rarely very bright: we estimate that 5% to 20% of GRBs exhibit a bright optical flash (R<14) during the prompt gamma-ray emission, and that more than 50% of the GRBs have an optical emission fainter than R=15.5 when the gamma-ray emission is active. We study the apparent optical brightness distribution of GRBs at 1000 s showing that our observations confirm the distribution derived by other groups. The combination of these results with those obtained by other rapid slewing telescopes allows us to better characterize the early optical emission of GRBs and to emphasize the importance of very early multi-wavelength GRB studies for the understanding of the physics of the ejecta.