Optical Limits on Precursor Emission from Gamma-Ray Bursts with Known Redshift

Abstract

Making use of virtual observatory data, we present the first comprehensive sample of optical observations conducted before the explosion times of all gamma-ray bursts (GRBs) with known redshifts. In total, the fields of 11 such GRBs were observed by the Near-Earth Asteroid Tracking (NEAT) program from years to hours before the bursts. Although the typical limiting magnitudes from these observations are Råisebox-0.5ex 20 mag, we find no evidence for a significant detection of a precursor. The deepest nondetection of precursor emission is from GRB 030329, reaching down to an absolute B-band magnitude of M${}_B$i̊sebox-0.5ex -18 mag from 6 to 1500 days (rest frame) before the burst. This is of comparable brightness to supernovae, which in some scenarios for GRB progenitors are predicted to predate a GRB on similar timescales. Since sources cannot be localized to better than rs̊ebox-0.5ex 500 mas (3 σ) with current large-area surveys, unrelated supernovae or active galactic nucleus activity in GRB hosts could be mistaken for genuine precursor emission. This possibility motivates the need for not only deep wide-field imaging, but imaging at high spatial resolution.