Prompt optical observations of gamma-ray bursts

The Robotic Optical Transient Search Experiment (ROTSE) seeks to measure si multaneous and early afterglow optical emission from gamma-ray bursts (GRBs ). A search for optical counterparts to six GRBs with localization errors o f 1 deg(2) or better produced no detections. The earliest limiting sensitiv ity is m(ROTSE) > 13.1 at 10.85 s (5 s exposure) after the gamma-ray rise, and the best limit is m(ROTSE) > 16.0 at 62 minutes (897 s exposure). These are the most stringent limits obtained for the GRB optical counterpart bri ghtness in the first hour after the burst. Consideration of the gamma-ray f luence and peak flux for these bursts and for GRB 990123 indicates that the re is not a strong positive correlation between optical flux and gamma-ray emission.