Prompt observations of gamma -ray burst afterglows with ROTSE-III.

Abstract

Gamma-ray bursts (GRBs) are some of the most energetic explosions in the universe, releasing over 10<super>51</super> ergs in gamma-rays in tens of seconds. About half of all GRBs have optical afterglows that peak around R &sim; 16 and fade very rapidly over the course of several days. The late time afterglow of the GRB can provide information about the circumburst environment, but prompt optical observations are essential to probe into the central engine of the burst. Until 2002, only one afterglow had been detected promptly. This dissertation describes the prompt and early afterglow detections of the Robotic Optical Transient Search Experiment (ROTSE)-III telescope array from February 2003 through May 2005. The four 0.45-m ROTSE-III telescopes are located in Australia, Texas, Namibia, and Turkey, and can respond to GRB triggers from satellite experiments such as HETE-2 and <italic>Swift</italic> in 5-10 seconds from the receipt of the GRB alert. The ROTSE-III instruments have responded to over 30 GRB triggers with detections of 10 afterglows, including one prompt counterpart detected during the gamma-ray emission. The early afterglow lightcurves show great diversity, although most of them are dimmer at the early time than a backward extrapolation of the later observations would predict. I address various models that can explain the observed diversity, as well as the predictions for early spectral evolution of the lightcurves from these models. Finally, I describe a search for untriggered GRB afterglows. This is the first large area survey with the sensitivity to be able to find typical afterglows without depending on satellite detections of the prompt gamma-rays.