GRB Flares: A New Detection Algorithm, Previously Undetected Flares, and Implications on GRB Physics

Abstract

Flares in GRB light curves have been observed since shortly after the discovery of the first GRB afterglow. However, it was not until the launch of the Swift satellite that it was realized how common flares are, appearing in nearly 50% of all X-ray afterglows as observed by the XRT instrument. The majority of these observed X-ray flares are easily distinguishable by eye and have been measured to have up to as much fluence as the original prompt emission. Through studying large numbers of these X-ray flares it has been determined that they likely result from a distinct emission source different than that powering the GRB afterglow. These findings could be confirmed if similar results were found using flares in other energy ranges. However, until now, the UVOT instrument on Swift seemed to have observed far fewer flares in the UV/optical than were seen in the X-ray. This was primarily due to poor sampling and data being spread across multiple filters, but a new optimal co-addition and normalization of the UVOT data has allowed us to search for flares in the UV/optical that have previously gone undetected. Using a flare finding algorithm based on the Bayesian Information Criterion, we have analyzed the light curves in the Second UVOT GRB Catalog and present the finding of at least 118 unique flares detected in 68 GRB afterglows.