AAS 207th Meeting, 8-12 January 2006
Session 75 Early Results from Swift: The Gamma Ray Burst Bonanza
Poster, Tuesday, 9:20am-6:30pm, January 10, 2006, Exhibit Hall

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[75.07] Maximizing Signal-to-Noise for Dark Gamma-ray Burst Afterglows

A. N. Morgan, P. W. A. Roming, J. A. Nousek (Penn State)

Given the significant fraction of gamma-ray bursts with no detected optical afterglow, it is important to know how much data to sum to both have the best chance of getting a detection and to determine how ``dark'' such bursts actually are. The flux from gamma-ray burst afterglows can be approximated by a power-law decay, F \propto t-\delta where \delta is the temporal decay index. While the signal-to-noise ratio (S/N) for a constant source generally grows as texp1/2, this is not the case for sources that decay with time according to a power law. For such sources, co-adding additional data eventually becomes detrimental as the flux fades beyond the background level. In this poster paper we present a method for determining how far to co-add gamma-ray burst data to achieve the maximum S/N given the initial brightness, temporal decay index, background flux of each exposure, and start and stop times of each exposure relative to the time of burst. We also present the results of applying this method to Swift Ultraviolet/Optical Telescope (UVOT) data, focusing primarily on bursts with no detected optical afterglows for which we assumed values for the initial source brightness and temporal decay index. Funding for the Swift program at Penn State is provided by NASA under the contract NAS5-00136.

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