Evidence of Exponential Decay Emission in the Swift Gamma - ray Bursts

We present a systematic study of the steep decay emission from gamma-ray bursts (GRBs) observed by the Swift X-Ray Telescope (XRT). In contrast to the analysis described in recent literature, we produce composite Burst Alert Telescope (BAT) and XRT light curves by extrapolating the XRT data (2–10 keV) into the BAT energy range (15–25 keV) rather than extrapolating the BAT data into the XRT energy band (0.3–10 keV). Based on the fits to the composite light curves, we have confirmed the existence of an exponential decay component which smoothly connects the BAT prompt data to the XRT steep decay for several GRBs. We also find that the XRT steep decay for some of the bursts can be well fit by a combination of a power-law with an exponential decay model. We discuss this exponential component within the frame work of both the internal and the external shock model. Subject headings: Gamma-ray Burst NASA Goddard Space Flight Center, Greenbelt, MD 20771 Oak Ridge Associated Universities, P.O. Box 117, Oak Ridge, Tennessee 37831-0117 CRESST NASA Goddard Space Flight Center, Greenbelt, MD 20771 Universities Space Research Association, 10211 Wincopin Circle, Suite 500, Columbia, MD 21044-3432 Department of Physics, Hiroshima University, Higashi-Hiroshima, Hiroshima, 739-8526, Japan Institute of Space and Astronautical Science, JAXA, Kanagawa 229-8510, Japan Department of Physics, North Carolina Agricultural and Technical State University, 1601 East Market Street,Greensboro, North Carolina 27411 Department of Physics and Astronomy, University of Leicester, LE1, 7RH, UK