Evidence for a Canonical GRB Afterglow Light Curve in the Swift XRT Data

We present new observations of the early X-ray afterglows of the first 27 gamma-ray bursts (GRBs) well-observed by the Swift X-ray Telescope (XRT). The early X-ray afterglows show a canonical behavior, where the light curve broadly consists of three distinct power law segments: (i) an initial very steep decay (∝ t with 3 . α1 . 5) , followed by (ii) a very shallow decay (0.5 . α2 . 1.0), and finally (iii) a somewhat steeper decay (1 . α3 . 1.5). These power law Department of Astronomy & Astrophysics, Pennsylvania State University, University Park, PA 16802, USA NASA/Marshall Space Flight Center, National Space Science Technology Center, XD-12, 320 Sparkman Drive, Huntsville, AL 35805, USA. Department of Physics and Astronomy, University of Leicester, Leicester, UK Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, P.O. Box 20450, MS 29, Stanford, CA 94309. Institute for Advanced Study, Einstein Drive, Princeton, NJ 08540, USA. Chandra Fellow. IPA with NASA/MSFC through Universities Space Research Association INAFIstituto di Fisica Spazialee Fisica Cosmica sezione di Palermo, Palermo, Italy INAF – Osservatorio Astronomico di Brera, Merate, Italy ASI Science Data Center, Frascati, Italy Università degli studi di Milano-Bicocca, Dipartimento di Fisica, Milan, Italy NASA/Goddard Space Flight Center, Greenbelt, MD