Swift observations of the prompt X-ray emission and afterglow from GRB050126 and GRB050219A

¹ Department of Physics and Astronomy, University of Leicester, LE1 7RH, UK e-mail: mrg@star.le.ac.uk
² INAF – Osservatorio Astronomica di Brera, via Bianchi 46, 23807 Merate, Italy
³ Department of Astronomy & Astrophysics, 525 Davey Lab., Pennsylvania State University, University Park, PA 16802, USA
⁴ Universita degli studi di Milano-Bicocca, P.za dell Scienze 3, 20126 Milano, Italy
⁵ NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁶ Department of Physics, University of Nevada, BOX 454002, Las Vegas, NV 891, USA
⁷ ASI Science Data Center, via Galileo Galilei, 00044 Frascati, Italy
⁸ INAF – Instituto di Astrofisica Spaziale e Cosmica, via Ugo La Malfa 153, 90146, Palermo, Italy
⁹ Universities Space Research Association, 10211 Wincopin Circle, Suite 500, Columbia, MD, 21044-3432, USA

Received: 1 November 2005
Accepted: 23 November 2005

Abstract

We report on the temporal and spectral characteristics of the early X-ray emission from the Gamma Ray Bursts GRB050126 and GRB050219A as observed by Swift . The X-ray light-curves of these 2 bursts both show remarkably steep early decays (), breaking to flatter slopes on timescales of a few hundred seconds. For GRB050126 the burst shows no evidence of spectral evolution in the 20–150 keV band, and the spectral index of the γ-ray and X-ray afterglows are significantly different suggesting a separate origin. By contrast the BAT spectrum of GRB050219A displays significant spectral evolution, becoming softer at later times, with Γ evolving toward the XRT photon index seen in the early X-ray afterglow phase. For both bursts, the 0.2–10 keV spectral index pre- and post-break in the X-ray decay light-curve are consistent with no spectral evolution. We suggest that the steep early decline in the X-ray decay light-curve is either the curvature tail of the prompt emission; X-ray flaring activity; or external forward shock emission from a jet with high density regions of small angular size (). The late slope we associate with the forward external shock.