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Issue A&A
Volume 449, Number 1, April I 2006
Page(s) 89 - 100
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20054457

A&A 449, 89-100 (2006)
DOI: 10.1051/0004-6361:20054457

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

M. R. Goad1, G. Tagliaferri2, K. L. Page1, A. Moretti2, J. P. Osborne1, S. Kobayashi3, P. Kumar3, P. I. Mészáros3, G. Chincarini2, 4, T. Sakamoto5, B. Zhang6, S. D. Barthelmy5, A. P. Beardmore1, D. N. Burrows3, S. Campana2, M. Capalbi7, L. Cominsky5, G. Cusumano8, N. Gehrels5, P. Giommi7, O. Godet1, J. E. Hill3, 5, 9, J. A. Kennea3, H. Krimm5, V. La Parola8, V. Mangano8, T. Mineo8, D. C. Morris3, K. Mukerjee1, J. A. Nousek5, P. T. O'Brien1, C. Pagani2, 3, M. Perri7, P. Romano2 and A. A. Wells1

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 ( $F(t)\propto t^{-3}$), 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 $\gamma$-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 $\Gamma$ 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 ( ${>}\Gamma^{-1}$). The late slope we associate with the forward external shock.


Key words: gamma rays: bursts -- gamma rays: observations -- black hole physics

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