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Issue A&A
Volume 442, Number 2, November I 2005
Page(s) L1 - L5
Section Letters
DOI http://dx.doi.org/10.1051/0004-6361:200500173



A&A 442, L1-L5 (2005)
DOI: 10.1051/0004-6361:200500173

Letter

Swift XRT observations of the breaking X-ray afterglow of GRB 050318

M. Perri1, P. Giommi1, 2, M. Capalbi1, L. Amati3, F. Frontera3, 4, G. Chincarini5, 6, S. Campana5, A. Moretti5, P. Romano5, G. Tagliaferri5, G. Cusumano7, V. La Parola7, V. Mangano7, T. Mineo7, D. N. Burrows8, J. E. Hill9, 10, J. A. Kennea8, D. C. Morris8, C. Pagani5, 8, J. A. Nousek8, L. Angelini9, N. Gehrels9, M. Still9, A. P. Beardmore11, M. R. Goad11, O. Godet11, P. T. O'Brien11, J. P. Osborne11 and K. L. Page11

1  ASI Science Data Center, via Galileo Galilei, 00044 Frascati, Italy
    e-mail: perri@asdc.asi.it
2  Agenzia Spaziale Italiana, Unità Osservazione dell'Universo, Viale Liegi 26, 00198 Roma, Italy
3  INAF - Istituto di Astrofisica Spaziale e Fisica Cosmica, Sezione di Bologna, via Gobetti 101, 40129 Bologna, Italy
4  Università degli Studi di Ferrara, Dipartimento di Fisica, via Saragat 1, 44100 Ferrara, Italy
5  INAF - Osservatorio Astronomico di Brera, via Bianchi 46, 23807 Merate, Italy
6  Università degli Studi di Milano-Bicocca, Dipartimento di Fisica, Piazza delle Scienze 3, 20126 Milano, Italy
7  INAF - Istituto di Astrofisica Spaziale e Fisica Cosmica, Sezione di Palermo, via La Malfa 153, 90146 Palermo, Italy
8  Department of Astronomy & Astrophysics, Pennsylvania State University, University Park, PA 16802, USA
9  NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA
10  Universities Space Research Association, 10211 Wincopin Circle, Suite 500, Columbia, MD, 21044-3432, USA
11  Department of Physics & Astronomy, University of Leicester, Leicester LE1 7RH, UK

(Received 11 July 2005 / Accepted 14 August 2005)

Abstract
We report the results of Swift X-Ray Telescope (XRT) observations of GRB 050318. This event triggered the Burst Alert Telescope (BAT) aboard Swift and was followed-up with XRT and UVOT for 11 consecutive orbits starting from 54 min after the trigger. A previously unknown fading X-ray source was detected and accurately monitored. The source was found to decrease in intensity with time and a clear temporal break occurring at ~18 000 s after the trigger was observed. The X-ray light curve was found to be consistent with a broken power-law with decay indices $-1.17\pm 0.08$ and -2.10+0.22-0.24 before and after the break. The spectrum of the X-ray afterglow was well described by a photoelectrically absorbed power-law with energy index of $-1.09\pm0.09$. No evidence of spectral evolution was found. We compare these results with those obtained with UVOT and separately reported and refine the data analysis of BAT. We discuss our results in the framework of a collimated fireball model and a synchrotron radiation emission mechanism. Assuming the GRB redshift derived from the farthest optical absorption complex (z = 1.44), the event is fully consistent with the $E_{\rm {p}}$- $E_{\rm {iso}}$ correlation.


Key words: gamma rays: bursts -- X-rays: individual: GRB 050318

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