Long-term monitoring of the X-ray afterglow of GRB 050408 with Swift/XRT

¹ ASI Science Data Center, via G. Galilei, 00044 Frascati, Italy e-mail: capalbi@asdc.asi.it
² International School for Advanced Studies (SISSA-ISAS), via Beirut 2-4, 34014 Trieste, Italy
³ ASI, Unità Osservazione dell'Universo, viale Liegi 26, 00198 Roma, Italy
⁴ INAF – Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate, Italy
⁵ INAF – Istituto di Astrofisica Spaziale e Fisica Cosmica, Sezione di Palermo, via U. La Malfa 153, 90146 Palermo, Italy
⁶ Università degli Studi di Milano-Bicocca, Dipartimento di Fisica, Piazza delle Scienze 3, 20126 Milano, Italy
⁷ NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁸ Department of Physics and Astronomy, The Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218, USA
⁹ Department of Astronomy and Astrophysics, Pennsylvania State University, University Park, PA 16802, USA
¹⁰ Department of Physics and Astronomy, University of Leicester, Leicester LE1 7RH, UK
¹¹ Universities Space Research Association, 10211 Wincopin Circle, Suite 500, Columbia, MD 21044-3432, USA

Received: 7 April 2006
Accepted: 12 October 2006

Abstract

We present observations of the X-ray afterglow of GRB 050408, a gamma-ray burst discovered by HETE-II. Swift began observing the field 42 min after the burst, performing follow-up over a period of 38 d (thus spanning three decades in time). The X-ray light curve showed a steepening with time, similar to many other afterglows. However, the steepening was unusually smooth, over the duration of the XRT observation, with no clear break time. The early decay was too flat to be described in terms of standard models. We therefore explore alternative explanations, such as the presence of a structured afterglow or of long-lasting energy injection into the fireball from the central GRB engine. The lack of a sharp break puts constraints on these two models. In the former case, it may indicate that the angular energy profile of the jet was not a simple power law, while in the second model it implies that injection did not stop abruptly. The late decay may be due either to a standard afterglow (that is with no energy injection), or to a jetted outflow still being refreshed. A significant amount of absorption was present in the X-ray spectrum, corresponding to a rest-frame hydrogen column density  cm^-2, indicative of a dense environment.