Swift observations of GRB 050904: the most distant cosmic explosion ever observed

¹ INAF – IASFPA, via Ugo La Malfa 153, 90146 Palermo, Italy e-mail: giancarlo.cusumano@ifc.inaf.it
² INAF – Osservatorio Astronomico di Brera, via Bianchi 46, 23807 Merate, Italy
³ Università degli studi di Milano-Bicocca, Dip. di Fisica, Piazza delle Scienze 3, 20126 Milan, Italy
⁴ Los Alamos National Laboratory, PO Box 1663, Los Alamos, NM 87545, USA
⁵ Department of Astronomy & Astrophysics, Pennsylvania State University, University Park, PA 16802, USA
⁶ NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁷ National Research Council, 2001 Constitution Avenue, NW, TJ2114, Washington, DC 20418, USA
⁸ Department of Physics and Astronomy, University of Leicester, University Road, Leicester LE1 7RH, UK
⁹ Department of Physics and Astronomy, Sonoma State University, Rohnert Park, CA 94928-3609, USA
¹⁰ ASI Science Data Center, via Galileo Galilei, 00044 Frascati, Italy
¹¹ UC Berkeley Space Sciences Laboratory, Berkeley, CA 94720-7450, USA
¹² MSSL, University College London, Holmbury St. Mary, Dorking, RH5 6NT Surrey, UK
¹³ Department of Physics, Pennsylvania State University, PA 16802, USA
¹⁴ Department of Physics, University of Nevada, Box 454002, Las Vegas, NV 89154-4002, USA

Received: 9 March 2006
Accepted: 5 October 2006

Abstract

Context. Swift discovered the high redshift () GRB 050904 with the Burst Alert Telescope (BAT) and began observing with its narrow field instruments 161 s after the burst onset. This gamma-ray burst is the most distant cosmic explosion ever observed. Because of its high redshift, the X-ray Telescope (XRT) and BAT simultaneous observations provide 4 orders of magnitude of spectral coverage (0.2-150 keV; 1.4-1090 keV in the source rest frame) at a very early source-frame time (22 s). The X-ray emission was monitored by the XRT up to 10 days after the burst.

Aims. We present the analysis of BAT and XRT observations of GRB 050904 and a complete description of its high energy phenomenology.

Methods. We performed time resolved spectral analysis and light curve modeling.

Results. GRB 050904 was a long, multi-peaked, bright GRB with strong variability during its entire evolution. The light curve observed by the XRT is characterized by the presence of a long flaring activity lasting up to 1-2 h after the burst onset in the burst rest frame, with no evidence of a smooth power-law decay following the prompt emission as seen in other GRBs. However, the BAT tail extrapolated to the XRT band joins the XRT early light curve and the overall behavior resembles that of a very long GRB prompt. The spectral energy distribution softens with time, with the photon index decreasing from -1.2 during the BAT observation to -1.9 at the end of the XRT observation. The dips of the late X-ray flares may be consistent with an underlying X-ray emission arising from the forward shock and with the properties of the optical afterglow reported by Tagliaferri et al. (2005b, A&A, 443, L1).

Conclusions. We interpret the BAT and XRT data as a single continuous observation of the prompt emission from a very long GRB. The peculiarities observed in GRB 050904 could be due to its origin within one of the first star-forming regions in the Universe; very low metallicities of the progenitor at these epochs may provide an explanation.