X-ray flares in the early Swift observations of the possible naked gamma-ray burst 050421

¹ X-ray and Observational Astronomy Group, Department of Physics & Astronomy, University of Leicester, LE1 7RH, UK e-mail: og19@star.le.ac.uk
² Goddard Space Flight Center, Greenbelt, Maryland 20771, USA
³ Universities Space Research Association, 10211 Wincopin Circle, Suite 500, Columbia, MD, 21044-3432, USA
⁴ Department of Physics, University of Nevada, Box 454002, Las Vegas, NV 89154-4002, USA
⁵ ASI Science Data Center, via G. Galilei, 00044 Frascati, Italy
⁶ National Research Council, 2101 Constitution Ave NW, Washington, DC 20418, USA
⁷ Department of Astronomy & Astrophysics, 525 Davey Lab, Pennsylvania State University, University Park, PA 16802, USA
⁸ INAF - Istituto di Astrofisica Spaziale e Fisica Cosmica Sezione di Palermo, Via U. La Malfa 153, 90146 Palermo, Italy
⁹ INAF - Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807, Merate (LC), Italy

Received: 28 October 2005
Accepted: 26 February 2006

Abstract

We present the Swift observations of the faint burst GRB 050421. The X-ray light-curve shows at least two flares: the first flare peaking at ~110 s after the BAT trigger (T₀) and the second one peaking at ~154 s. The first flare presents a flux variation of and a short timescale ratio . The second flare is smaller and presents a flux variation of and a short timescale ratio . We argue that the mechanism producing these flares is probably late internal shocks. The X-ray light-curve is consistent with a rapid decline with a temporal index , which decays from ~10^-9 erg cm^-2 s^-1 at T₀₊₁₀₀ s to < erg cm^-2 s^-1 at T₀₊₉₀₀ s. A possible spectral softening is also observed with time, from to ∼1.2. A good joint fit to the BAT and XRT spectra before T₀₊₁₇₁ s with indicates that the early X-ray and Gamma-ray emissions are likely produced by the same mechanism. We argue that the X-ray spectral softening, if any, is due to a shift of the peak of the prompt emission spectrum down to lower energies, and that the rapid decline of the X-ray emission is probably the tail of the prompt emission. This suggests that the X-ray emission is completely dominated by high latitude radiation and the external shock, if any, is extremely faint and below the detection threshold. GRB 050421 is likely the first “naked burst” detected by Swift.