GRB 050822: detailed analysis of an XRF observed by Swift

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 Department of Physics, University of Nevada, Box 454002, Las Vegas, NV 89154-4002, USA
³ Department of Astronomy & Astrophysics, 525 Davey Lab, Pennsylvania State University, University Park, PA 16802, USA
⁴ NASA Goddard Space Flight Center, Greenbelt, MD 20771, USA
⁵ INAF – Osservatorio Astronomico di Brera, via E. Bianchi 46, 23807 Merate (LC), Italy

Received: 9 February 2007
Accepted: 24 May 2007

Abstract

We report on the temporal and spectral characteristics of the early X-ray emission from the GRB 050822 as observed by Swift. This burst is likely to be an XRF showing major X-ray flares in its XRT light-curve. The quality of the data allows a detailed spectral analysis of the early afterglow in the X-ray band. During the X-ray flares, a positive correlation between the count rate and the spectral hardness (i.e. the higher the count rate, the harder the spectrum) is clearly seen for the X-ray flares. This behaviour, similar to that seen for Gamma-ray pulses, indicates that the energy peak of the spectrum is in the XRT energy band and it moves towards lower energies with time. We show evidence for the possible detection of the emergence of the forward-shock emission, produced at a radius larger than 4  10¹⁶ cm in the case of a CBM afterglow model (a formation region clearly different from that producing the prompt emission). Finally, we show that the null detection of a jet break up to T₀₊₄  10⁶ s in the X-ray light curve of this XRF can be understood: i) if the jet seen on-axis is uniform with a large opening angle (); or ii) if the jet is a structured Gaussian-like jet with the line-of-sight outside the bright Gaussian core.