Late internal shock model for bright X-ray flares in Gamma-ray Burst afterglows and GRB 011.pdf

a r X i v : a s t r o - p h / 0 5 0 6 1 5 5 v 4 1 S e p 2 0 0 5 Mon. Not. R. Astron. Soc. 000, 000–000 (0000) Printed 2 February 2008 (MN LATEX style file v2.2) Late internal shock model for bright X-ray flares in Gamma-ray Burst afterglows and GRB 011121 Y. Z. Fan 1,2,3? and D. M. Wei 1,2 ? 1Purple Mountain Observatory, Chinese Academy of Science, Nanjing 210008, China 2National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, China 3Dept. of Physics, University of Nevada, Las Vegas, NV 89154, USA. Accepted Received ; in original form ABSTRACT We explore two possible models which might give rise to bright X-ray flares in GRBs afterglows. One is an external forward-reverse shock model, in which the shock param- eters of forward/reverse shocks are taken to be quite different. The other is a so called “late internal shock model”, which requires a refreshed unsteady relativistic outflow generated after the prompt γ?ray emission. In the forward-reverse shock model, after the time t× at which the RS crosses the ejecta, the flux declines more slowly than (t⊕/t×) ?(2+β), where t⊕ denotes the observer’s time and β is the spectral index of the X-ray emission. In the “late internal shock model”, decaying slopes much steeper than (t⊕/te,⊕) ?(2+β) are possible if the central engine shuts down after te,⊕ and the observed variability timescale of the X-ray flare is much shorter than te,⊕. The sharp decline of the X-ray flares detected in GRB 011121, XRF 050406, GRB 050502b, and GRB 050730 rules out the external forward-reverse shock model directly and favors the “late internal shock model”. These X-ray flares could thus hint that the central engine operates again and a new unsteady relativistic outflow is generated just a few minutes after the intrinsic hard burst. Key words: Gamma Rays: bursts?ISM: jets and outflows–radiation mechanisms: nonthermal?X-rays: general 1 INTRODUCTION GRB 011121 was simultaneously detected by BeppoSAX GRBM and WFC (Piro