Accepted...... Received......; in original form...... 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 parameters 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
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