Late internal‐shock model for bright X‐ray flares in gamma‐ray burst afterglows and GRB 011121

We explore two possible models which might give rise to bright X‐ray flares in gamma‐ray burst (GRB) 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 reverse shock 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 ⊕ / t e,⊕ ) −(2+β) are possible if the central engine shuts down after t e,⊕ and the observed variability time‐scale of the X‐ray flare is much shorter than t e,⊕ . 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 favours the ‘late internal‐shock model’. These X‐ray flares could thus hint that the central engine has begun to operate again and a new unsteady relativistic outflow is generated just a few minutes after the intrinsic hard burst.