2005ApJ...628..853G

Recently, iron emission features have been observed in several X-ray afterglows of GRBs. It is found that the energy obtained from the illuminating continuum that produces the emission lines is much higher than that of the main burst. The observation of an SN-GRB association indicates that a fallback disk is formed after the supernova explosion. The disk is optically thick, advection-dominated, and dense. We suggest that delayed-injection energy after the initial main burst, much higher than the energy of the main burst, causes a reburst appearance in GRB afterglows, illuminates the region of the disk surface with τ~1 (τ is the optical depth for the Thomson scatter), and produces an iron emission line whose luminosity can be up to 10⁴⁵ ergs/s. The duration of the iron line emission can be 10⁴-10⁵ s. This model can explain the appearance of rebursts and emission lines in GRB afterglows and the disappearance of the iron emission lines, and can also naturally solve the problem of the energy of the illuminating continuum being higher than that of the main burst. This scenario is different from the models previously put forward to explain the emission lines and can be tested by the Swift satellite.