A New Model for Iron Emission Lines and Re-Burst in GRB X-Ray Afterglows

Abstract

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 which produces the emission lines is much higher than that of the main burst.The observation of SN-GRB association indicates a fallback disk should be formed after the supernovae explosion. The disk is optically thick and advection-dominated and dense. We suggest that the delayed injection energy after the initial main burst, much higher than energy of the main burst, causes the re-burst appearance in GRB afterglow and illuminates the region of the disk surface with $\tau\approx1$($\tau$ is the optical depth for the Thomson scatter ) and produces the iron emission line whose luminosity can be up to $10^{45}$ erg$s^{-1}$. The duration of the iron line emission can be $10^{4}-10^{5}$ s. This model can explain the appearance of re-burst and emission lines in GRB afterglow and disappearance of the iron emission lines, and also can naturally solve the problem of higher energy of the illuminating continuum than that of the main burst. This scenario is different from the models put forward to explain the emission lines before, that can be tested by SWIFT satellite.