The X-ray afterglows of GRB 020813 and GRB 021004 with Chandra HETGS: Possible evidence for a supernova prior to GRB 020813

We report on the detection of an emission line near 1.3 keV, which we associate with blueshifted hydrogen-like sulfur (S XVI), in a 76.8 ks Chandra HETGS spectrum of the afterglow of GRB 020813. The line is detected at 3.3 σ significance. We also find marginal evidence for a line possibly due to hydrogen-like silicon (Si XIV) with the same blueshift. A line from Fe is not detected, although a very low significance Ni feature may be present. A thermal model fits the data adequately, but a reflection model may provide a better fit. There is marginal evidence that the equivalent width of the S XVI line decreases as the burst fades. We infer from these results that a supernova likely occurred ≳2 months prior to the γ-ray burst. We find no discrete or variable spectral features in the Chandra HETGS spectrum of the GRB 021004 afterglow.