Early X-ray/UV Line Signatures of GRB Progenitors and Hypernovae

Abstract

We calculate the X-ray/UV spectral line signatures expected from the interaction of a gamma-ray burst afterglow and a dense pre-burst environment produced by the progenitor. We explore the conditions under which Fe line and edge equivalent widths of $\sim$ 1 keV can arise, and discuss the possibility of gaining information about possible progenitor scenarios using X-ray metal line spectra in the first few days of a burst. A wind or supernova shell around the burst produces an X-ray absorption line spectrum and later emission lines, while a hypernova funnel model produces mainly emission lines. The Fe \ked can in some cases be more prominent than the Fe \kal line. Under simple assumptions for the input continuum luminosity, current reports of observed Fe line luminosities are compatible with an Fe-enriched funnel model, while lower values are expected in shell models.