Broad and Shifted Iron‐Group Emission Lines in Gamma‐Ray Bursts as Tests of the Hypernova Scenario

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Abstract
In the hypernova/collapsar model of γ-ray bursts, it is natural that radiation is emitted by the inner engine for some time after the burst. This has been discussed as a possible source of the X-ray line emission observed in some afterglows. We show here that the natural geometry of a hypernova—a source of radiation at the bottom of a deep funnel—has very significant consequences for the shape and central energy of the observed emission lines; the lines acquire a very broad scattering wing on the low-energy side and a characteristic second peak 1 Compton wavelength away from the initial energy due to the once- and twice-scattered photons. We suggest that this explains the large width of the observed lines. Furthermore, the downscattering lowers the central line energy (by up to 1 keV in the source rest frame, before the lines become unrecognizable) so that the observed line energies can become consistent with originating from cobalt and nickel, as expected in few-day-old supernova ejecta.
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