  In the hypernova/collapsar model of gamma-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 one 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.
