A matter-antimatter annihilation engine would be expected to produce a large mass of positrons. This would in turn create the primary gamma-ray emission in the line at 0.511 MeV and in the positronium continuum below this energy.
Assuming the spacecraft has structural material, the produced gamma-rays could be reprocessed into x-rays by such material. This would result in another source of continuum at lower energies. Furthermore, if the annihilation temperature is approximately that of fusion temperatures, the 0.511 MeV line will be broadened to about 100 KeV. This would result in a further continuum contribution from the thermal bremmstrahlung or free-free electron emission at energies up to about 1 MeV. The resulting spectrum would show a steeply-sloping continuum from the x-ray absorption regime (about 10 KeV) to about 1 MeV with a single emission line superimposed on the continuum.
Spectra of GRBs are observed to show such steeply sloping continua between 10 KeV and 1 MeV, or in some cases higher. The observed continua can be explained by thermal bremsstrahlung emission at a temperature of about 300 million degrees kelvin. Another possible source is a thermal synchrotron process at lower temperatures if strong magnetic fields are present. Furthermore, many spectra also show single emission features at energies of a few hundred KeV. These features have been interpreted as Doppler-shifted 0.511 MeV annihilation lines. The line widths are known to range from the energy resolution limit, which is a few tens of KeV, up to about 250 KeV, in this region. The Doppler-shifts are generally red and the existence of blue-shifted lines is very difficult to determine since the signal-to-noise ratio problem increases with energy.
However, if such lines are real, the similarity between GRB spectra and those expected from electron-positron annihilation spacecraft is remarkable.