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2.5. Spectral Lines

Both absorption and emission features have been reported by various experiments prior to BATSE. Absorption lines in the 20-40 keV range have been observed by several experiments - but never simultaneously. GINGA has discovered several cases of lines with harmonic structure [104, 105]. These lines were interpreted as cyclotron lines (reflecting a magnetic field of approx 1012 Gauss) and providing one of the strongest arguments in favor of the galactic neutron star model. Emission features near 400 keV have been claimed in other bursts [106]. These have been interpreted as red-shifted 511keV annihilation lines with a corresponding red-shift of approx 20% due to the gravitational field on the surface of the Neutron star. These provided additional evidence for the galactic neutron star model.

So far BATSE has not found any of the spectral features (absorption or emission lines) reported by earlier satellites [107, 108]. This can be interpreted as a problem with previous observations (or with the difficult analysis of the observed spectra) or as an unlucky coincidence. Given the rate of observed lines in previous experiments it is possible (at the approx 5% level) that the two sets of data are consistent [109].

Recently Mészáros & Rees [110] suggested that within the relativitic fireball model the observed spectral lines could have be blue shifted iron X-ray line.