Almost fifteen years ago Mazets et al. (1981, 1982) reported seeing single lines in the spectra of gamma-ray bursts at low energies (E <= 70keV). Later Heuter (1988) reported single lines at low energies in the spectra of two bursts seen by HEAO-1 A4. However, the statistical significance of the lines was modest.
More recently, equally-spaced lines were seen by Ginga in the spectra of three bursts (Murakami et al. 1988, Fenimore et al. 1988, Graziani et al. 1992, Yoshida et al. 1992) with high significance (Fenimore et al. 1988; Graziani et al. 1992a,b, 1993; Freeman et al. 1995a,b). Figure 17 shows the lines seen in two of the bursts. The line features in these three bursts have been studied extensively, and there is no doubt that they exist.
Lines have not been definitively seen by BATSE (Palmer et al. 1994), but this fact does not strongly contradict earlier observations (Teegarden et al. 1993, Fenimore et al. 1993, Band et al. 1994).
Similar line features are seen in the spectra of accretion-powered pulsars, which are known to be magnetic neutron stars (se Figure 18). The equally-spaced lines seen in gamma-ray bursts and accretion-powered pulsars are easily explained in terms of cyclotron resonant scattering in a strong magnetic field (Makashima and Mihara 1992).
Magnetic neutron stars in the Galactic corona appear able to produce cyclotron lines even though the luminosities of the bursts greatly exceed the so-called Eddington luminosity at which radiation pressure and gravity balance. Cyclotron lines may form, for example, in a relativistic wind flowing out from the magnetic poles of the neutron star (Miller et al. 1991), or at magnetic equator where hot plasma is trapped by magnetic field (Lamb 1982; Katz 1982, 1994).
In contrast, producing cyclotron lines is a difficult problem for
cosmological models. While no quantitative calculations have been
done, the violent release of 
ergs of energy which is
required to power cosmological bursts would seem to preclude the
formation of cyclotron lines lasting tens of seconds. "Femtolensing"
has been proposed (Stanek, Paczynski, and Goodman 1993, Ulmer and
Goodman 1994) but requires a very small source whereas gamma-ray burst
emission at cosmological distances requires a large source.
Thus the existence of cyclotron lines is weighty evidence favoring the Galactic hypothesis.