Evidence for cyclotron absorption from spectral features in gamma-ray bursts seen with Ginga

Abstract

Absorption features near 40 keV in the spectra of several gamma-ray burst sources, first reported by Mazets et al., have been widely ascribed to cyclotron absorption, implying that a strongly magnetized neutron star is the cause of the bursts. Most interpretations of the spectra have been hampered by instrumental limitations: the absorption features are usually at the lower end of the energy range covered by the detector, making spectral unfolding unreliable. Here we report new observations by the gamma-ray burst detector on board the Ginga satellite, which has two well-calibrated detectors covering a wide energy range, 1.5 to 375 keV. The spectral features we find are consistent with first and second cyclotron harmonics, which we take as strong evidence for the magnetized neutron star model.