Spectral evolution of the 5 March 1979 γ burst

The γ burst¹ of 5 March 1979 was observed by nine experiments^2-6 widely spaced on an interplanetary scale allowing an accurate position for a γ-burst source to be determined^2,6 for the first time. Spectral observations in the range 50-1,000 keV had sufficient time resolution to resolve spectral changes within the main burst. Early in the main burst, the typical photon energies declined from a spectrum similar to a 30 keV black body to that of a ~26 keV black body. Thereafter, at the shoulder on the main burst the spectrum became harder (KT ~31 keV). The γ burst apparently triggered an X-ray pulsar with a period of ~8.0 s (refs 3-5,7) and with a softer spectrum (KT ~= 12 keV). We show here that, although the source is probably a neutron star with an obliquely rotating magnetic field, simple cooling of residual hot spots does not completely explain the pulsations.