The 1979 March 5 gamma-ray transient: Was it a classic gamma-ray burst?

The 1979 March 5 gamma-ray transient has long been thought to be fundamentally different from the classic gamma-ray bursts (GRBs). It had recurrences, pulsations, and a soft spectral component unlike classic GRBs. With the exception of the soft component reported from the KONUS experiment, the unusual characteristics of the March 5 transient were detectable primarily because it was extremely bright. Computer limitations, satellite transmission effects or pulse pileup, and dead-time effects have prevented, until now, the analysis of spectra from the International Cometary Explorer (ICE) and the Pioneer Venus Orbiter (PYO). The ICE-PVO spectrum of the main peak differs markedly from the published KONUS spectrum. Rather than being dominated by a soft component similar to that observed in the soft gamma repeaters (SGRs), the ICE-PVO spectrum appears to be consistent with a classic GRB spectrum, especially above 100 keV. Above 100 keV, the spectrum is consistent with thermal bremsstrahlung with a temperature of similar to 200 keV, somewhat soft but within the range of classic GRBs. We believe that, given the ICE-PVO spectral observations, the March 5 transient would have been classified as a classic GRB when it was discovered. Although a formal analysis has not been done, the pulsations and recurrences might still be unique features that distinguish the March 5 transient from GRBs. The ICE spectrum provides evidence for a low-energy cutoff at 100 keV. If high-velocity neutron stars are born as misaligned rotators with their velocities aligned with their spin axes and if their emissions are beamed, then when they are young their spatial distribution will be similar to the SGRs. If torques can align the field with the spin axis, then when they are old their spatial distribution will be isotropic like classic GRBs. Thus, the SGRs and GRBs could be consanguineous: high-velocity neutron stars initially produce SGR events (and, occasionally a GRB like the March 5 transient) and when they are older and in the galactic corona, they go through a GRB phase. The March 5 event demonstrates that high-velocity neutron stars at distances of tens of kiloparsecs are capable of producing events like classic GRBs. This reanalysis has revised the March 5 transient intensity upward such that the peak luminosity at an assumed distance of 55 kpc is 1.9 x 10(45) ergs s(-1). However, the March 5 event is consistent with the classic GRB log N-log P distribution and is not necessarily extraordinarily bright.