Supernovae in Helium Star–Neutron Star Binaries: A Possible γ-ray Burst Mechanism

Helium star–neutron star binaries are widely believed to be the progenitors of the observed double pulsar systems. In these, the second neutron star is presumed to be the compact remnant of the helium star supernova. Here, the observational implications of such a supernova are discussed, and in particular are explored as a candidate γ-ray burst mechanism. In this scenario the supernova results in a transient period of rapid accretion onto the neutron star, extracting via magnetic torques its rotational energy at highly super-Eddington luminosities in the form of a narrowly beamed, strongly electromagnetically dominated jet. Compton scattering of supernova photons advected within the ejecta, and photons originating at shocks driven into the ejecta by the jet, will cool the jet and can produce the observed prompt emission characteristics, including the peak–inferred isotropic energy relation, X-ray flash characteristics, subpulse light curves, energy dependent time lags and subpulse broadening, and late time spectral softening. The duration of the burst is limited by the rate of Compton cooling of the jet, eventually creating an optically thick, moderately relativistically expanding fireball which can produce the afterglow emission. If the neutron star stays bound to a compact remnant, late term light curve variability may be observed as in SN 2003dh. It is also noted that a failed supernova will result in a short burst (limited instead by the accretion time of the ejected envelope) without a subsequent afterglow or supernova, yielding a natural explanation for the bimodal time distribution within a unified burst model.