Are Gamma-Ray Bursts in Star-Forming Regions?

Abstract

The optical afterglow of the gamma-ray burst GRB 970508 (z=0.835) was a few hundred times more luminous than any supernova. Therefore, the name "hypernova" is proposed for the whole GRB/afterglow event. There is tentative evidence that the GRBs 970228, 970508, and 970828 were close to star-forming regions. If this case is strengthened with future afterglows, then the popular model in which GRBs are caused by merging neutron stars will have to be abandoned, and a model linking GRBs to cataclysmic deaths of massive stars will be favored. The presence of X-ray precursors, first detected with Ginga, is easier to understand within a framework of a "dirty" rather than a "clean" fireball. A very energetic explosion of a massive star is likely to create a dirty fireball rather than a clean one. A specific speculative example of such an explosion is proposed: a microquasar. Its geometrical structure is similar to the "failed supernova" of Woosley: the inner core of a massive, rapidly rotating star collapses into a ~10 M_☉ Kerr black hole with ~5×10⁵⁴ ergs of rotational energy, while the outer core forms a massive disk/torus. A superstrong ~10¹⁵ G magnetic field is needed to make the object operate as a microquasar similar to the Blandford & Znajek model. Such events must be vary rare, 10⁴-10⁵ times less common than ordinary supernovae, if they are to account for the observed GRBs.