Are Gamma-Ray Bursts in Star-Forming Regions?
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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, a 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 be 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 (1993a): the inner core of a massive, rapidly rotating star collapses into a ~ 10 solar mass Kerr black hole with ~ 5 x 10^54 erg of rotational energy, while the outer core forms a massive disk/torus. A superstrong ~ 10^15 G magnetic field is needed to make the object operate as a microquasar similar to the Blandford & Znajek (1977) model. Such events must be vary rare, 10^4 - 10^5 times less common than ordinary supernovae, if they are to account for the observed GRBs.
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