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A. I. MacFadyen (California Institute of Technology)
A collapsar is a massive rotating star (Mms > 25 Msun) the core of which collapses to form a black hole. In a rapidly rotating star, the accretion of the rest of the star into the newly-formed black hole may produce an energetic long-duration gamma-ray burst (GRB) accompanied by a Type Ib/c supernova. An accretion disk forms as the outer layers of the star fall into the black hole (3 < {Mhole}/Msun < 10) at its center. Rapid accretion of stellar matter into the hole at rates of up to 0.1\, Msun\,{s}-1 releases large amounts of energy (~ 1051 erg s-1) some of which is deposited in the low density rotation axis of the star. The heated gas at the pole expands in a jet-like fireball which penetrates the surface of the star, escapes to large distances (> 1000 stellar radii), and makes the observed gamma-ray photons and lower energy afterglow. Since collapsars naturally form jetted explosions beamed to a small fraction of the sky, the energetic requirement is typically hundreds of times less than the observed ``isotropic equivalent energy.'' Supernova-like energies of ~1051 ergs, as calculated in models to be presented, are therefore sufficient to explain GRBs with a range of isotropic equivalent energies up to, and exceeding, 1054 ergs depending on the beaming angle. Outflowing winds blown from the accretion disk can produce stellar explosions independant of any GRB-producing jet which may also occur. These wind-driven explosions may occur in stellar explosions which do not make a GRB and constitute a new class of supernova explosion. A key feature of the collapsar winds is that they are capable of producing radioactive elements (e.g., 56Ni) necessary to power a long-duration supernova light curve.
The observational signatures of collapsars are diverse and depend on the beaming of the explosion, the amount of radioactive 56Ni produced and mixed into the stellar envelope, the angular momentum of the progenitor star and its radius at core collapse. Recent observational evidence, both the close association of well-localized GRBs with star-forming regions and possible direct links between GRBs and supernovae, supports the collapsar model.
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Bulletin of the American Astronomical Society, 34
© 2002. The American Astronomical Soceity.