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A.I. MacFadyen (UC Santa Cruz)
A collapsar is a massive rotating star (M\rm ms \gtrsim 25 M\odot) whose core 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 produces 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 \lesssim {\rm Mhole}/M\odot \lesssim 10) at its center. Rapid accretion of stellar matter into the hole at rates of up to 0.1\, M\odot\,{\rm s}-1 releases large amounts of energy (\approx 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, and makes the observed gamma-ray photons and lower energy afterglow at large distances (> 1000 stellar radii). Since collapsars naturally form jetted explosions beamed to approximately 1% of the sky, the energetic requirement is typically hundreds of times less than the observed ``isotropic equivalent energy.'' Supernova-like energies of 1051-1052 ergs, as calculated in detailed 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.
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. This work is supported in part by the DOE ASCI-2 program.
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