A collapsar is a massive rotating star (M_ms > 25 M_sun) 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 < M_hole/M_sun < 10) at its center. Rapid accretion of stellar matter into the hole at rates of up to 0.1\, M_sun\,s^-1 releases large amounts of energy (approx. 10^51 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 10^51-10^52 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, 10^54 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.
[Y2.002] Gamma-Ray Burst Progenitors: Merger Model
Maximilian Ruffert (University of Edinburgh)
The mergers of neutron stars and black holes remain a viable
model for gamma-ray burst central engines, at least for the
class of short bursts: their time scales, occurrence rates
and energy output seem to be consistent with observations.
We will present results of our latest simulations showing
how the orbit of a neutron star around a black hole shrinks
due to gravitational radiation, how the neutron star's
matter gets accreted by the black hole, and how the tidal
forces of the black hole finally shred the neutron star into
a thick disk. In this process, huge amounts of energy are
radiated away by gravitational waves and by neutrinos
emitted from the hot disk. The neutrino luminosities are so
large that an appreciable fraction (some few percent!) of
neutrinos annihilate with antineutrinos creating the clean
fireball necessary to power gamma-ray bursts.
[Y2.003] Gamma-ray Burst Afterglows: Probing the Burst Energetics and Environment
Fiona Harrison (California Institute of Technology)
The detection of afterglows (long-lived emission) from
gamma-ray bursts (GRBs) has revolutionized the field. As a
result of the discovery of these fading counterparts we now
know the GRB distance and energy scale, and are beginning to
gain an understanding of the properties of the host
galaxies. As is the case with supernovae, studying the
evolution of the broadband emission with time in principal
enables us to probe the geometry of the explosion, the
density of the medium immediately surrounding the
progenitor, as well as the properties of the interstellar
medium in the host galaxy. Accomplishing this requires a
model for the physics of the relativistic shock, an
understanding of the absorbing properties of the
insterstellar medium, and high-quality radio through X-ray
measurements. In this talk I will focus on several
well-studied events, describing what broadband afterglow
observations can contribute in principal, comparing with
what has to-date been securely understood from the data.
[Y2.004] GRBs, their Hosts and High-Redshift Galaxies
Andrew Fruchter (Space Telescope Science Institute)
I will discuss the properties of the hosts of gamma-ray bursts. I will show that the nature of these galaxies, as well as the locations of the GRBs on them suggests that the long-duration GRBs, which are the only type which have so far been localized, are associated with massive stars. I will also discuss how GRBs may allow us to better study star formation at high redshift.