1,457 research outputs found
Gamma-Ray Bursts: The Central Engine
A variety of arguments suggest that the most common form of gamma-ray bursts
(GRBs), those longer than a few seconds, involve the formation of black holes
in supernova-like events. Two kinds of ``collapsar'' models are discussed,
those in which the black hole forms promptly - a second or so after iron core
collapse - and those in which formation occurs later, following ``fallback''
over a period of minutes to hours. In most cases, extraction of energy from a
rapidly accreting disk (and a rapidly rotating black hole) is achieved by
magnetohydrodynamical processes, although neutrino-powered models remain viable
in cases where the accretion rate is >0.05 solar masses per second. GRBs are
but one observable phenomenon accompanying black hole birth and other
possibilities are discussed, some of which (long, faint GRBs and soft x-ray
transients) may await discovery. Since they all involve black holes of similar
mass accreting one to several M\sun, collapsars have a nearly standard total
energy, around 10**52 erg, but both the fraction of that energy ejected as
highly relativistic matter and the distribution of that energy with angle can
be highly variable. An explanation is presented why inferred GRB luminosity
might correlate inversely with time scales and arguments are given against the
production of ordinary GRBs by supergiant stars.Comment: 10 pages, 2 figures, Fifth Huntsville Conference on Gamma-Ray Bursts
eds. R. M. Kippen, R.S. Mallozzi, & V. Connaughton, AI
High energy transients
A meeting was convened on the campus of the University of California at Santa Cruz during the two-week interval July 11 through July 22, 1983. Roughly 100 participants were chosen so as to give broad representation to all aspects of high energy transients. Ten morning review sessions were held in which invited speakers discussed the current status of observations and theory of the above subjects. Afternoon workshops were also held, usually more than one per day, to informally review various technical aspects of transients, confront shortcomings in theoretical models, and to propose productive courses for future research. Special attention was also given to the instrumentation used to study high energy transient and the characteristics and goals of a dedicated space mission to study transients in the next decade were determined. A listing of articles written by various members of the workshop is included
On the Progenitors of Collapsars
We study the evolution of stars that may be the progenitors of common
(long-soft) GRBs. Bare rotating helium stars, presumed to have lost their
envelopes due to winds or companions, are followed from central helium ignition
to iron core collapse. Including realistic estimates of angular momentum
transport (Heger, Langer, & Woosley 2000) by non-magnetic processes and mass
loss, one is still able to create a collapsed object at the end with sufficient
angular momentum to form a centrifugally supported disk, i.e., to drive a
collapsar engine. However, inclusion of current estimates of magnetic torques
(Spruit 2002) results in too little angular momentum for collapsars.Comment: 3 pages, 5 figures, in Proc. Woods Hole GRB meeting, ed. Roland
Vanderspe
Long Gamma-Ray Transients from Collapsars
In the collapsar model for common gamma-ray bursts, the formation of a
centrifugally supported disk occurs during the first 10 seconds following
the collapse of the iron core in a massive star. This only occurs in a small
fraction of massive stellar deaths, however, and requires unusual conditions. A
much more frequent occurrence could be the death of a star that makes a black
hole and a weak or absent outgoing shock, but in a progenitor that only has
enough angular momentum in its outermost layers to make a disk. We consider
several cases where this is likely to occur - blue supergiants with low mass
loss rates, tidally-interacting binaries involving either helium stars or giant
stars, and the collapse to a black hole of very massive pair-instability
supernovae. These events have in common the accretion of a solar mass or so of
material through a disk over a period much longer than the duration of a common
gamma-ray burst. A broad range of powers is possible, to
erg s, and this brightness could be enhanced by beaming. Such
events were probably more frequent in the early universe where mass loss rates
were lower. Indeed this could be one of the most common forms of gamma-ray
transients in the universe and could be used to study first generation stars.
Several events could be active in the sky at any one time. A recent example of
this sort of event may have been the SWIFT transient Sw-1644+57.Comment: submitted to Astrophysical Journa
The Central Engines of Gamma-Ray Bursts
Leading models for the "central engine" of long, soft gamma-ray bursts (GRBs)
are briefly reviewed with emphasis on the collapsar model. Growing evidence
supports the hypothesis that GRBs are a supernova-like phenomenon occurring in
star forming regions, differing from ordinary supernovae in that a large
fraction of their energy is concentrated in highly relativistic jets. The
possible progenitors and physics of such explosions are discussed and the
important role of the interaction of the emerging relativistic jet with the
collapsing star is emphasized. This interaction may be responsible for most of
the time structure seen in long, soft GRBs. What we have called "GRBs" may
actually be a diverse set of phenomena with a key parameter being the angle at
which the burst is observed. GRB 980425/SN 1988bw and the recently discovered
hard x-ray flashes may be examples of this diversity.Comment: 8 pages, Proc. Woods Hole GRB meeting, Nov 5 - 9 WoodsHole
Massachusetts, Ed. Roland Vanderspe
Type Ia Supernova: Burning and Detonation in the Distributed Regime
A simple, semi-analytic representation is developed for nuclear burning in
Type Ia supernovae in the special case where turbulent eddies completely
disrupt the flame. The speed and width of the ``distributed'' flame front are
derived. For the conditions considered, the burning front can be considered as
a turbulent flame brush composed of corrugated sheets of well-mixed flames.
These flames are assumed to have a quasi-steady-state structure similar to the
laminar flame structure, but controlled by turbulent diffusion. Detonations
cannot appear in the system as long as distributed flames are still
quasi-steady-state, but this condition is violated when the distributed flame
width becomes comparable to the size of largest turbulent eddies. When this
happens, a transition to detonation may occur. For current best estimates of
the turbulent energy, the most likely density for the transition to detonation
is in the range 0.5 - 1.5 x 10^7 g cm^{-3}.Comment: 12 pages, 4 figure
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