Models (2) ...
Gamma ray bursts can produce energies
as large or even larger than that of a supernova explosion.
In addition, as we have already noted, this enormous energy must be released in a matter
of
seconds in the highest energy part of the spectrum.
That is a very tall order!
A Gravitational Source?
Almost the only way that we know to explain such a rapid release of that amount of energy
is from a collapse involving a compact gravitational source. Some popular
ideas for the specifics of how this could happen include
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A binary neutron star system radiates orbital energy as gravitational waves
(see the discussion of the Binary Pulsar in Chapter 14), causing the two
neutron stars to spiral together and merge.
Such a merger of two neutron stars, with jet outflow perpendicular to the merger plane,
could produce a burst of gamma rays
as the neutron stars collapse to a rotating black hole. Here is a
supercomputer simulation (Rosswog animation)
of a neutron star merger.
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Gravitational wave emission could likewise cause the
merger of two black holes or a neutron star and black hole in a binary
system,
with jet outflow perpendicular to the
merger plane to produce the gamma ray burst.
The top right animation on this page
illustrates a neutron star and black hole merger that
produces jet outflow and triggers
a series of smaller gamma ray pulses followed by a very large burst.
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In a variant of the core-collapse supernova mechanism,
a spinning very massive star might collapse gravitationally to a
Kerr black hole (a distorted, spinning black hole) and the
jet outflow from the region surrounding this
collapsed object could produce a burst of gamma rays. Such an event
has been given the name hypernova. This
animation (Erin hypernova) illustrates a hypernova
engine for a gamma ray burst.
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None of these mechanisms have yet been ruled out by the data.
As noted in the right frame, it is possible that
there may be more than one engine that drives gamma ray bursts.
Evidence That Hypernovae Power Longer Bursts
For those gamma ray bursts where a transient afterglow has been detected, the
afterglows allow us to pinpoint the location of the burst and to study this
region at other wavelengths. Based on such studies,
there is now strong evidence that gamma ray bursts associated with
observed afterglows are originating in star-forming regions
of distant galaxies.
Since core collapse of massive young stars is expected to occur preferentially in
star-forming regions, this favors the hypernova model of the central engine.
In addition, in some of these cases there is evidence that
a gamma ray burst may be correlated in some way with a supernova explosion in the
same galaxy. Conversely, the mergers of neutron stars or black holes are expected
to be most common among old populations of stars, because for these collapsed
objects to merge in a binary system they must first spend a very long time
radiating alway orbital energy as gravitational waves.
Therefore, there is growing evidence that at least those gamma ray
bursts for which an afterglow has been detected may be powered by
some variant of the hypernova
mechanism.
However, afterglows can presently be detected only for longer-lived gamma ray
bursts because of the technology used to detect them. This means that we
still do not know where the
shorter-lived gamma ray bursts originate, so it is possible that they correspond to
a different mechanism than the longer-lived bursts that have been studied
extensively over the last several years. One common current hypothesis is
that the longer gamma ray bursts are powered primarily by hypernovae and the shorter
ones primarily
by neutron star mergers, but we still do not have enough information to
test this idea very rigorously.
