and Cosmology

9.7 Gamma-Ray Bursts
Hypernovae. At the present time, the sources of longduration
GRBs have pretty much been identified. With
the accurate location obtained from observations of their
afterglows, it was found that they occur in star-forming
galaxies, not in passive early-type galaxies. This finding
is similar to that of core-collapse supernovae which
are also found only in galaxies with star-formation activity.
It was therefore speculated that the origin of
GRBs is closely linked to star formation. The discovery
of a coincidence of several GRBs with supernova
explosions suggests that long-duration GRBs are extraordinarily
energetic explosions of stars, so-called
hypernovae. Even if the emission is highly anisotropic,
as expected from the fireball model, the corresponding
energy released by the hypernovae is very large.
For the purpose of identifying GRBs, the SWIFT
satellite was launched in November 2004. This satellite
is equipped with three instruments: a wide-field
gamma-ray telescope to discover the GRBs, an X-ray
telescope, and a UV-optical telescope. Within a few
seconds of the discovery of a GRB, the satellite targets
the location of the burst, so that it can be observed by
the latter two telescopes, obtaining an accurate position.
This information is then immediately transmitted
to the ground, where other telescopes can follow the afterglow
emission. SWIFT is expected to discover about
100 GRBs per year and to obtain significantly improved
statistics of their afterglow light curves and redshifts.
Already in its first year of operation, a large number
of GRBs were found by SWIFT, including the one at
z = 6.3.
Counterparts of Short-Duration GRBs. SWIFT has
allowed the identification of four short-duration GRBs
in 2005. In contrast to the long-duration bursts, some
of these seem to be associated with elliptical galaxies;
this essentially precludes any association with supernova
explosions. In fact, for one of these short burst,
very sensitive limits on the optical brightness explicitly
rules out any contribution from a supernova explosion.
Furthermore, the host galaxies of short bursts are at substantially
lower redshift, z ∼ 0.2. Given that both kinds
of GRBs have about the same observed flux (or energy),
this implies that short-duration bursts are less energetic
than long-duration ones, by approximately two orders
of magnitude. All of these facts clearly indicate that
short- and long-duration GRBs are due to different populations
of sources. The lower energies of short bursts
and their occurrence in early-type galaxies with old stellar
populations are consistent with them being due to the
merging of compact objects, either two neutron stars, or
a neutron star and a black hole.
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