and Cosmology

9. The Universe at High Redshift
404
Fig. 9.42. Distribution of gamma-ray bursts on the sphere
as observed by BATSE, an instrument on-board the CGROsatellite,
during the about nine year mission; in total, 2704
GRBs are displayed. The color of the symbols represents the
observed strength (fluence, or energy per unit area) of the
bursts. One can see that the distribution on the sky is isotropic
to a high degree
ferent distribution of sources is required, hence also
a different kind of source.
The only way to obtain an isotropic distribution for
sources which are typically more distant than the disk
scale-height is to assume sources at distances considerably
larger than the distance to the Virgo Cluster,
hence D ≫ 20 Mpc; otherwise, one would observe an
overdensity in this direction. In addition, the deviation
from the N(> F) ∝ F −3/2 -law means that we observe
sources up to the edge of the distribution (or, more precisely,
that the curvature of spacetime, or the cosmic
evolution of the source population, induces deviations
from the Newtonian counts), so that the typical distance
of GRBs should correspond to a relatively high redshift.
This implies that the total energy in a burst has to be
E ∼ 10 51 to 10 54 erg. This energy corresponds to the
rest mass Mc 2 of a star. The major part of this energy
is emitted within ∼ 1 s, so that GRBs are, during this
short time-span, more luminous than all other γ -sources
in the Universe put together.
Identification and Afterglows. In February 1997, the
first identification of a GRB in another wavelength band
was accomplished by the X-ray satellite Beppo-SAX.
Within a few hours of the burst, Beppo-SAX observed
the field within the GRB error box and discovered
a transient source, by which the positional accuracy was
increased to a few arcminutes. In optical observations of
this field, a transient source was then detected as well,
very accurately defining the position of this GRB. The
optical source was identified with a faint galaxy. Optical
spectroscopy of the source revealed the presence
of absorption features at redshift z = 0.835; hence, this
GRB must have a redshift equal or larger than this.
For the first time, the extragalactic nature of GRBs was
established. In fast progression, other GRBs could be
identified with a transient optical source, and some of
them show transient radiation also at other wavelengths,
from the radio band up to X-rays. The lower-energy radiation
of a GRB after the actual burst in gamma-rays
is called an afterglow.
GRBs can be broadly classified into short- and
long-duration bursts, with a division at a duration of
t burst ∼ 2 s. The spectral index of the short-duration
bursts is considerably harder at γ -ray energies that
that of long-duration bursts. Until 2005, only afterglows
from long-duration bursts had been discovered.
Long-duration bursts occur in galaxies at high redshift,
typically z ∼ 1 or higher, with the highest-redshift burst
identified to date having a redshift of z = 6.3. In one
case, an optical burst was discovered about 30 seconds
after the GRB, with the fantastic brightness of
V ∼ 9 mag, at a redshift of z = 1.6. For a short period
of time, this source was apparently more luminous than
any quasar in the Universe. Thus, during or shortly after
the burst at high energies, GRBs are also very bright
in the optical.
Fireball Model. Whereas the distance of the sources,
and therefore also their luminosity, was then known,
the question of the nature of GRBs still remained unanswered.
One model of GRBs quite accurately describes
their emission characteristics, including the afterglow.
In this fireball model, the radiation is released in the relativistic
outflow of electron–positron pairs with a Lorentz
factor of γ ≥ 100. However, different hypotheses exist
as to how this fireball is produced, hence what the physical
origin of a GRB might be. One of these states that
a GRB is caused by the merger of two neutron stars, or
a neutron star and a black hole. In this case, the emission
will probably be highly anisotropic, so that estimates of
the luminosity from the observed flux, based on the
assumption of isotropic emission, may not be correct.