The year 1997 marked the beginning of the "afterglow era" in the
Gamma-Ray Burst (GRB) studies. The fast and accurate
localizations of GRBs by the BeppoSAX observatory were crucial for
establishing the connection of GRBs with the sources of decaying
X-ray, optical, and radio emission. Identification of GRB counterparts
at other wavelengths and, finally, the discovery of host galaxies
was a breakthrough in determining the distance to the GRB sources and
marked the first step in unraveling their mystery.
The most distant presently known GRB source is located at a redshift
of at least z=3.4, that is at a distance of about 12 billion light years. The
cosmological distances to the GRB sources imply that they are
associated with phenomena as energetic as approx. 1053 erg
and are the most luminous among the presently known objects in the
GRB 920723 light curve in the soft gamma-ray band. The
vertical dashed line at t=6 sec indicates the moment of an abrupt
change of the spectral properties, presumably associated with the
onset of the afterglow emission.
A quick release of such a huge energy in a compact volume would
be enough to power a relativistic fireball - a plasma cloud
expanding into the surrounding interstellar medium with
Interaction of this fireball with the interstellar medium results
in an afterglow emission which follows the main
event and can last for days and even months (Figs.1 and 2).
Observations immediately after a burst, when the expansion velocity of
the fireball is at the maximum are of great interest for GRB theory.
Archival search of the data of the international GRANAT
X-ray/gamma-ray observatory performed in collaboration with MPA
researchers resulted in the discovery of such early afterglows
for several bright GRBs. Moreover, an abrupt change of
the radiation spectrum, presumably associated with the onset of
the afterglow emission was found in one of the events
Study of the properties of the early afterglow emission led to an
estimate of the initial fireball expansion velocity for this GRB event
-- 99.999% of the speed of light.
The afterglow light curve in the soft gamma-ray band.
Zero time is at the moment of an abrupt change of the spectrum
(shown by dashed vertical lines in Figs.1 and 2). The main burst is not shown here because it is at t<0
for the choice of time zero.
Time history of the spectral index in the 8-200
keV energy band. Zero time is the same as in Fig.1.
GRANAT/SIGMA Observation of the Early Afterglow from GRB920723 in
R.Burenin, A.Vikhlinin, M.Gilfanov et al.
Letters, 1999, 344, L53 Preprint
Gamma-ray burst afterglows, M.Gilfanov and R.Burenin,
of the MPA, 1998, pp.14-15 (ps,
Last modified: Thu Jul 1 09:40:39 MDT 1999
by Markus Rampp