Gamma-ray burst afterglows

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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 Universe

Figure 1:
    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 ultrarelativistic velocity. 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 (Fig.3). 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.

Figure 2:
    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.

Figure 3:
    Time history of the spectral index in the 8-200 keV energy band. Zero time is the same as in Fig.1.

M.Gilfanov, R.Burenin

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Last modified: Thu Jul 1 09:40:39 MDT 1999 by Markus Rampp
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