Astron. Astrophys. 358, L41-L44 (2000)

3. Conclusions and discussion

The PHEBUS instrument aboard the GRANAT observatory detected soft gamma-ray afterglow from the GRB 920723 and GRB 910402. These events are the brightest bursts detected by the PHEBUS instrument. In both cases the light curve of the burst makes smooth transition from the main burst into the afterglow emission (Fig. 3 and 5). The best fit power law indices of time for the afterglow are during after GRB 910402 and during after GRB 920723 (at confidence level). In both cases the beginning of the afterglow emission () coincides within statistical errors with the abrupt softening of the emission (). Just after GRB event the energy spectrum of the afterglow emissions is much softer than the energy spectrum of the main GRB events. The average hardness ratio of the main GRB event (ratio of count rates in to ) is equal to . This corresponds to the power-law spectrum with the photon index of . After abrupt transition to the afterglow both bursts have much softer spectra with the photon index of .

Thus the moments corresponding to the beginning of domination of the emission fading as the power law during afterglow in both cases coincide within statistical errors with the abrupt softening of the energy spectra emitted by the both gamma-ray burst sources in soft gamma-rays .

In the internal/external shock scenario we can suggest that we found the moment when the much softer afterglow emission (connected with external shock) is starting to dominate over harder GRB emission (connected with internal shocks).

During the afterglow of GRB 910402 the statistically significant hardening of the emission spectra is observed (Fig. 4). The afterglow spectrum just after the gamma-ray burst has the photon index of . As one can see from Fig. 4 after the afterglow photon index was already . This is the first observation of hardening of the afterglow spectra with time. The possibility of such hardening is under discussion in different GRB afterglow models.

The afterglow emission intensities in energy range are rather faint in comparison with gamma-ray bursts. In the case of GRB 920723 the afterglow emission contains of the total burst energy that was emitted during the interval of in the energy range. In the case of GRB 910402 only of the GRB energy was released during the observed afterglow in time interval.

© European Southern Observatory (ESO) 2000

Online publication: June 8, 2000
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