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

1. Introduction

Recent observations of the X-ray, optical and radio afterglow emission (e.g. Costa et al. 1997; Van Paradijs et al. 1997; Frail et al. 1997) of Gamma-Ray Bursts as well as measurements of the high redshifts of the GRB sources in optical give a new impact to the gamma-ray burst astronomy. It was shown that the sources of some of the detected GRB events are located at cosmological distances. X-ray and optical afterglow emission is fading as power law of time. Such behaviour is consistent with the relativistic fireball model of GRB (Mészáros&Rees 1993, 1997).

The results of earlier observations indicated that afterglow might be present right after gamma-ray burst events in x-rays (Sunyaev et al. 1990, Murakami et al. 1991, Terekhov et al. 1993, Sazonov et al. 1998), soft gamma-rays (Klebesadel 1992, Tkachenko et al. 1995), gamma-rays (Hurley et al. 1994).

The GRANAT observatory was launched into a high-apogee orbit with the PROTON carrier rocketon December 1, 1989. Its four-day orbit with an initial apogee is of the kind that the satellite enters the Earth radiation belts only for a short interval (several hours). The satellite was outside the Earth magnetosphere and the radiation belts during 3 days in every orbit. This ensures almost constant background level during observations in absence of bright solar flares.

The PHEBUS instrument is the part of the payload of the observatory. It consists of six cylindrical ( in diameter and in height) BGO detectors surrounded by a plastic anticoincidence shield to reject the background connected with the charged particles. The detectors were placed on different sides of the GRANAT satellite, parallel to the axes of the Cartesian coordinate system in such a way that with the probability of at least two detectors were able to observe a GRB event with no absorption by the satellite mechanical structure. The instrument was sensitive to photons in the broad energy range with the intrinsic total efficiency to gamma-rays equal to or greater 0.78. The field of view of the instrument was . Each of the six BGO detectors was equipped with a trigger system to detect bursts. The trigger system activates electronics of the instrument to transit to the "burst mode" automatically if the count rate exceeds the background level by at least 8 standard deviations, in at least two PHEBUS detectors.

We present the results of observations of the soft gamma-ray early afterglows with energy more than from two bright Gamma-ray bursts GRB 920723 and GRB 910402 detected by the PHEBUS instrument. Both gamma-ray bursts are strong events and give statistically significant count rate in all 6 detectors of the PHEBUS instrument. GRB 910402 is the brightest burst observed by PHEBUS. The burst Universal trigger time is . Fig. 1 shows the background-subtracted GRB 910402 light curve in energy range.

Fig. 1. The light curve of GRB 910402 in the energy range. .

GRB 920723 has been detected by three instruments of the GRANAT observatory: SIGMA, WATCH and PHEBUS in energy range (Terekhov et al. 1995). For this burst the WATCH instrument detected fading afterglow emission for more than after the end of the main event (Terekhov et al. 1993). Further analysis of the SIGMA data revealed soft gamma-ray afterglow lasted for . The abrupt change of the GRB spectra was observed during transition to the afterglow emission in the energy range (Burenin et al. 1999). The PHEBUS burst Universal trigger time of this event is . In Fig. 2 the background subtracted light curve of GRB 920723 is shown in energy range.

Fig. 2. The light curve of GRB 920723 in the energy range. .

BATSE did not see either of these events. GINGA has an event at that time on 910402 and PVO did see GRB 920723 (Fenimore 2000).

© European Southern Observatory (ESO) 2000

Online publication: June 8, 2000
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