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Astron. Astrophys. 344, L53-L56 (1999)

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Letter to the Editor

GRANAT/SIGMA observation of the early afterglow from GRB 920723 in soft gamma-rays

R.A. Burenin 1,2, A.A. Vikhlinin 1, M.R. Gilfanov 1,3, O.V. Terekhov 1,2, A.Yu. Tkachenko 1,2, S.Yu. Sazonov 1,2, E.M. Churazov 1,3, R.A. Sunyaev 1,3, P. Goldoni 4, A. Claret 4, A. Goldwurm 4, J. Paul 4, J.P. Roques 5, E. Jourdain 5, F. Pelaez 6 and G. Vedrenne 5

1 Space Research Institute, Russian Academy of Sciences, Profsoyuznaya 84/32, 117810 Moscow, Russia
2 visiting Max-Planck-Institut für Astrophysik
3 Max-Planck-Institut für Astrophysik, Karl-Schwarzschild-Strasse 1, D-85740 Garching bei München, Germany
4 CEA/DSM/DAPNIA/SAp, Centre d'Etudes de Saclay, F-91191 Gif-sur-Yvette, Cedex, France
5 Centre d'Etude Spatiale des Rayonnements (CNRS/UPS) 9, avenue du Colonel Roche, B.P. 4346, F-31028 Toulouse Cedex, France
6 Department of Physics and Astronomy, Mississippi State University, MS 39762, USA

Received 1 February 1999 / Accepted 25 February 1999

Abstract

We present a GRANAT/SIGMA observation of the soft gamma-ray afterglow immediately after GRB 920723. The main burst is very bright. After [FORMULA]s, the burst light curve makes a smooth transition into an afterglow where flux decays as [FORMULA]. The power-law decay lasts for at least 1000 s; beyond this time, the afterglow emission is lost in the background fluctuations. At least [FORMULA] of main burst energy is emitted in the afterglow. At approximately [FORMULA]s after the trigger, we also observe an abrupt change in the burst spectrum. At [FORMULA]s, the ratio of 8-20 and 75-200 keV fluxes corresponds to the power law spectral index [FORMULA]. At [FORMULA]s, the value of [FORMULA] increases to [FORMULA] and stays at this level afterwards. The observed afterglow characteristics are discussed in connection with the relativistic fireball model of gamma-ray bursts.

Key words: gamma rays: bursts

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© European Southern Observatory (ESO) 1999

Online publication: March 18, 1999
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