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Astron. Astrophys. Suppl. Ser. 138, 451-452

Physical parameters of GRB 970508 from its afterglow synchrotron emission

T.J. Galama 1 - R.A.M.J. Wijers 2 - P.M. Vreeswijk 1 - P.J. Groot 1 - J. Van Paradijs 1,3 - C. Kouveliotou 4 - C. Robinson 4 - M. Bremer 5 - R.G. Strom 1,6 - N. Tanvir 7

Send offprint request: T.J. Galama

1 - Astronomical Institute "Anton Pannekoek"/CHEAF, Kruislaan 403, 1098 SJ, Amsterdam, The Netherlands
2 - Dept. of Physics & Astronomy, SUNY, Stony Brook, NY, U.S.A.
3 - Physics Department, UAH, Huntsville, AL 35899, U.S.A.
4 - USRA at NASA/MSFC, Code ES-84, Huntsville AL 35812, U.S.A.
5 - Institut de Radio Astronomie Millimétrique, 300 rue de la Piscine, F-38406 Saint-Martin d'Hères, France
6 - NFRA, Radio Observatory, Postbus 2, 7990 AA, Dwingeloo, The Netherlands
7 - Institute of Astronomy, Madingley Road, Cambridge, UK

Received December 29, 1998; accepted May 4, 1999


For the case of an adiabatically expanding blast wave, which is applicable to observed gamma-ray burst (GRB) afterglows at late times, we give expressions to infer the physical properties of the afterglow from the measured spectral features.
We show that enough data exist for GRB970508 to compute unambiguously the ambient density, n=0.03 cm-3, and the blast wave energy per unit solid angle, ${\cal
E}=4\ 10^{52}$ erg/4$\pi$ sr. We also compute the energy density in electrons and magnetic field. We find that they are 12% and 9%, respectively, of the nucleon energy density.

Key words: gamma-rays: bursts -- radiation mechanisms: non-thermal

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