ar X iv : a st ro - p h / 99 04 28 6 v 1 2 1 A pr 1 99 9 The X - ray , Optical and Infrared Counterpart to GRB 980703

We report on X-ray, optical and infrared follow-up observations of GRB980703. We detect a previously unknown X-ray source in the GRB error box; assuming a power law decline we find for its decay index α < –0.91 (3σ). We invoke host galaxy extinction to match the observed spectral slope with the slope expected from ‘fireball’ models. We find no evidence for a spectral break in the infrared to X-ray spectral range on 1998 July 4.4, and determine a lower limit of the cooling break frequency: νc > 1.3 × 10 17 Hz. For this epoch we obtain an extinction of AV = 1.50 ± 0.11. From the X-ray data we estimate the optical extinction to be AV = 20.2 +12.3 −7.3 , inconsistent with the former value. Our optical spectra confirm the redshift of z=0.966 found by Djorgovski et al. (1998). We compare the afterglow of GRB980703 with that of GRB970508 and find that the fraction of the energy in the magnetic field, ǫB < 6× 10 , is much lower in the case of GRB980703, which is a consequence of the high frequency of the cooling break. Subject headings: Gamma-rays bursts—gamma-rays:observations—radiation mechanisms:non-thermal