The X-ray, optical, and infrared counterpart to GRB 980703

We report on X-ray, optical, and infrared follow-up observations of GRB 980703. 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 (3 σ). 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, v_c > 1.3 × 10¹⁷ Hz. For this epoch we obtain an extinction of A_V = 1.50 ± 0.11. From the X-ray data we estimate the optical extinction to be A_V = 20.2_-7.3^+12.3, inconsistent with the former value. Our optical spectra confirm the redshift of z = 0.966. We compare the afterglow of GRB 980703 with that of GRB 970508 and find that the fraction of the energy in the magnetic field, ∈_B < 6 × 10^-5, is much lower in the case of GRB 980703, as a consequence of the high frequency of the cooling break.