The X‐Ray, Optical, and Infrared Counterpart to GRB 980703
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Abstract
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 alpha<-0.91 (3 sigma). 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, nu_c>1.3x10^17 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^+12.3_-7.3, 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_B<6x10^-5, is much lower in the case of GRB 980703, as a consequence of the high frequency of the cooling break.