The X‐Ray, Optical, and Infrared Counterpart to GRB 980703

Abstract

We report on X-ray, optical and infrared follow-up observations of GRB\n980703. We detect a previously unknown X-ray source in the GRB error box;\nassuming a power law decline we find for its decay index alpha < -0.91\n(3-sigma). We invoke host galaxy extinction to match the observed spectral\nslope with the slope expected from `fireball' models. We find no evidence for a\nspectral break in the infrared to X-ray spectral range on 1998 July 4.4, and\ndetermine a lower limit of the cooling break frequency: nu_c > 1.3 x 10^{17}\nHz. For this epoch we obtain an extinction of A_V = 1.50 +/- 0.11. From the\nX-ray data we estimate the optical extinction to be A_V = 20.2 +12.3 -7.3,\ninconsistent with the former value. Our optical spectra confirm the redshift of\nz = 0.966 found by Djorgovski et al. (1998). We compare the afterglow of GRB\n980703 with that of GRB 970508 and find that the fraction of the energy in the\nmagnetic field, epsilon_B < 6x10^{-5}, is much lower in the case of GRB 980703,\nwhich is a consequence of the high frequency of the cooling break.\n