Optical study of the counterpart to GRB 990712

Quasi-simultaneous BVR-band observations performed from New Zealand and Argentina ∼16 hr after the burst clearly detected the optical counterpart to GR-B 990712. Based on these measurements we construct the optical multi-band spectrum. We report that the spectrum between the R and B bands follows a power law $F_v{\mathrm{\propto \nu }}^{\beta }$ with index $\text{β=−0.50±0.16.}$ The spectrum is consistent with a stretch of an afterglow spectrum between the peak frequency, ${\nu }_m,$ and the cooling break, ${\nu }_c.$ The photon index derived following the model of Sari et al. (1998), $\text{p=2.36±0.08}$ is compatible with β and the power law decay, α, only if no absorption is introduced. Thus, our results support that GRB 990712 occurred in a low density region, resembling GRB 970508.