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<SUB>v</SUB>~ν<SUP>β</SUP> with index β=-0.50+/-0.16. The spectrum is consistent with a stretch of an afterglow spectrum between the peak frequency, ν<SUB>m</SUB>, and the cooling break, ν<SUB>c</SUB>. The photon index derived following the model of Sari et al. (1998), 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. .