3 Optical and infrared observations

We observed the field of GRB 980703 with the Wise Observatory 1-m telescope (in I); the 3.5-m New Technology Telescope (NTT; in V, I and H), the 2.2-m (in H and Ks) and Dutch 90-cm (in gunn i) telescopes of ESO (La Silla); the CTIO 0.9-m telescope (in R) and UKIRT (in H, J and K).

  

Figure: V, R, I, J, H and K light curves of GRB 980703. The filled symbols denote our data, while the open symbols represent data taken from the literature (see Vreeswijk et al. 1998 for the references). At the epochs t1-t4 we have reconstructed the spectral flux distribution of the OT

The light curves of the OT, which was independently discovered by Frail et al. (1998) and Zapatero Osorio et al. (1998), are shown in Fig. 1 (see Vreeswijk et al. for the photometric calibration and a log of the observations). In view of the flattening of the light curves after $t\sim 5$ days we fitted a model $F_{\nu} = F_{0} \cdot t^{\alpha} + F_{\rm gal}$ to our own I and H band light curves. The weighted mean value of $\alpha$ for the I and H-bands equals $-1.61 \pm 0.12$. For the V, R, J and K bands we fixed $\alpha$ at this value, included also data from the literature, and only fitted m₀ and $m_{\rm gal}$. The fits are shown as solid lines in Fig. 1. For the first epoch (t₁) we fitted the resulting spectral flux distribution with a power law, $F_\nu \propto \nu^{\beta}$, and find $\beta = -2.71 \pm 0.12$.