TITLE: GCN GRB OBSERVATION REPORT NUMBER: 233 SUBJECT: GRB990123, Optical BVRI Observations DATE: 99/01/27 19:24:25 GMT FROM: Elena Pian at ITESRE-CNR,Bologna GRB 990123 BVRI Photometry N. Masetti, E. Palazzi, E. Pian, F. Frontera (ITESRE-CNR, Bologna), C. Bartolini, A. Guarnieri, A. Piccioni (Astronomy Dept., Univ. Bologna), G. Valentini (Teramo Astr. Obs.) and E. Costa (IAS-CNR) report: "We have imaged the field of GRB990123 with the Bologna Astronomical Observatory 1.5m telescope equipped with BFOSC, using Johnson B and V, and Cousins R and I filters in January 24-26. Calibrations were done referring to GCN 206 for the B image, to GCN 204 for the R image, and using the standard star BD+252534 (Taylor et al. 1989, AJ 97, 1798) for the V and I images. In the appended table the preliminary magnitudes of the optical transient (GCN 201) are reported along with the observation dates. Power-law fits to the R and V band data yield temporal decay indices of alpha = 1.20 +- 0.08 and alpha = 1.25 +- 0.16, respectively (errors are 1 sigma), with satisfactory chi square values. No indication is found of a flattening of the light curve due to an underlying galaxy. Suggestion of a slight flux increase is seen on January 25 in both R and V bands, as opposed to the overall fading trend. This is however not significant. The BVRI spectral continuum of the optical transient in the first night (average time of Jan 24.16), dereddened with E(B-V) = 0.016 (GCN 207), has a power-law shape (f_nu = k*nu^{-beta}) with best-fit slope beta = 1.1 +- 0.2 (1 sigma). The BVRI light curves and the radio-to-X-ray spectral energy distribution of the transient are also shown at the Web site http://tonno.tesre.bo.cnr.it/~masetti/grb990123.html Multiwavelength observations, particularly in the near-infrared and millimetric bands, are needed and urged. Date (UT) mag error band ----------------------------------- 1998 Jan 24.122 19.36 0.20 I 24.142 19.79 0.10 R 24.164 19.97 0.16 V 24.194 20.64 0.07 B 24.216 19.92 0.10 R 25.138 20.91 0.10 R 25.159 20.77 0.10 R 25.181 21.01 0.20 V 25.203 20.93 0.20 V 26.154 21.77 0.16 V 26.154 21.56 0.10 R This message can be cited."