TITLE: GCN GRB OBSERVATION REPORT NUMBER: 218 SUBJECT: GRB 990123: New Optical Observations, Decay Measure DATE: 99/01/25 10:34:08 GMT FROM: Josh Bloom at CIT GRB 990123: New Optical Observations, Decay Measure J. S. Bloom, S. R. Kulkarni, S. G. Djorgovski, S. C. Odewahn (CIT), R. Sagar, A. K. Pandey, Neelakshi, R. K. S. Yadav (U. P. State Observatory, India) report on behalf of the Caltech-NRAO-CARA GRB collaboration: "On 24.0 Jan 1999 UT, we imaged the optical transient (GCN #201) of GRB 990123 (GCN #199) with the 1.04-m UPSO telescope, India. In one 50-minute B-band image the transient is well-detected at B = 20.16 +/- 0.15 (Jan 23.958 UT). The transient is also well-detected in a 60-minute stacked R-band exposure at gunn-r = 20.02 +/- 0.11. The B-band image was calibrated to the zeropoint of GCN #206 with a set of secondary stars in the OT field. The R-band photometry was tied the gunn-r system using the Palomar 60-inch photometry (#207). The error associated with the calibration between the two somewhat different bandpasses is small compared to the statistical uncertainties. These measurements have not been corrected for Galactic extinction which would amount to A_B = 0.069 and A_r = 0.041 (GCN #207). With the Galactic extinction correction, the transient still blue (ie. has a positive spectral index beta ~= 0.3) at this second epoch (see GCN #207). Assuming a power-law decay (f_nu[nu] = const*t^alpha) we find alpha_B = -0.90 +/- 0.11 (between Jan 23.58 and Jan 23.95) alpha_r = -1.04 +/- 0.14 (between Jan 23.56 and Jan 24.00) This decay is significantly flatter than alpha = -1.5 found at earlier times (ie. t <= Jan 23.5; GCN #208). A recalibration of the transient magnitude reported by Garnavich et al. (GCN #215) using the reference star from Gal et al. (GCN #207), gives r = 21.0 (errors not reported) on Jan 24.547 UT. This implies a decay of alpha = -1.26, consistent with that found above. An updated light-curve may be obtained at http://astro.caltech.edu/~jsb/GRB/grb990123.html This message may be cited."