TITLE: GCN GRB OBSERVATION REPORT NUMBER: 205 SUBJECT: GRB990123, early optical counterpart detection DATE: 99/01/23 23:15:16 GMT FROM: Carl Akerlof at U.Michigan C. W. Akerlof and T. A. McKay (Univ. of Michigan) report on behalf of the ROTSE collaboration (Michigan/LANL/LLNL): We observed the error box of GRB 990123 provided by the BACODINE Burst Position Notice dated 23-Jan-99 09:46:59 using the ROTSE-I telephoto camera array located at Los Alamos, New Mexico. The first exposure began at 9:47:18.30, 22.18 seconds after the nominal burst trigger time. A rapidly fading object was discovered at the coordinates, RA = 231.3754, DEC = 44.7666 (J2000) which is within 1/3 of a pixel of the optical counterpart reported by Odewahn et al. (GCN #201). The light curve for this object is relatively complex: the luminosity increases by 3 magnitudes between the first and second exposures. Estimated magnitudes for the first six exposures are given below: UTC exposure m_v 9:47:18.3 5 secs. 11.82 9:47:43.5 5 secs. 8.95 9:48:08.8 5 secs. 10.08 9:51:37.5 75 secs. 13.22 9:54:22.8 75 secs. 14.00 9:57:08.1 75 secs. 14.53 Note that the ROTSE-I detector system uses an unfiltered broadband CCD so that magnitude estimates are based on comparisons to catalog values for nearby stars. Sky patrol images of the same coordinates taken 133 minutes earlier showed no evidence of the transient to a limit of at least two magnitudes deeper. A more extensive analysis of this data will be available in the near future. The discovery images will be posted on the ROTSE Web page at: http://www.umich.edu/~rotse/gifs/grb990123/990123.gif This message is quotable in publications. [GCN OP NOTE: This archived copy has had a typo fixed. The original version had the 9:48:08.8 timestamp for the 3rd exposure go out as 9:47:08.8.]