TITLE: GCN GRB OBSERVATION REPORT NUMBER: 313 SUBJECT: GRB 990510 optical observations DATE: 99/05/11 02:15:39 GMT FROM: Titus Galama at U.Amsterdam T.J. Galama, P.M. Vreeswijk, E. Rol (U. of Amsterdam), N. Tanvir (Institute of Astronomy, Cambridge, UK), E. Palazzi, E. Pian, N. Masetti, F. Frontera (ITESRE, CNR, Bologna), K. Pollard (U. of Canterbury), J. Menzies (SAAO), P. Sackett (U. of Groningen), K. Sahu (STScI), J. van Paradijs (U. of Amsterdam and U. of Alabama in Huntsville), and C. Kouveliotou (USRA/MSFC) report on behalf of the PLANET microlensing team, and the Bologna and Amsterdam/Huntsville GRB optical follow-up teams: Comparison of European Southern Observatory 2.2m and SAAO Sutherland 1.0m telescope (South Africa) R-band observations, of the error box of GRB 990510 (Dadina et al. 1999; IAU Circular 7160), shows that the proposed optical counterpart to GRB 990510 (Vreeswijk et al., GCN #310) is decaying in brightness. We obtain the following magnitudes (measured with respect to the USNO star at RA = 13:38:00.82, DEC=-80:29:11.7, for which we have assumed R = 17.2): R mag err May (UT) Telescope 19.11 0.02 10.73 SAAO 1m 19.17 0.02 10.76 SAAO 1m 19.25 0.02 10.79 SAAO 1m 19.61 0.01 10.99 ESO 2.2m The absolute uncertainty in the calibration is not included. The decay is consistent with a power law, with temporal decay index -0.85 +- 0.04. The light curve can be found at http://www.astro.uva.nl/~titus/grb990510.html This message is citeable.