TITLE: GCN GRB OBSERVATION REPORT NUMBER: 315 SUBJECT: GRB990510 DATE: 99/05/11 12:56:04 GMT FROM: Brian Schmidt at Australian Nat. U. Tim Axelrod, Jeremy Mould, and Brian Schmidt (Research School of Astronomy and Astrophysics, The Australian National University) report the following optical photometry of GRB990510 using the Mount Stromlo 50inch + Macho Camera (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, cf. Galama et al GCN 313). The following observations are Macho R (effective lambda = 700nm), but transform almost identically to R_c. R mag err May (UT) Telescope 18.24 0.02 10.514 MSO 50inch 18.31 0.02 10.522 MSO 50inch 18.30 0.02 10.529 MSO 50inch 19.23 0.07 10.775 MSO 50inch 19.31 0.07 10.783 MSO 50inch 19.25 0.04 10.791 MSO 50inch The absolute uncertainty in the calibration is not included. The decay is consistent with a power law, with temporal decay index -0.88 +- 0.03. Simultaneous observations in Macho-B (effective lambda = 510nm) show a powerlaw decay of index -0.94 +- 0.04. This message is citeable. [GCN OP NOTE: This circular was received at 20:26 11 May 99 UT, but was delayed in distribution due to not having a prior vetting account.]