TITLE: GCN GRB OBSERVATION REPORT NUMBER: 320 SUBJECT: GRB 990510 Optical Observations DATE: 99/05/12 14:43:50 GMT FROM: Jens Hjorth at U.Copenhagen GRB 990510 Optical Observations J. Hjorth (Copenhagen), I. Burud (Liege), A. Pizzella (ESO Chile), H. Pedersen (Copenhagen), A. O. Jaunsen (Oslo), B. Lindgren (Copenhagen) report: "Thirty-one 300-sec R-band images of the optical transient (OT) related to GRB 990510 (GCN #310, 312, 313) were obtained with the DFOSC at the Danish 1.54-m telescope on La Silla between 11.14 May and 11.37 May 1999 UT. The seeing FWHM ranged from 1.2" to 1.6". An astrometric plate solution (USNO-A2.0) yielded the OT coordinates RA = 13:38:07.11, Dec = -80:29:48.2 (J2000) with an uncertainty of +- 0.44". Photometry was carried out using the deconvolution code described by Magain, Courbin & Sohy (ApJ, 494, 452, 1998). Simultaneous deconvolution of the 31 images resulted in a light curve with an rms scatter of 0.016 mag. The lightcurve is consistent with a power law decay with index -1.27 in this small interval. The combined lightcurve including data from Axelrod et al. (GCN #315), Galama et al. (GCN #313) and Pietrzynski & Udalski (GCN #316) is inconsistent with a single power law. Our results thus confirm the steepening of the light curve noted by Stanek et al. (GCN #318). The faint object 2.05" south and 0.56" east of the OT is clearly seen on the deconvolved image. The object does not appear extended towards the OT. Images, deconvolved images and light curves are posted at http://www.astro.ku.dk/~jens/grb990510/ ."