GCN Circular 477
GRB 991216 Optical Observations A. Diercks (Caltech), L. Ferrarese (UCLA), J. S. Bloom (Caltech) report on behalf of the Caltech-NRAO-CARA GRB collaboration: "We imaged the field of GRB 991216 in Gunn-i on UT Dec. 17.214 and Dec. 17.462 UT using the Palomar 200-inch with the COSMIC imager under photometric conditions. At each epoch we took three exposures of 300s each centered on the RXTE-PCA error circle (Takeshima et al. 1999). The OT identified by Halpern (GCN #472) is well detected in each image and faded 0.48 +/- 0.04 mag between epochs. This corresponds to a power-law decay index of alpha = -1.2 +/- 0.1. Based on observations of Landolt standards in SA 94 and conversion to Gunn-i magnitudes (Wade et al., PASP 91:35), we estimate the brightness at i = 19.1 +/- 0.2 on Dec. 17.214 UT and i = 19.6 +/- 0.2 on Dec. 17.463 UT. Refinement of these absolute magnitudes will be possible as soon as better absolute calibrations are available. We note that the dust maps of Schlegal,Finkbinder, and Davis (ApJ, 500, 525) give E(B-V) = 0.634 at this position, (l,b) = (190.44,-16.63), corresponding to A(Gunn-i) = 1.3 for extra-galactic sources." This message may be cited.