TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  477
SUBJECT: GRB 991216 Optical Observations
DATE:    99/12/17 22:30:51 GMT
FROM:    Alan H. Diercks at Caltech  <ad@astro.caltech.edu>

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.