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GCN Circular 476

GRB 991216, optical observations
1999-12-17T22:18:03Z (24 years ago)
Saurabh Jha at CfA <>
S. Jha, R. Kirshner, K. Stanek, P. Garnavich, M. Garcia,
A. Szentgyorgyi (CfA), and J. Tonry (U. Hawaii) report:

We imaged the optical transient reported by Uglesich et al. (GCN 472)
with the U. Hawaii 2.2-m telescope on Dec 17.61 UT. The source is
clearly detected in each of two 300s R-band exposures. Though 
conditions were photometric, the observations were made at high 
airmass (~3), making precise absolute photometry difficult.

Differential photometry with respect to 30 USNO-A2.0 stars in the 
images yields a magnitude estimate for the transient of R = 19.4 +/- 
0.1 (internal error). This is the same magnitude derived by Uglesich 
et al. for their Dec. 17.312 UT observation, but it is possible that 
our two zeropoints are not identical.

With respect to the reference star used by Henden et al. (GCN 473),
we find an R magnitude difference of 4.29 magnitudes. This together
with the data of Henden et al. gives an R-band power-law decay 
index of 1.0 +/- 0.2.

Given the lack of a precise calibration for comparison stars in the
field, we also list the R magnitude difference between the transient
and two stars very near the transient from our Dec 17.61 UT images:

         Star                   Position (J2000)        Transient - Star 
A (USNO-A2.0 975.01388002)  05:09:29.799 +11:17:08.47       +4.12 mag
B (anonymous)               05:09:32.132 +11:17:23.75       -0.04 mag

A finder chart with the proposed comparison stars labelled is available 

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