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GCN Circular 454

GRB 991208 Optical Decay
1999-12-12T04:19:33Z (25 years ago)
Brian Lindgren Jensen at U.of Copenhagen <>
GRB 991208 Optical Decay

B. L. Jensen, J. Hjorth, H. Pedersen (University of Copenhagen),
H. E. Kristen (CfA), L. Tomassi (Universita di Milano), 
E. Pian (ITESRE, Bologna) and K. Hurley (UCB SSL) report on behalf of a
larger European collaboration:

"We have imaged a significant fraction of the errorbox of GRB 991208
(Hurley & Cline GCN #450) with the 2.5-m Nordic Optical Telescope. Three
5-min R-band exposures were obtained with ALFOSC on 10.27 December 1999
UT and three 3-min R-band exposures were obtained with StanCam on 11.85
December 1999 UT. On both occasions the data were obtained during
twilight, at high (3 - 4) airmass and in bad seeing (2.5" - 3" FWHM). At
the location of the new radio (Frail GCN #451) and optical
(Castro-Tirado et al. GCN #452) source we detect a transient optical
source [at RA(J2000) = 16:33:53.52, Dec = +46:27:21.0  (+- 0.4")], thus
confirming its proposed identification with the afterglow of GRB 991208.
 We have performed PSF photometry on the images using DAOPHOT II/ALLSTAR
and tied our photometric zeropoint to the USNO-A2.0 star 1350-08916561
which has R = 15.8. On this system we find that the magnitudes of the
optical transient were R = 18.7 +- 0.1 on 10.27 Dec and 20.0 +- 0.2 on
11.85 Dec. We caution however that the latter data point may be affected
by systematic uncertainties due to the difficult observing conditions.
If the optical transient follows a power law decay, the decay index is
-2.15 and the predicted magnitude on 12.25 Dec is R = 20.2 +- 0.3. If we
discard the 11.85 Dec data point and use R = 19.1 +- 0.1 on 11.25 Dec
(GCN #452), the decay index is -0.98 and the predicted magnitude on
12.25 Dec is R = 19.4 +- 0.2.
The NOT images will be posted at ."
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