TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  171
SUBJECT: GRB 981220
DATE:    98/12/27 18:20:07 GMT
FROM:    Fredrick J. Vrba at USNO  <fjv@nofs.navy.mil>

The U.S. Naval Observatory GRB team (F. J. Vrba, C. B. Luginbuhl,
A. A. Henden, S. E. Levine, J. Munn, B. Canzian, H. H. Guetter), 
D. H. Hartmann (Clemson Univ.), and M. C. Jennings (IGPP, UCR visitor) 
report the results of photometric reductions of the I-band observations of 
of GRB 981220 taken at the U. S. Naval Observatory 1.55-m telescope.
(See preliminary reports and details of observations in GCN 164 and 166.)
One hour integrations on two consecutive nights were centered in time
at UT 1998 December 23.280 and 24.266 and reached limiting magnitudes on
each night of I = 24.5 for photometric errors of < 0.20 mag. Standardization
was accomplished by observation of two nearby Landolt standards and assuming
a mean extinction coefficient for the site, adding about 0.008 mag error
to the total error budget (details can be obtained by contacting 
the authors at fjv@nofs.navy.mil). Photometry was obtained for all objects in 
an area centered on the intersection of the 5.0-arcmin wide (90% confidence) 
RXTE localization (GCN 159) and the 2.4-arcmin wide (99% confidence) IPN 
annulus (GCN 160), but with dimensions 3.4-arcmin by 10.5-arcmin in order 
to be confident of covering the actual GRB location. Photometry for a total 
of 267 objects brighter than I = 24.5 was obtained, of which approximately 
100 objects lie within the formal RXTE/IPN localization intersection. 
No object was found to vary by more than 2-sigma of its standard error of
unit weight (approximately 0.01 mag for I < 21.0; 0.05 mag for I = 23.0,
0.15 mag for I = 24.0). Specifically, the object suggested as a possible
counterpart and marked "1" on the ARC 3.5-m image taken by Dierks and Deutsch
(http://www.astro.washington.edu/deutsch/grb/grb981220/) had I-band magnitudes
of 21.674 +/- 0.020 and 21.667 +/- 0.020 on UT December 23 and 24, respectively. 
We find no counterpart to the detection limit of our frames estimated to be
I = 25 for the radio source reported in GCN 168 and suggested to be the radio
afterglow in GCN 170.
  
This GCN note can be cited.