GCN Circular 194
F. J. Vrba reports, on behalf of the U.S. Naval Observatory GRB team, that an error was made in the calibration of GRB 981220 photometry reported in GCN 171. An incorrect exposure time was used with the result that all photometry reported in GCN 171 should be brightened by 1.945 magnitudes. For convenience, we provide a corrected version below and apologize for any confusion that may have resulted. We 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 = 22.6 for photometric errors of < 0.20 mag. Standardization was accomplished by observation of two nearby Landolt fields comprising nine standard stars 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 email@example.com). 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 = 22.6 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 < 19.1; 0.05 mag for I = 21.1, 0.15 mag for I = 22.1). 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 19.729 +/- 0.020 and 19.722 +/- 0.020 on UT December 23 and 24, respectively. We find no counterpart to the detection limit of our frames estimated to be I = 23.1 for the radio source reported in GCN 168 and suggested to be the radio afterglow in GCN 170. This GCN note can be cited.