{
  "bibcode": "1999GCN...194....1V",
  "body": "F. J. Vrba reports, on behalf of the U.S. Naval Observatory GRB\nteam, that an error was made in the calibration of GRB 981220 photometry \nreported in GCN 171. An incorrect exposure time was used with the\nresult that all photometry reported in GCN 171 should be\nbrightened by 1.945 magnitudes. For convenience, we provide a corrected\nversion below and apologize for any confusion that may have resulted. \n\nWe report the results of photometric reductions of the I-band observations of \nof GRB 981220 taken at the U. S. Naval Observatory 1.55-m telescope.\n(See preliminary reports and details of observations in GCN 164 and 166.)\nOne hour integrations on two consecutive nights were centered in time\nat UT 1998 December 23.280 and 24.266 and reached limiting magnitudes on\neach night of I = 22.6 for photometric errors of < 0.20 mag. Standardization\nwas accomplished by observation of two nearby Landolt fields comprising\nnine standard stars and assuming a mean extinction coefficient for the site, \nadding about 0.008 mag error to the total error budget (details can be obtained \nby contacting the authors at fjv@nofs.navy.mil). Photometry was obtained for \nall objects in an area centered on the intersection of the 5.0-arcmin wide \n(90% confidence) RXTE localization (GCN 159) and the 2.4-arcmin wide (99% confidence) \nIPN annulus (GCN 160), but with dimensions 3.4-arcmin by 10.5-arcmin in order \nto be confident of covering the actual GRB location. Photometry for a total \nof 267 objects brighter than I = 22.6 was obtained, of which approximately \n100 objects lie within the formal RXTE/IPN localization intersection. \nNo object was found to vary by more than 2-sigma of its standard error of\nunit weight (approximately 0.01 mag for I < 19.1; 0.05 mag for I = 21.1,\n0.15 mag for I = 22.1). Specifically, the object suggested as a possible\ncounterpart and marked \"1\" on the ARC 3.5-m image taken by Dierks and Deutsch\n(http://www.astro.washington.edu/deutsch/grb/grb981220/) had I-band magnitudes\nof 19.729 +/- 0.020 and 19.722 +/- 0.020 on UT December 23 and 24, respectively. \nWe find no counterpart to the detection limit of our frames estimated to be\nI = 23.1 for the radio source reported in GCN 168 and suggested to be the radio\nafterglow in GCN 170.\n  \nThis GCN note can be cited.",
  "circularId": 194,
  "createdOn": 916345801000,
  "email": "fjv@nofs.navy.mil",
  "subject": "GRB981220",
  "submitter": "Fredrick J. Vrba at USNO  <fjv@nofs.navy.mil>",
  "eventId": "GRB 981220"
}