{
  "bibcode": "1999GCN...196....1B",
  "body": "Optical Observations of GRB 981220\n\nJ. S. Bloom, S. G. Djorgovski, S. R. Kulkarni (CIT), J. Brauher (IPAC),\nD. A. Frail (NRAO), R. Goodrich, F. Chaffee (CARA) report on behalf of\nthe Caltech-NRAO-CARA GRB Collaboration:\n\n\"On 14 January 1999 UT we imaged the region of the radio transient\n(GCN #168; GCN #170) of GRB 981220 (GCN #159; GCN #160) with the Keck\nII 10-m telescope.  Total integration time in R-band was 1300-s in\n~0.6 arcsec seeing (FWHM).  An astrometric plate solution was obtained\non the stacked image using the USNO-A2.0 catalogue.  The r.m.s. errors\nof the 26 tie stars surrounding the radio position was 0.24 arcsec (in\nra) and 0.18 arcsec (in dec).  A Landolt standard field (PG0231+051)\nwas also observed for photometric calibration.\n\nOur Keck images can be found at the web location,\nhttp://astro.caltech.edu/~jsb/GRB/grb981220.html.  There are two\nsources in the vicinity of the radio transient, 'J' and 'K'. Source\n'J' coincides with the radio position as reported earlier (GCN #176).\nOn Jan 14.30, the magnitudes of source 'J' and 'K' are respectively\n26.4 (+/- 0.3 statistical; +/- 0.4 zero-point error) and R = 25.6 (+/-\n0.2 statistical; +/- 0.4 zero-point error); the zero-point error\nincludes uncertainties in the local sky level determination, aperture\ncorrection, and the photometric zero-point of the standard stars.\n\nWe note that on Dec 23.4 1998 (GCN #176) both 'J' and 'K' have\ncomparable brightness whereas 'J' is clearly fainter than 'K' in the\nJan 14.30 image.  We conclude that the optical source coincident with\nthe radio transient position has faded by ~0.8 mag between the two\nepochs. Since the inferred decay is shallower than that found in other\nGRB afterglow, we suggest that the first epoch flux measure of the\noptical transient (GCN #176) was partly contaminated by the host\ngalaxy.\n\nMasetti et al. (GCN #179) had claimed that the source at the radio\nposition had a 4-arcsec extension. As can be seen from our images, the\nextension is likely an artifact resulting from poorer seeing in the\nB-band image of Masetti et al.\n\nWe conclude that source 'J' is the host galaxy of GRB 981220 and it is\nnot unusually large but has typical angular size of other host\ngalaxies.\n\nThis message can be cited.\"",
  "circularId": 196,
  "createdOn": 916859753000,
  "email": "jsb@astro.caltech.edu",
  "subject": "Optical Observations of GRB 981220",
  "submitter": "Josh Bloom at CIT  <jsb@astro.caltech.edu>",
  "eventId": "GRB 981220"
}