GCN Circular 196

Optical Observations of GRB 981220

J. S. Bloom, S. G. Djorgovski, S. R. Kulkarni (CIT), J. Brauher (IPAC),
D. A. Frail (NRAO), R. Goodrich, F. Chaffee (CARA) report on behalf of
the Caltech-NRAO-CARA GRB Collaboration:

"On 14 January 1999 UT we imaged the region of the radio transient
(GCN #168; GCN #170) of GRB 981220 (GCN #159; GCN #160) with the Keck
II 10-m telescope.  Total integration time in R-band was 1300-s in
~0.6 arcsec seeing (FWHM).  An astrometric plate solution was obtained
on the stacked image using the USNO-A2.0 catalogue.  The r.m.s. errors
of the 26 tie stars surrounding the radio position was 0.24 arcsec (in
ra) and 0.18 arcsec (in dec).  A Landolt standard field (PG0231+051)
was also observed for photometric calibration.

Our Keck images can be found at the web location,
http://astro.caltech.edu/~jsb/GRB/grb981220.html.  There are two
sources in the vicinity of the radio transient, 'J' and 'K'. Source
'J' coincides with the radio position as reported earlier (GCN #176).
On Jan 14.30, the magnitudes of source 'J' and 'K' are respectively
26.4 (+/- 0.3 statistical; +/- 0.4 zero-point error) and R = 25.6 (+/-
0.2 statistical; +/- 0.4 zero-point error); the zero-point error
includes uncertainties in the local sky level determination, aperture
correction, and the photometric zero-point of the standard stars.

We note that on Dec 23.4 1998 (GCN #176) both 'J' and 'K' have
comparable brightness whereas 'J' is clearly fainter than 'K' in the
Jan 14.30 image.  We conclude that the optical source coincident with
the radio transient position has faded by ~0.8 mag between the two
epochs. Since the inferred decay is shallower than that found in other
GRB afterglow, we suggest that the first epoch flux measure of the
optical transient (GCN #176) was partly contaminated by the host
galaxy.

Masetti et al. (GCN #179) had claimed that the source at the radio
position had a 4-arcsec extension. As can be seen from our images, the
extension is likely an artifact resulting from poorer seeing in the
B-band image of Masetti et al.

We conclude that source 'J' is the host galaxy of GRB 981220 and it is
not unusually large but has typical angular size of other host
galaxies.

This message can be cited."