GCN Circular 182

GRB 981226: Optical Observations

J. S. Bloom, R. R. Gal, and J. Meltzer (Caltech), on behalf of the
Caltech/NRAO GRB collaboration, report:

"On 27.1 and 28.1 December 1998 UT we obtained optical images of the X-ray
localization (GCN #174; GCN #178) of GRB 981226 at the Palomar 60-inch.
Total integration time was 1800-s and 3450-s with an approximate gunn-i
object detection threshold of 23.5 and 24.3 (on 27.1 Dec, 28.1 Dec,
respectively).  An astrometric plate solution was obtained by comparing
field stars to that from USNO-A2 Catalogue; the rms fit in either axes was
found to be 0.18 arcsec.  We assumed the standard zero-point for the
instrument; for comparison, this zero-point gives gunn-i = 20.47 for the
star at ra: 23:29:43.5, dec: -23:54:13.8 (J2000).

The J-band source reported by Castro-Tirado et al. (GCN #173) is not
detected at either epoch, implying a J - i of greater than about 3.5 mag
on 27.1 Dec UT.  The source reported by Galama et al. (GCN #172) did not
vary significantly (delta m = 0.12 mag, consistent within the photometric
error of r.m.s. = 0.1 mag) and is not likely to be the optical transient
of this GRB.  These statements are consistent with those by Rhoads et al.
(GCN #181).
 
We do find that the object at ra: 23:29:27.33, dec: -23:56:53.7 (+/- 0.18
arcsec), reported as possibly variable by Wozniak (GCN #177), to have
faded by 0.46 (+/- 0.09 rms) mag between the two epochs.  Our preliminary
photometry gives i=22.9 +/- 0.2 and i=23.4 +/- 0.2 on the first and second
epochs, respectively.  We note that this source is well-outside (~1.5
arcmin) the formal error box of the X-ray NFI position (GCN #178), so its
connection with GRB 981226 remains to be established.  If indeed this
source is the transient afterglow of the GRB, then the measured time decay
constant is alpha = -0.5; this is shallower than those found in other
afterglow but can be reconciled if the underlying galaxy host is of
comparable brightness to the transient.

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