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
Not found
 
 
; GCN #178
Not found
 
 
) 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
Not found
 
 
) 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
Not found
 
 
) 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
Not found
 
 
).
 
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
Not found
 
 
), 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
Not found
 
 
), 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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