"I revise here the report in GCN 4381 of an optical object at the Swift XRT
position of the possible GRB 051221B (Boyd et al. GCN 4376, Page et al.
GCN 4378, Fenimore et al. GCN 4382), prompted by the non-detections at this
position by Milne (GCN 4420) and Sharapov et al. (GCN 4612). The position
and magnitude given in GCN 4381 were both in error as a result of using an
incorrect entry in the USNO-B1.0 catalog for the quoted comparison star.
The only object that is consistent in position, within 4.5", of the fading
XRT source (Page et al. GCN 4378), is located at
RA(J2000) = 20h 49m 34.86s
Dec(J2000) = +53d 02' 08.3"
It is also faintly visible on the digitized R and I-band POSS-II plates,
and clearly on the image of Sharapov et al.
Since it was not ruled out that this event was a hard X-ray transient,
it is still possible to test that hypothesis by searching for an optical
counterpart. However, there is no evidence in MDM images that the
above object has varied. Observations of it from from 5.5 to 8 hours after
the event using the MDM 1.3m, and again at 30.6 hours using the MDM 2.4m,
are all consistent with its appearance on the POSS, and with measurement
on the CCD images of R = 20.63+/-0.03, referenced to a USNO B1.0 star
at 20h 49m 36.16s, +53d 01' 26.4" having R2 = 17.54. The MDM 2.4m image
further sets a limit of R > 23.8 on any other object in the XRT error circle,
and is shown here:
Recall that Galactic extinction in this direction, (l,b) = (91.2,+5.8),
is considerable. E(B-V) = 1.37 mag, corresponding to A_R = 3.66 mag
(Schlegel et al. 1998).
This message may be cited."