TITLE:   GCN CIRCULAR
NUMBER:  31415
SUBJECT: GRB 220107A: Swift/UVOT Detection
DATE:    22/01/09 00:24:42 GMT
FROM:    Mike Siegel at PSU/Swift MOC  <mhs18@psu.edu>

M. H. Siegel (PSU) reports on behalf of the Swift UVOT team:

The Swift/UVOT began settled observations of the field of GRB 220107A 26.7 ks after
the Fermi detection (GCN Circ. 31399). An uncatalogued fading source consistent with 
the GUANO localization (DeLaunay et al., GCN Circ. 31402) and the XRT position
(Burrows et al., GCN Circ. 31410) is detected in the initial UVOT exposures.
This appears consistent with the optical transient reported by Hosokawa et al.
(GCN Circ. 31413).

The preliminary UVOT position is:
    RA  (J2000) =  11:19:13.71 =  169.80713 (deg.)
    Dec (J2000) = +34:10:14.5  =  34.17070 (deg.)
with an estimated uncertainty of 0.4 arc sec. (radius, 90% confidence).
 
Preliminary detections and 3-sigma upper limits using the UVOT photometric
system (Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the early exposures
are:

Filter        T_start(ks)  T_stop(ks)      Exp(s)           Mag

white             26.7         28.5         1834         20.16 +/- 0.08
white             32.4         33.0         2708         20.35 +/- 0.07
white             37.6         39.4          247         20.43 +/- 0.24

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.056 in the direction of the burst
(Schlegel et al. 1998).