GCN Circular 26574

N. P. M. Kuin (UCL-MSSL) and S. L. Laha (GSFC/UMBC/CRESST)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 191221B
92 s after the BAT trigger (Laha et al., GCN Circ. 26534
Not found
 
 
).
A source consistent with the XRT position (Beardmore et al. GCN Circ. 26540
Not found
 
 
)
and ALMA position (Laskar et al., GCN Circ. 26564
Not found
 
 
)  was detected in the
initial UVOT exposures.

The preliminary UVOT position is:
    RA  (J2000) =  10:19:19.24 = 154.83017 (deg.)
    Dec (J2000) = -38:09:27.64 = -38.15768 (deg.)
with an estimated uncertainty of 0.42 arc sec. (radius, 90% confidence).

Preliminary detections using the UVOT photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the early exposures
are:

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

white               92          241          147         see below
u                  304          554          246         12.30 +/- 0.02
b                  560          579           20         13.54 +/- 0.04
uvw2               610          630           20         13.20 +/- 0.06
v                  635          655           20         13.42 +/- 0.05
uvm2               659          679           20         12.79 +/- 0.06
uvw1               684          704           20         12.82 +/- 0.05

Aperture photometry shows initially a completely saturated detector,
leading
to wh < 13.8 mag. The first exposure in the white filter (tstart=T0+92s)
shows a readout streak but the readout streak traverses also another
relatively bright star which leads to a saturated readout streak
and precludes using that method of determining the magnitude (Page
et al, 2013 MNRAS 436, 1684).   Analysis continues.

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.08 in the direction of the burst
(Schlegel et al. 1998).