GCN Circular 29436

M. H. Siegel (PSU) and A. Y. Lien (GSFC/UMBC)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 210207B
82 s after the BAT trigger (Lien et al., GCN Circ. 29420
Not found
 
 
).
A source consistent with the XRT position
(Lien et al. GCN Circ. 29420
Not found
 
 
) is detected in the initial UVOT exposures,
as reported in the initial circular and confirmed by ARIES (Kumar et al.,
GCN Circ. 29421
Not found
 
 
), KAIT (Zheng & Filippenko, GCN Circ. 29430
Not found
 
 
) and LCO
(Strausbaugh & Cucchiara, GCN Circ. 29431
Not found
 
 
). We note that the lack
of detection in the two bluer NUV passbands is suggestive of a redshift in
the 1.4-1.9 range.

The preliminary UVOT position is:
    RA  (J2000) =  18:02:32.40 = 270.63500 (deg.)
    Dec (J2000) = +53:40:55.8  =  53.68217 (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 (fc)          83          233          147      13.66+/-1.13
white             4081         5717          393      17.91+/-0.04
white            61938        62307          361      21.06+/-0.32
v                 4492         4692          196      17.61+/-0.10
v                 9433        10341          885      18.54+/-0.09
v                56912        74907         1297     >19.84
b                 3876         5512          393      17.95+/-0.05
b                44686        45593          885     >20.99
b                61025        79012         1770      20.79+/-0.18
u (fc)             295          410          113      14.42+/-0.03
u                 5106         5306          196      17.80+/-0.09
u                43773        44680          885      20.47+/-0.26
u                51795        69087          612     >19.80
uvw1              4901         5101          196      18.23+/-0.18
uvw1             11254        11428          171     >18.90
uvw1             39917        40544          616     >19.72
uvw1             50889        68646         1771     >20.53
uvm2              4697         4896          196     >19.12
uvm2             10346        11246          885     >20.23
uvw2              4287         5871          342     >19.39
uvw2             56005        74346         1771     >20.68

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.041 in the direction of the burst
(Schlegel et al. 1998).

[GCN OPS NOTE:  Per author's request the rerferece to Circ 294311
was changed to the correct value Circ 29431.]