GCN Circular 22221

N. P. M. Kuin (UCL-MSSL) and and S. J. LaPorte (PSU) report on
behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 171209A
107 s after the BAT trigger (LaPorte et al., GCN Circ. 22213
Not found
 
 
).
The proposed possible transient position was confirmed by independent
observations of the LCO telescopes in Siding Springs in the SDSS r and
i filters (Guidorzi et al, GCN Corc. 22214) and by the XRT (d'Avanzo
et al. GCN Circ. 22218
Not found
 
 
).

Due to the nearby star (at 3.7"), the count rates were determined using
a smaller than normal aperture of 2.5 arcsec, and for the reported
magnitudes no aperture correction has been applied.
The brightness is seen to rise to 16th magnitude around 400s past the
trigger at which time there is a break in the UVOT observations until
4000s past the trigger at which time the source is seen to be decaying.
After binning the white filter data in 10s bins, half a magnitude large
variability is seen up to 175s after the trigger.
There is no measurable flux in the uvw1,uvm2, and uvw1 filters
which were observed after the gap, which suggests a redshifted source
with a redshift in the range z = 1.8 - 2.8.

Preliminary 3-sigma upper limits using the UVOT photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) are:

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

white              175          185          541         17.4
v                 4630         6265          393         17.3
b                 4014         5649          393         18.2
u                  320          350           30         16.5
w1                5039         6480          201         >19.6
m2                4834         6470          393         >20.2
w2                4425         6060          393         >20.1

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.15 in the direction of the burst
(Schlegel et al. 1998).