TITLE:   GCN CIRCULAR
NUMBER:  21846
SUBJECT: GRB 170906B: Swift/UVOT Upper Limits
DATE:    17/09/07 15:06:30 GMT
FROM:    Mike Siegel at PSU/Swift MOC  <siegel@swift.psu.edu>

M. H. Siegel (PSU) and B. Sbarufatti (PSU)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 170906B
81 s after the BAT trigger (Sbarufatti et al., GCN Circ. 21822).
No optical afterglow consistent with the XRT position
(Osborne et al. GCN Circ. 21823)
is detected in the initial UVOT exposures.
Preliminary 3-sigma upper limits using the UVOT photometric system
(Breeveld et al. 2011, AIP Conf. Proc. 1358, 373) for the first
finding chart (FC) exposure and subsequent exposures are:

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

white_FC            81          231          147         >20.2
u_FC               293          543          246         >19.3
white               81          930          243         >20.6
v                  623          815           39         >17.5
b                  548          741           39         >18.7
u                  293          716          265         >19.3
w1                 672          865           39         >18.4

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.42 in the direction of the burst
(Schlegel et al. 1998).