TITLE:   GCN CIRCULAR
NUMBER:  28343
SUBJECT: GRB 200901B: Swift/UVOT Detection
DATE:    20/09/01 20:17:40 GMT
FROM:    Alice Breeveld at MSSL-UCL  <a.breeveld@ucl.ac.uk>

A. A. Breeveld (UCL-MSSL) and B. Sbarufatti (PSU) report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 200901B 176 s after the BAT trigger 
(Sbarufatti et al., GCN Circ. 28337).
A source consistent with the XRT position (Sbarufatti et al. GCN Circ. 28337)is detected in the 
initial UVOT exposures in the u filter only, but many exposures are smeared, so the magnitudes may 
not be reliable.

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(s)   T_stop(s)      Exp(s)           Mag

v                  482          801           58        >18.1
b                  431          750           58        >19.1
u                  176          426          245         19.3 ± 0.3
w1                 531          851           58        >18.1
m2                 506          826           58        >17.9
w2                 457          776           58        >18.2

The magnitudes in the table are not corrected for the Galactic extinction due to the reddening of 
E(B-V) = 0.09 in the direction of the burst (Schlegel et al. 1998).