GCN Circular 37123

S.P.R. Shilling (Lancaster U.) and P. A. Evans (U. Leicester)
report on behalf of the Swift/UVOT team:

The Swift/UVOT began settled observations of the field of GRB 240809A
98 s after the BAT trigger (Evans et al., GCN Circ. 37110
Not found
 
 
).
A source consistent with the enhanced XRT position
(Osborne et al., GCN Circ. 37115
Not found
 
 
) is detected in the initial UVOT exposures.

The preliminary UVOT position is:
    RA  (J2000) =  15:50:10.52 = 237.54385 (deg.)
    Dec (J2000) = -02:19:03.0  =  -2.31751 (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               98          248          147         14.42 +/- 0.02
v                  640          660           19         16.11 +/- 0.14
b                  565          585           19         16.28 +/- 0.09
u                  310          560          246         15.13 +/- 0.03
w1                 689          709           19         16.19 +/- 0.17
m2                 665         1263           78         17.31 +/- 0.18
w2                 616         1387           97         18.33 +/- 0.24


The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.249 in the direction of the burst
(Schlegel et al. 1998).