GCN Circular 18122

Paul Kuin (MSSL/UCL) 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 150811A
147 s after the BAT trigger (Evans et al., GCN Circ. 18119
Not found
 
 
). A source
consistent with the XRT position and the optical transient reported
by Butler et al.(GCN Circ. 18120
Not found
 
 
) and Zheng et al. (GCN Circ. 18121
Not found
 
 
)
is detected in the promptly available UVOT exposures.

The preliminary UVOT position is:
    RA  (J2000) =  19:25:21.4
    Dec (J2000) = -15:25:31.1
with an estimated uncertainty of 0.6 arc sec. (radius, 90% confidence).

The transient source is at a distance of 4.6 arcsec of a weak nearby
source.

Preliminary detections for the White and U finding charts 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           147          297          147         20.43 +/- 0.23
u_FC               306          549          241         18.24 +/- 0.11

The magnitudes in the table are not corrected for the Galactic extinction
due to the reddening of E(B-V) = 0.17 in the direction of the burst
(Schlegel et al. 1998).