GCN Circular 12222

S. R. Oates (MSSL-UCL) reports on behalf of the Swift-UVOT team

The Swift/UVOT began settled observations of the field of GRB 110731A 75s 
after the trigger (Oates et al., GCN Circ. 12215
Not found
 
 
). We detect a fading 
source in the white, v, b and u filters at a refined position of 
RA(J2000), DEC(J2000)= 280.50413 deg, -28.537167 deg. This is equivalent to:

    RA (J2000)  18:42:00.99
   Dec (J2000) -28:32:13.8

with an estimated uncertainty of 0.5 arcsec (radius, 90% confidence).
This is consistent with the UVOT position reported in Oates et al. 
(GCN Circ. 12215
Not found
 
 
) and with the enhanced position of the X-ray afterglow 
(Beardmore et al., GCN Circ. 12219
Not found
 
 
). The non-detection in the uv filters is 
consistent with a redshift of between 2 and 3.

Preliminary magnitudes using the UVOT photometric system (Poole et al. 2008, 
MNRAS, 383, 627) for the finding chart (FC) and summed exposures at the 
location of the optical afterglow are:

Filter  T_start(s)  T_stop(s)   Exp(s)       Mag
#####################################################
wh (FC)     75         225       147    15.85+/-0.02
wh          567        587       19     17.64+/-0.11
v           790        810       19     16.90+/-0.23
b           542        562       19     17.87+/-0.23 
u (FC) 	    287        537       246    17.71+/-0.08
u           1118       1138      19     18.37+/-0.44
uvw1        666        1286      78     >18.88   
uvm2        642        1261      78     >18.77   
uvw2        592        1385      97     >19.24   
#####################################################

We note that photometry is difficult due to the field being crowded and the presence of a faint 
contaminating source in the source region. The values quoted above are not 
corrected for the non-negliable Galactic extinction due to the reddening 
of E(B-V) = 0.18 in the direction of the burst (Schlegel et al. 1998).