GCN Circular 9074

P. Schady (MSSL-UCL) and S.R.Oates (MSSL-UCL) reports on behalf of the 
Swift/UVOT team:

Swift UVOT began settled exposures of GRB090401B (Schady et al, GCN 9066
Not found
 
 
) 
82 s after the burst trigger, and detected a bright optical transient at 
the position reported in by Schady et al. (GCN 9067
Not found
 
 
), consistent with the 
enhanced XRT error circle (Osborne et al, GCN 9070
Not found
 
 
).

The source is detected in the white, v, b and u filter, but not in any of 
UV filters, placing an upper limit on the redshift of z < 3. We note that 
the afterglow lies close to a known source, which may be contaminating our 
phometry. The light curve decays steeply at early times at a rate of 
alpha_1~1.5, and then breaks at ~T+440s to a slope of alpha_2~0.7.

Using 2.5arcsec aperture, the magnitudes and 3 sigma upper
limits to our observations our reported below:

Filt    T_start (s)  T_stop (s)  Exp (s)  Mag/3-sig UL
white      82           232       147     16.83+/-0.02
white      874          1024      147     19.14+/-0.08
v          623          1248      78      18.31+/-0.20
v          4753         4953      197     19.10+/-0.25
b          549          1346      58      19.48+/-0.26
b          5573         5773      197     20.39+/-0.28
u          293          543       246     17.99+/-0.07
u          698          1312      58      18.67+/-0.22
uvw1       672          1297      78        > 19.42
uvm2       647          1272      78        > 18.99
uvw2       599          1387      78        > 19.39

The above magnitudes have not been corrected for the Galactic extinction 
corresponding to E(B-V) = 0.67 (Schlegel et al 1998). The photometry is on 
the UVOT flight system described in Poole et al (2008, MNRAS, 383,627)