GCN Circular 3923

A. Cucchiara (PSU), J. Cummings (GSFC), S. Holland (GSFC), C. Gronwall 
(PSU),
A. Blustin (MSSL), F. Marshall (GCFS), A. Smale (NASA HQ), L. Cominsky 
(Sonoma State U.),
N. Gehrels (GSFC) on behalf of the Swift UVOT team

Swift/UVOT began observing the field of GRB050904 at 01:54:25 UT,
164 s after the BAT trigger (Cummings, et al., GCN 3910
Not found
 
 
) in all the six 
filters.
In the first 100 s V-band image (T+ 214 s), no new source is detected with
respect to the DSS down to a 3 sigma upper limit of 18.9 mag in the XRT 
error
circle (GCN 3910
Not found
 
 
).
At T+90 min after the trigger, in a single 900 s V-band image the upper 
limit
was 20.1.

The instrument continued to collect data during all the other orbits
detecting no new source down to the following upper limits:

Filter  Start_Time(UT)  Stop_Time(UT)  Exp(s) 3sig_limit
 
V         01:54          16:20         4887    20.9
B         01:56          18:12         4773    21.9
U         01:56          17:57         4686    21.6
UVW1      01:56          16:49         4292    21.1
UVM2      01:56          16:35         4684    21.4
UVW2      01:57          18:26         4331    21.4

Where Start_Time and Stop_Time are the time range over wich the summed 
images
were accumulated and Exp is the total exposure time (in seconds) of the 
summed image.
The magnitude upper limits are not corrected for extinction.
These non-detections are consistent with a high redshift GRB,
as reported by J. Haislip, et al. (GCN 3914
Not found
 
 
 and 3919
Not found
 
 
).
These magnitudes are based on preliminary zero-points, measured
in orbit, and will require refinement with further calibration.

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