TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  3887
SUBJECT: GRB050826: Swift/UVOT early upper limits
DATE:    05/08/26 20:18:52 GMT
FROM:    Alexander Blustin at MSSL-UCL  <ajb@mssl.ucl.ac.uk>

A. J. Blustin (UCL-MSSL), V. Mangano (INAF-IASF),
F. Marshall (GSFC), M. Ajello (MPE), M. Trippico (GSFC-SSAI),
N. Gehrels (GSFC) on behalf of the Swift/UVOT team

The Swift/UVOT began observing the field of GRB050826 at
06:19:55 UT, 105 s after the BAT trigger (Mangano et al.,
GCN 3884). No new source with respect to the DSS image was
detected in the 100 s V-band UVOT Image down to a 3-sigma
upper limit (in the XRT error circle: Mangano et al., GCN
3885) of 17.9 mag.

There was also no optical/UV counterpart detected in summed
images from all filters, down to the following 3-sigma magnitude
upper limits:

Filter T_range (s) Exp (s) 3sig UL

V      105-6545    777     19.4
B      251-16450   1857    21.2
U      236-12331   823     19.0
UVW1   223-11570   968     19.7
UVM2   209-10662   968     19.3
UVW2   266-17318   1818    20.3

Where T_range is the time range post-trigger over which the
summed images were accumulated and Exp is the total exposure time
of the summed image. The magnitude upper limits are not corrected
for extinction.

These magnitudes are based on preliminary zero-points, measured
in orbit, and will require refinement with further calibration.