TITLE:   GCN CIRCULAR
NUMBER:  3227
SUBJECT: GRB 050408: Early Optical/Ultraviolet Observations with Swift/UVOT
DATE:    05/04/10 22:54:37 GMT
FROM:    Stephen Holland at USRA/NASA/GSFC/SSC  <sholland@milkyway.gsfc.nasa.gov>

S. T. Holland (GSFC-USRA), M. Capalbi (ASDC), A. Morgan, S. Kobayashi,
(PSU), A. Breeveld (MSSL), P. Boyd (GSFC-UMBC), N. Gehrels (GSFC), D.
Hinshaw (GSFC-SPSYS), K. Mason (MSSL), J. Nousek (PSU), and A. Wells
(Leicester) on behalf of the Swift UVOT team report:

     The Swift Ultra-Violet/Optical Telescope (UVOT) observed the field of
GRB 050408 (Sakamoto et al., GCN 3189) starting at 17:07:08 UT on 8 April
2005.  There is a faint source at the location of the optical afterglow
(de Ugarte Postigo et al., GCN 3192) visible in the coadded U-band image.  
This source has U = 21.30 (-0.32,+0.45) mag and the total coadded exposure
time is 2927 s.  This source is at the 3-sigma detection limit of the
coadded image and is only marginally consistent with being a point source.  
There is no detection in the other filters.  The 3-sigma limiting
magnitudes in a 3.5 arcsecond radius circular aperture , and the coadded
exposure times, are: V = 21.3 (3866 s), B = 22.2 (2910 s), UVW1 = 17.8
(3223 s), UVM2 = 17.6 (1989 s), and UVW2 = 17.82 (3402 s).  The magnitudes
reported in this Circular are based upon preliminary flight calibrations.

     The first Swift/UVOT image of the field was a 100 s V-band exposure
centred 45.117 minutes after the BAT trigger.  No source is detected in
this image down to a 3-sigma limiting magnitude of V = 19.9.