TITLE:   GCN GRB OBSERVATION REPORT
NUMBER:  3186
SUBJECT: Swift-UVOT Optical Counterpart Candidate to XRF 050406
DATE:    05/04/07 01:19:53 GMT
FROM:    Pete Roming at PSU  <roming@astro.psu.edu>

E. Rol (University Leicester), P. Schady (MSSL/PSU), S. Hunsberger (PSU), A. 
Breeveld (MSSL), P. Boyd (GSFC/UMBC), C. Kouveliotou (MSFC), P. Roming (PSU),
J. Nousek (PSU), N. Gehrels (GSFC), on behalf of the Swift UVOT team.

Starting with data taken 88 s after the BAT trigger (Parsons et al; GCN 3180),
we have co-added the early time data of GRB 050406 in each of the Swift
Ultra-Violet/Optical Telescope (UVOT) U, B, and V bands. We detect a source
within the XRT error circle (M. Capalbi et al; GCN 3184) at the 4.3 (19.0
magnitude), 3.0, and 2.5-sigma detection levels in the U, B, and V
respectively. We do not detect a source in a second deeper U band observation
taken 1.3 hours after the burst. The source is located at RA = 02:17:52.2, DEC
= -50:11:15.8. The UVOT source location and fading is consistent with the
Magellan candidate by Berger et al (GCN 3185) strongly suggesting that this is
the optical counterpart of XRF 050406. More detailed analysis of the UVOT data
is underway.