TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3180 SUBJECT: Swift-BAT detection of GRB 050406 DATE: 05/04/06 17:04:17 GMT FROM: Ann M. Parsons at NASA/GSFC/Swift A. Parsons (GSFC), S. Barthelmy (GSFC), J. Cummings (GSFC/NRC), E. Fenimore (LANL), N. Gehrels (GSFC), D. Hullinger (UMD), S. Hunsberger (PSU), H. Krimm (GSFC/USRA), C. Markwardt (GSFC/UMD), D. Palmer (LANL), T. Sakamoto (GSFC/NRC), G. Sato (ISAS), M. Suzuki (Saitama), J. Tueller (GSFC) D. Burrows, J. Kennea, J. Nousek (PSU), G. Chincarini, P. Romano (INAF-OAB) on behalf of the Swift team: At 15:58:48.40 UT Swift-BAT triggered on burst GRB 050406 (trigger 113872). The BAT-derived position is RA,Dec= 34.428, -50.178 (J2000). Significant emission has been detected for at least 3 sec with possible low significance peaks over an additional 30 sec. The peak count rate was 800 cnts/sec. The source was detected at 15 degrees off the bore-site and thus is in the fully-coded field-of-view. Swift slewed promptly and the UVOT imaged the field at the end of the slew and did not find a bright optical source in the 8 arcmin square binned field of view centered on the BAT position. The XRT also imaged the field promptly and did not find a bright X-ray source within the field of view. Based on previous experience, this lack of prompt x-ray afterglow suggests either that this trigger is not a GRB, or that it is a very interesting and unusual GRB. The BAT rate trigger and image are significant enough to lean toward an interpretation of an unusual GRB.