TITLE: GCN GRB OBSERVATION REPORT NUMBER: 3082 SUBJECT: Swift-BAT trigger 107873 (GRB050309): possible X-ray counterparts DATE: 05/03/09 23:09:32 GMT FROM: Scott Barthelmy at NASA/GSFC S. Barthelmy (GSFC), D.Burrows (PSU), L. Barbier (GSFC), J. Cummings (GSFC/NRC), E. Fenimore (LANL), N. Gehrels (GSFC), D. Hullinger (GSFC/UMD), J. Kennea (PSU), H. Krimm (GSFC/USRA), D. Malesani (SISSA), C. Markwardt (GSFC/UMD), A. Moretti (INAF-OAB), K. Page (U. Leicester), D. Palmer (LANL), A. Parsons (GSFC), P. Romano (INAF-OAB), T. Sakamoto (GSFC), G. Sato (ISAS), M. Suzuki (Saitama), J. Tueller (GSFC) on behalf of the Swift-BAT/-XRT teams: Further analysis of Swift data on BAT trigger #107873 has led to an ambiguous situation regarding the reality of this marginal BAT trigger. The Swift-BAT triggered (#107873) at 10:43:21 UT. The trigger occurred on the rising edge of the SAA. Ground analysis has confirmed the very low image-domain detection significance of this trigger. A visual inspection of the mask-tagged light curve also shows very marginal emission. As a result, a retraction GCN Notice was issued earlier today. We are now ammending that Notice. The BAT position was within the Swift Earth limb constraint at the time of the burst, so Swift did not execute a prompt automated slew. The spacecraft executed a delayed slew to the BAT location at about 11:33 UT and XRT observations began at 11:35:54 UT. Two orbits of data were collected by the XRT, for a total of 4157s of exposure. Preliminary analysis indicates that there are four X-ray sources in the XRT field of view, two of which fall inside the BAT error circle. These are: Source #1: RA(J2000) = 12 10 29.1, Dec(J2000) = +77 37 04.8. This is a very weak source (about 0.006 cps). It appears to be roughly constant in intensity, but the source is so faint that we cannot determine with confidence whether or not it is varying in flux. Source #2: RA(J2000) = 12 09 37.2, Dec(J2000) = +77 35 56.5. This source is brighter (about 0.01 cps) and may be fading. Again, we cannot be certain with the data currently in hand. The other two sources are a bright star and a very faint object well outside the BAT error circle. The estimated uncertainty in the XRT positions is 6 arcseconds radius. A 20 ks Target of Opportunity observation has been initiated to provide a second set of data beginning at about 19:45 UT. This should help us determine the likelihood that either XRT source is related to the BAT trigger. In light of these XRT observations, the BAT team cannot rule out the possibility that this trigger represents a threshold-level GRB.