TITLE: GCN GRB OBSERVATION REPORT NUMBER: 2991 SUBJECT: GRB 050128: Swift XRT prompt afterglow position DATE: 05/01/28 09:17:28 GMT FROM: David Burrows at PSU/Swift L. A. Antonelli (INAF-OAR), S. Campana, G. Tagliaferri, C. Pagani, A. Moretti, P. Romano, G. Chincarini (OAB), G. Cusumano, V. Mangano (ISAF/Palermo), P. Giommi (ASI), D. N. Burrows, J. E. Hill, J. A. Kennea, J. Racusin, J. A. Nousek (PSU), J. Osborne, K. Page, M. Goad, A. Beardmore, A. Wells (U. Leicester), K. Mason (MSSL), N. White, N. Gehrels (NASA/GSFC), report on behalf of the Swift XRT team: The Swift BAT instrument detected an apparent burst at 04:20:04 UT on 2005 January 28. The spacecraft initiated a prompt automated slew and observation sequence. The XRT began taking data on the burst at 2005-01-28 04:23:51 and continued until 2005-01-28 05:55:27. The XRT was in Photon Counting mode performing calibration observations of PG1311+129 when the burst occurred, and continued to collect data in Photon Counting mode during the slew and after settling onto the BAT field. As a result, no prompt XRT position was reported to the ground. However, analysis of the XRT data shows a bright, rapidly fading point source at: RA(J2000): 14 38 17.87 DEC(J2000): -34 45 56.6 These positions were processed through the ground pipeline and have an estimated uncertainty of 8 arcseconds, dominated by systematic uncertainties in the XRT boresight, which is not yet fully calibrated. We note, however, that our final position for GRB050126 was 30 arcseconds from the optical candidate (GCN 2985) in spite of our estimated uncertainty of 8 arcseconds. The XRT count rate is 1.29+/-0.28 c/s (0.5-10 keV) in a 1745 s exposure, which corresponds roughly to a 1 mCrab source. The XRT is continuing to observe this burst when it is visible to the Swift observatory. [GCN OPS NOTE (28jan05): Per D.Burrows request, the authors G. Cusumano, V. Mangano were added to the list.]