TITLE: GCN CIRCULAR NUMBER: 17679 SUBJECT: Swift triggered on a source in Terzan 5 (trigger=637212) DATE: 15/04/05 18:45:15 GMT FROM: Scott Barthelmy at NASA/GSFC S. D. Barthelmy (GSFC), W. H. Baumgartner (GSFC/UMBC), V. D'Elia (ASDC), L. Izzo (URoma/ICRA), J. A. Kennea (PSU) and K. L. Page (U Leicester) report on behalf of the Swift Team: At 18:15:06 UT, the Swift Burst Alert Telescope (BAT) triggered and located a source within Terzan 5 (trigger=637212). Swift slewed immediately to the source. The BAT on-board calculated location is RA, Dec 267.000, -24.772 which is RA(J2000) = 17h 48m 00s Dec(J2000) = -24d 46' 18" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). As is typical for image trigger, there is nothing in the real-time TDRSS BAT light curve. The XRT began observing the field at 18:24:47.4 UT, 580.8 seconds after the BAT trigger. XRT found a bright X-ray source located at RA, Dec 267.0200, -24.7801 which is equivalent to: RA(J2000) = +17h 48m 4.80s Dec(J2000) = -24d 46' 48.4" with an uncertainty of 5.4 arcseconds (radius, 90% containment). This location is 71 arcseconds from the BAT onboard position, within the BAT error circle. This position is 1.44 arcseconds from a known X-ray source EXO 1745-248. No event data are yet available to determine the column density using X-ray spectroscopy. The initial flux in the 2.5 s image was 1.34e-09 erg cm^-2 s^-1 (0.2-10 keV). UVOT took a finding chart exposure of 150 seconds with the White filter starting 589 seconds after the BAT trigger. No credible afterglow candidate has been found in the initial data products. The 2.7'x2.7' sub-image covers 100% of the XRT error circle. The typical 3-sigma upper limit has been about 19.6 mag. The 8'x8' region for the list of sources generated on-board covers 100% of the XRT error circle. Because of the density of catalogued stars, further analysis is required to report an upper limit for any afterglow in the region. No correction has been made for the large, but uncertain extinction expected. We note that the source EXO 1745-248 within Terzan 5 is currently in outburst, so we suggest this may be the cause of the trigger.