TITLE: GCN CIRCULAR NUMBER: 10722 SUBJECT: An SGR-like burst from the magnetar 1E 1841-04.5 in Kes 73 DATE: 10/05/06 15:30:01 GMT FROM: David Palmer at LANL A. P. Beardmore (U Leicester), D. N. Burrows (PSU), S. Campana (INAF-OAB), S. T. Holland (CRESST/USRA/GSFC), J. A. Kennea (PSU), W.B Landsman (GSFC), K. L. Page (U Leicester), D. M. Palmer (LANL), P. Romano (INAF-IASFPA), M. H. Siegel (PSU), M. Stamatikos (OSU/NASA/GSFC), M. A. Stark (PSU) and R. L. C. Starling (U Leicester) report on behalf of the Swift Team: At 14:37:44 UT, the Swift Burst Alert Telescope (BAT) triggered and located a short soft burst (trigger=421262) consistent with the location of the SNR Kes 73. Because this source was identified on board, it did not reach an automatic merit sufficient to cause a slew. The BAT on-board calculated location is RA, Dec 280.331, -4.921 which is RA(J2000) = 18h 41m 19s Dec(J2000) = -04d 55' 15" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). The BAT light curve shows a single high bin at the 64 ms timescale, with an effective count rate of 4000 count/s 0.1 s after the nominal trigger time. The count rate during the 16 ms trigger interval averaged 12000 counts/s. When it was realized that this SNR contains a known magnetar AXP, and that this burst is consistent with Soft Gamma Repeater activity, we manually upgraded this target to ensure slewing. The XRT began observing the field at 15:09:15.9 UT, 1891.3 seconds after the BAT trigger. Using promptly downlinked data we find an X-ray source located at RA, Dec 280.33059, -4.93731 which is equivalent to: RA(J2000) = 18h 41m 19.34s Dec(J2000) = -04d 56' 14.3" with an uncertainty of 4.4 arcseconds (radius, 90% containment), which is 3.2 arcseconds from the location of 1E 1841-04.5 (AKA PSR J1841-0456). This position may be improved as more data are received; the latest position is available at http://www.swift.ac.uk/sper. A power-law fit to a spectrum formed from promptly downlinked event data gives a column density consistent with the Galactic value of 1.57e+22 cm^-2 (Kalberla et al. 2005). UVOT took a finding chart exposure of 150 seconds with the White filter starting 1895 seconds after the BAT trigger. The full 2.7'x2'7 sub-image has not been received and the partial image does not cover the XRT error circle. The 8'x8' region for the list of sources generated on-board confirms a known optical source within the XRT circle. The preliminary UVOT position is RA, Dec 280.33001, -4.93763 which is RA(J2000) = 18h 41m 19.20s Dec(J2000) = -04d 56' 15.5" with an estimated uncertainty of 1.1 arcsec (radius, 90% confidence, statistical + systematic). The white magnitude is 17.98 +- 0.35 No correction has been made for the large, but uncertain extinction expected.