TITLE: GCN CIRCULAR NUMBER: 16330 SUBJECT: Swift detection of SFXT IGR J17544-2619 DATE: 14/05/25 22:51:03 GMT FROM: Scott Barthelmy at NASA/GSFC S. D. Barthelmy (GSFC), S. B. Cenko (GSFC), V. D'Elia (ASDC), A. Y. Lien (NASA/GSFC/ORAU), V. Mangano (PSU), F. E. Marshall (NASA/GSFC), A. Maselli (INAF-IASFPA), P. Romano (INAF-IASFPA), B. Sbarufatti (INAF-OAB/PSU) and M. H. Siegel (PSU) report on behalf of the Swift Team: At 22:25:47 UT, the Swift Burst Alert Telescope (BAT) triggered twice on an SFXT IGR J17544-2619, (triggers=599954 and 599955). Swift slewed immediately to the source. The BAT on-board calculated location is RA, Dec 268.617, -26.339, which is RA(J2000) = 17h 54m 28s Dec(J2000) = -26d 20' 19" with an uncertainty of 3 arcmin (radius, 90% containment, including systematic uncertainty). As is typical for image triggers the real-time BAT light curve shows nothing significant. The XRT began observing the field at 22:33:07.4 UT, 439.5 seconds after the BAT trigger. XRT found a bright X-ray source located at RA, Dec 268.6050, -26.3322 which is equivalent to: RA(J2000) = +17h 54m 25.20s Dec(J2000) = -26d 19' 55.9" with an uncertainty of 4.9 arcseconds (radius, 90% containment). This location is 45 arcseconds from the BAT onboard position, within the BAT error circle. 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 3.58e-09 erg cm^-2 s^-1 (0.2-10 keV). Previously, Swift observed bright flares from this source on 2007 November 8 (Krimm et al. 2007, Atel #1265), 2008 March 31 (Sidoli et al. 2009, ApJ, 690, 120), 2008 September 4 (Sidoli et al. 2009, MNRAS, 397, 1528), 2009 March 15 (Krimm et al. 2009, Atel #1971), 2009 June 6 (Romano et al. 2011, MNRAS, 410, 1825), 2010 March 4 (Romano et al. 2011, MNRAS, 412, L30), 2011 March 24 (Farinelli et al. 2012, MNRAS, 424, 2854), 2012 April 12 (Romano et. al. 2012, ATel #4040), 2012 July 24 (Romano et al. 2012, ATel #4275), 2013 June 28 (Romano et al. 2013, ATel #5179), and 2013 September 11 (Romano et al. 2013, ATel #5388). The historical light curve from the BAT hard X-ray transient monitor (Krimm et al, 2013, ApJS, 209, 14; 15-50 keV) can be found at http://swift.gsfc.nasa.gov/docs/swift/results/transients/weak/IGRJ17544-2619 .