Swift/XRT observations of an outburst of the SFXT IGRJ17544-2619

ATel #2069; P. Romano (INAF-IASF Palermo), L. Sidoli (INAF-IASF Milano), S. Vercellone (INAF-IASF Palermo), M. Chester (PSU), P. A. Evans (U. Leicester), P. Esposito (INAF-IASF Milano), J. A. Kennea (PSU), V. La Parola (INAF-IASF Palermo), H. A. Krimm (CRESST/GSFC/USRA), D. N. Burrows (PSU), N. Gehrels (GSFC)
on 6 Jun 2009; 14:01 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Pat Romano (romano@ifc.inaf.it)

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Swift has observed a new outburst from IGR J17544-2619, a Supergiant Fast X-ray Transient discovered in September 2003 with INTEGRAL (Sunyaev et al., ATel#190). The Swift Burst Alert Telescope (BAT) triggered on it on 2009 June 06 at 07:48:59 UT (image trigger=354221). Swift immediately slewed to the target, so that the NFIs started observing about 164s after the trigger.

The initial Swift/XRT data (WT mode, 170-446 s since the trigger) show a decaying light curve with a count rate that started at about 20 counts/s. The following PC data (448-521s) seamlessly continue the decaying trend down to about 2 counts/s.

The peak of the outburst (WT data) has an unabsorbed 0.3-10 keV flux of 1E-9 erg/cm2/s, and shows a hard spectrum, fitted with a power-law model with a photon index of 1.12(-0.13,+0.14), absorbed with a column density of NH=(1.2+/-0.2)E+22 cm-2. The PC spectrum (unabsorbed 0.3-10 keV flux ~2E-10 erg/cm2/s) fitted with a power-law model (Cash statistics) yields a photon index of 1.10(-0.64,+0.70) and an absorbing column density of 1.3(-0.9,+1.1)E+22 cm-2, consistent with the WT data fit.

Previously, Swift caught 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,in press,arXiv:0905.2815), and 2009 March 15 (Krimm et al. 2009, Atel #1971).

We would like to thank the Swift Team for making these observations possible, in particular the duty scientists as well as the science planners.