Swift observations of a bright outburst of the SFXT IGR J17544-2619

ATel #2463; P. Romano (INAF-IASF Palermo), H. A. Krimm (CRESST/GSFC/USRA), D. M. Palmer (LANL), S. D. Barthelmy (GSFC), D. N. Burrows (PSU), J. R. Cummings (NASA/UMBC), G. Cusumano (INAF-IASF Palermo), P. Esposito (INAF-IASF Milano), P. A. Evans (U. Leicester), J. A. Kennea (PSU), V. La Parola (INAF-IASF Palermo), S. Vercellone (INAF-IASF Palermo), N. Gehrels (GSFC)
on 5 Mar 2010; 14:30 UT
Distributed as an Instant Email Notice Request For Observations
Credential Certification: Hans A. Krimm (Hans.Krimm@nasa.gov)

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The Swift Burst Alert Telescope (BAT) triggered on a new outburst from the Supergiant Fast X-ray Transient (SFXT) IGR J17544-2619 on 2010 March 04 at 23:13:54 UT (image trigger=414875). Swift immediately slewed to the target, so that the narrow field instruments started observing about 395 s after the trigger.

Using the BAT data set from T-119 to T+183 sec we see a broad peak of approximate duration 150 seconds starting from ~T-50 relative to the start of the image trigger. The time-averaged spectrum from T+0.0 to T+183.1 sec is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 3.43 +- 0.81. The fluence in the 15-150 keV band is 3.2 +- 1.0 x 10-7 erg/cm2. All the quoted errors are at the 90% confidence level.

The XRT light curve shows a peak exceeding 25 counts/s, comparable to the bright flare observed on 2008 Sept 4 (Atel #1697). The count rate then decreases to about 0.5 counts/s and increases again up to about 20 counts/s at the end of the first orbit of observations. This behaviour has been previously observed in this source and most notably in IGR J08408-4503 (Romano et al., 2009, MNRAS, 392, 45). The XRT/WT spectrum (T+401 to T+839 s, integration time of 412 s) can be fitted with an absorbed power law (chi2_red=1.1 for 125 dof), resulting in a photon index of 1.2+/-0.1, an absorbing column density of NH=(1.2+/-0.2)E+22 cm-2, and an average flux of ~1.5E-9 erg/cm2/s (2-10 keV, unabsorbed). The XRT/PC spectrum (T+840 to T+2242 s, integration time of 1402 s) shows a power law shape with the following parameters: NH=1.9(+0.7,-0.5)E+22 cm-2, photon index of 1.5+/-0.4 (chi2_red=0.6 for 17 dof) and an average flux of ~2E-10 erg/cm2/s (2-10 keV, unabsorbed).

Previously, Swift observed 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), and 2009 June 6 (Romano et al. 2009, ATel #2069). Onboard detections include 2009-06-20 at 02:45:39, 2009-07-12 at 10:59:55, and 2009-08-20 at 03:19:54.

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