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

ATel #3235; P. Romano (INAF-IASF Palermo), S. D. Barthelmy (GSFC), P. Esposito (INAF-OAC),P. A. Evans (LU), V. Mangano (INAF-IASF Palermo), D. N. Burrows (PSU),M. M. Chester (PSU), G. Cusumano (INAF-IASFPA), R. Farinelli (INAF-IASFPA, U. Ferrara), J. A. Kennea (PSU), H. Krimm (CRESST/GSFC/USRA), V. La Parola (INAF-IASFPA), S. Vercellone (INAF-IASFPA), N. Gehrels (GSFC)
on 24 Mar 2011; 14:34 UT
Credential Certification: Pat Romano (romano@ifc.inaf.it)

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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 2011 March 24 at 01:56:57 UT (image trigger=449907). Swift immediately slewed to the target, so that the narrow field instruments started observing about 126.4 s after the trigger.

Currently, BAT telemetry has been downlinked for the time interval covering from T-120 to T+180 s since the trigger. The BAT on-board light curve shows clear evidence of emission both prior to T=0 s and past T+180 s. The spectrum from the portion of the event for which data are available (T+0.0 to T+64.0 s) is best fit by a simple power-law model, with a photon index of 3.55+/-0.38. The fluence in the 64-s interval in the 15-150 keV band is (3.8+/-0.6)E-07 erg/cm2. All the quoted errors are at the 90% confidence level.

The Swift/XRT light curve shows the descending part of one bright flare that reached 50 counts/s, a factor of two brighter than the previous one, observed on 2010 March 4 (Romano et al 2011, MNRAS, 412, L30). The count rate then decreases to about 2 counts/s by the end of the first orbit of observations (T+881 s). During the second orbit (up to T+2 hr) the count rate is steady at about 1 count/s.

The XRT/WT spectrum (T+133 to T+783 s, integration time of 650 s) can be fitted with an absorbed power law (chi2_red=1.02 for 556 dof), with a photon index of 0.82+/-0.04, and an absorbing column density of NH=(1.0+/-0.1)E+22 cm-2. The mean flux is ~2.9E-9 erg/cm2/s (2-10 keV, unabsorbed), which translates into a luminosity of 4.5E+36 erg/s (assuming the optical counterpart distance of 3.6 kpc, Rahoui et al. 2008, A&A, 484, 801). The XRT/PC spectrum (T+784 to T+881 s, integration time of 96 s) shows a power law shape with the following parameters: NH=2.2(-1.0,+1.4)E+22 cm-2, photon index of 1.7-0.7+0.8 (25% of 10E4 montecarlo realizations with Cash statistic > 74.19) and an average flux of ~1.8E-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), 2009 June 6 (Romano et al. 2011, 410, 1825), and 2010 March 4 (Romano et al 2011, MNRAS, 412, L30). In the XRT band we infer a luminosity a factor of 2 brighter during this outburst than in the previous ones, as in this instance Swift performed an earlier slew.