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Swift/XRT observes a bright outburst from the SFXT IGRJ17544-2619

ATel #1697; P. Romano, G. Cusumano (INAF-IASF Palermo), L. Sidoli, S. Vercellone(INAF-IASF Milano), E. Kuulkers (ESA/ESAC, on behalf of the INTEGRAL Galactic bulge monitoring team), J. A. Kennea(PSU), V. Mangano (INAF-IASF Palermo), H. A. Krimm (CRESST/GSFC/USRA), D. N. Burrows (PSU), N. Gehrels (GSFC)
on 4 Sep 2008; 15:17 UT
Distributed as an Instant Email Notice Request For Observations
Credential Certification: Pat Romano (romano@ifc.inaf.it)

Subjects: X-ray, Binary, Transient

Referred to by ATel #: 1929, 2463, 2820, 6227

During the monitoring of the Supergiant Fast X-ray Transient (SFXT) IGR J17544-2619 with Swift/XRT (Sidoli et al 2008a, ApJ, in press, arXiv0805.1808), we observed bright flaring activity, starting on 2008 September 4 at about 00:19:00 UT. This flare was preceded by intense activity in the previous few days as seen, e.g., in observations as part of the INTEGRAL Galactic bulge monitoring program (see ATel #1266) performed between UT 2008-08-30 00:29 to 04:11. The source was variable from pointing to pointing and reached a peak flux of about 50 mCrab (18-40 keV). The Swift/BAT did not trigger on it, so no event mode data are available.

The XRT light curve shows a peak exceeding 20 counts/s, implying a flare brighter than the one observed with Swift on March 31, at 20:53:27 UT (Sidoli et al. 2008, ATel#1454; Sidoli et al., 2008b, ApJ, in press, arXiv0808.3085).

The XRT spectrum (WT data, integration time of 217 s) can be fitted with an absorbed power law (chi2_red=1.265 for 81 dof), resulting in a photon index of 1.3+/-0.2, an absorbing column density of NH=1.8 (+0.4,-0.3)E+22 cm-2, and an average flux of ~8E-10 erg/cm2/s (2-10 keV, unabsorbed).

The fainter X-ray emission (PC data, integration time of ~630 s) shows a power law spectrum with the following parameters: NH=1.4 (+0.7 -0.5)E+22 cm-2, photon index of 0.8 (+0.4,-0.3) (chi2_red=1.394 for 21 dof) and an average flux of ~2E-10 erg/cm2/s (2-10 keV, unabsorbed).

We would like to thank the Swift Team for making these observations possible, in particular the duty scientists as well as the science planners, and the untiring work of the INTEGRAL Galactic bulge monitoring team.