A new outburst from the Supergiant Fast X-ray Transient IGRJ17544-2619 caught by Swift

ATel #1454; L. Sidoli (INAF-IASF Milano), P. Romano, V. Mangano (INAF-IASF Palermo), J. A. Kennea(PSU), G. Cusumano (INAF-IASF Palermo), S. Campana (OAB), S. Vercellone (INAF-IASF Milano), D. N. Burrows (PSU), N. Gehrels (GSFC)
on 1 Apr 2008; 15:52 UT
Distributed as an Instant Email Notice Request For Observations
Credential Certification: Lara Sidoli (sidoli@mi.iasf.cnr.it)

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The Swift satellite 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 March 31 at 20:50:45 UT, (trigger=308224). Swift immediately slewed to the target, so that the NFI onboard Swift started observing it about 162s after the trigger.

The BAT mask-weighted light curve shows a long lasting plateau emission from about T-107 s to T+553 s, with an approximately 8 s long peak centered at 546 s. The estimated T90 (15-350 keV) is 614+/-77s (estimated error including systematics). The time-averaged spectrum from T-107 to T+553 s is best fit by a simple power-law model. The photon index of the time-averaged spectrum is 4.7+/-0.6. The fluence in the 15-150 keV band is about 1.1E-06 erg/cm2. The flux in the 15-150 keV band is 1.6544E-09 ergs cm^-2 s^-1. All the quoted errors are at the 90% confidence level.

The Swift/XRT observations started on March 31, at 20:53:27 UT. The initial WT data (170-492s since the trigger) show a complex light curve with several flares and a count rate that reached about 20 counts/s. The following PC data (3724-5276s) show a count rate of (4.5+/-0.8)E-02 counts/s, so that a dynamic range of 400 is observed in about 1.5 hours.

The peak of the outburst (WT data) is at 1E-9 erg/cm2/s (unabsorbed, 2-10 keV), showing a hard spectrum, well fit with a powerlaw with a photon index of 0.74+/-0.11, and highly absorbed with a column density of NH=(1.1+/-0.2)E+22 cm-2 (chi2_red=1.035 for 143 dof). A fit with a cutoff power-law yields a column density of NH=(0.7+/-0.3)E+22 cm-2, a photon index of -0.24(-0.56,+0.57), and a cutoff energy of 4.6(-1.7,+6.4) keV (chi2_red=0.986 for 142; Ftest probability with respect to simple power law is 5E-03). A 3sigma upper limit to the equivalent width of an iron line at 6.7 keV can be placed at 63 eV.

The X-ray spectrum of the fainter emission (PC, ~5E-12 erg/cm2/s, unabsorbed) is significantly softer: fitted with a powerlaw model (Cash statistics), the resulting photon index is 1.53(-0.58,+0.70) and an absorbing column density of 1.1(-0.6,+0.9)E+22 cm-2.

Previously, several bright flares have been observed with INTEGRAL (see e.g. Grebenev et al. 2004, ATel #252; Sguera et al. 2006 ApJ, 646, 452; Walter et al., 2007, A&A, 476, 335; Kuulkers et al. 2007, ATel #1266), with Chandra (in't Zand 2005, A&A, 441, L1) and in archival BeppoSAX/WFC data (in't Zand et al., 2004, ESASP, 552, 427). The last outburst has been observed 144 days ago with Swift/BAT (Krimm et al., Atel#1265), beginning at 1:31:04 UT on November 8, 2007 (MJD 54412), but unfortunately observations with Swift/XRT were not possible because of Sun constraints.

The source position determined with XMM-Newton at arcsec level (Gonzalez-Riestra et al., IAUC 8202 ) was later refined with Chandra (R.A.(J2000) 17h 54m 25.284s, Dec (J2000)=-26deg 19' 52.62", 0.6 arcsec error radius; in't Zand 2005), which confirmed the association with the optical counterpart 2MASS J17542527-2619526 at first suggested by Rodriguez (2003, ATel#194), later identified as a O9Ib star (Pellizza et al. 2006) located at 3.6 kpc (Rahoui et al. 2008, arXiv.0802.1770).

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