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Swift observes a new outburst from the Supergiant Fast X-ray Transient XTEJ1739-302/IGRJ17391-3021

ATEL # 1466; P. Romano (INAF-IASF Palermo), L. Sidoli (INAF-IASF Milano), V. Mangano (INAF-IASF Palermo), H.A. Krimm (CRESST/GSFC/USRA), S. Barthelmy (GSFC), D.N. Burrows (PSU), G. Cusumano (INAF-IASF Palermo), N. Gehrels (GSFC), J.A. Kennea(PSU), A. Paizis (INAF-IASF Milano), J. Tueller (GSFC), S. Vercellone (INAF-IASF Milano)
on 9 Apr 2008; 15:16 UT
Distributed as an Instant Email Notice (Request for Observations)
Password Certification: Pat Romano (

Subjects: X-ray, Gamma Ray, Request for Observations, Binaries, Transients
Referred to by ATEL #: 1471

A new outburst from the source XTEJ1739-302/IGRJ17391-3021 (Smith et al. 1998, ApJ 501, L101), the prototype of the class of the Supergiant Fast X-ray Transients (SFXT; Smith et al. 2005 ApJ, 638, 974; Negueruela et al. 2006 ApJ, 638, 982), was observed by the Swift Burst Alert Telescope (BAT) on 2008-04-08 at 21:28:15 (trigger=308797). Swift immediately slewed to the target, so that the narrow-field instruments began observing the field 387 s after the BAT trigger.

Using the BAT data set from T-119 to T+950 s from the recent telemetry downlink, we obtained a nearly constant mask-weighted light curve at the level of 0.25 counts/cm2/s in the 15-150 keV band. The partial coding was 100%. We extracted two time-averaged spectra in the time intervals form T+0 s to T+341 s (before the slew to the target) and from T+373 s to T+850 s (after the slew, simultaneous with XRT observation). Both spectra can be fit with simple power law models with photon indices of 2.7 +/- 0.5 and 3.0 +/- 0.6, consistent with a constant slope. The 15-150 keV fluxes in the given time intervals are 1.8E-9 and 1.2E-9 ergs/cm2/s, respectively.

The Swift/XRT light curve shows two distinct flares of decreasing intensity; during the first one, which was caught in its descending part, the source reached ~10 counts/s; the second flare is roughly half as bright as the first one (< 4 counts/s). The Swift/XRT data consist of the initial WT data, and subsequent PC data, when the source was fainter. The XRT spectrum from WT data (393-468s since the trigger) can be fit with an absorbed powerlaw with a photon index of 1.29(-0.58,+0.67) and an absorbing column density of NH=(11-3+4)E+22 cm-2 (chi2_red=1.253 for 21 dof). The peak of the flare reached 1.7E-9 erg/cm2/s (unabsorbed, 2-10 keV), which translates into a luminosity of 1.4E+36 erg/s (assuming the optical counterpart distance of 2.7 kpc, Rahoui et al. 2008, arXiV 0802.1770). The fainter X-ray emission (PC data, 565-6649s since the BAT trigger) resulted in a photon index of 1.27(-0.73,+0.92), NH=(10-4+6)E+22 cm-2 (chi2_red=1.252 for 14 dof) and an unabsorbed flux of 4.3E-10 erg/cm2/s (2-10 keV).

In the past, several bright flares have been observed from this SFXT, mainly with RXTE (Smith et al. 2005 ApJ, 638, 974) and INTEGRAL (Sguera et al. 2005 A&A, 444, 221; Sguera et al. 2006 ApJ, 646, 452; Walter et al., 2007, A&A, 476, 335). The last outburst was observed with the IBIS/ISGRI instrument on-board INTEGRAL, about 403 days ago, on March 2, 2007 (Turler, et al., 2007 ATel#1019).

In the BAT hard X-ray transient monitor (15-50 keV), this source has shown six past episodes of emission above 150 mcrab and 3.0 sigma significance including an onboard trigger on June 18, 2007. The IGR J17391-3021 light curve from the BAT monitor can be found at

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

BAT Transient Monitor page: IGRJ17391-3021/

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R. E. Rutledge , Editor
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