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
Credential Certification: Pat Romano (romano@ifc.inaf.it)

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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 http://swift.gsfc.nasa.gov/docs/swift/results/transients/weak/IGRJ17391-3021/

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/