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Swift observes an outburst from the Supergiant Fast X-ray Transient SAX J1818.6-1703

ATEL # 2044; P. Romano (INAF-IASF Palermo), L. Sidoli (INAF-IASF Milano), H.A. Krimm (CRESST/GSFC/USRA), M.M. Chester (PSU), P.A. Evans (U. Leicester), S. Barthelmy (GSFC), S. Vercellone, V. La Parola, V Mangano (INAF-IASF Palermo), J.A. Kennea, D.N. Burrows (PSU), N. Gehrels (GSFC)
on 6 May 2009; 19:59 UT
Distributed as an Instant Email Notice (Request for Observations)
Password Certification: Pat Romano (

Subjects: X-ray, Gamma Ray, Request for Observations, Binaries, Transients

The Swift/BAT caught a new outburst from the Supergiant Fast X-ray Transient SAX J1818.6-1703 on May 6 at 14:02:11 UT. The time-averaged spectrum for the BAT data from T+0.0 to T+64.0 s is best fit by a simple power-law model. The power law index of the time-averaged spectrum is 3.34+/-0.62. The fluence in the 15-150 keV band is (2.1+/-0.6)E-07 erg/cm2. The averaged BAT light curve shows the source flux peaking between T-123 and T+69 at 0.05 +/- 0.006 ct/s/cm2 and falling to 0.03+/-0.005 ct/s/cm2 between T+309 and T+501.

Swift slewed immediately to the burst and the XRT began observing the field at 14:04:22.8 UT, 131.5 seconds after the BAT trigger. The Swift/XRT light curve of the first orbit of data shows several flares of decreasing intensity; during the first one, which was caught in its descending part, the source reached ~15 counts/s. The Swift/XRT data consist of the initial WT data, and subsequent PC data. The data are affected by contamination from single-reflection from stray light from outside the XRT field of view. The XRT spectrum from WT data (138-1320s since the trigger) can be fit with an absorbed powerlaw with a photon index of 1.1+/-0.1 and an absorbing column density of NH=(8+/-1)E+22 cm-2. The peak of the initial flare reached 2E-9 erg/cm2/s (unabsorbed, 0.2-10 keV), which translates into a luminosity of 1.5E+36 erg/s (assuming the optical counterpart distance of 2.5 kpc). The fainter X-ray emission (PC data, 1322-2299 s since the BAT trigger) resulted in a photon index of 1.0+/-0.4, NH=(9+/-2)E+22 cm-2 and an unabsorbed flux of 1.4E-9 erg/cm2/s (0.2-10 keV).

The SAX J1818.6-1703 light curve from the BAT hard X-ray transient monitor (15-50 keV) can be found at

SAX J1818.6-1703 is a Supergiant Fast X-ray Transient with a controversial nature (in't Zand et al., 2006, ATel #915; Bozzo et al., 2008, ATel #1493) discovered with WFC/BeppoSAX (in't Zand et al. 1998, IAUC # 6840 ). It was observed with IBIS/ISGRI in September and October 2003 (Grebenev & Sunyaev, 2005, AstL 31, 672; Sguera et al., 2005, A&A 444, 221) during brief outbursts lasting from 1 to about 3 hours. In September 2003, besides two bright outbursts reaching 230 and 173 mCrab, respectively, in the energy range 18-45 keV, the source showed a high energy activity lasting 22.5 hours (Grebenev & Sunyaev, 2005). More recently, further flaring activity have been detected with Swift/BAT (Barthelmy et al., GCN #7419) and with INTEGRAL (Grebenev & Sunyaev, 2008, ATel #1482). A periodicity (likely orbital) of 30-days was discovered (Bird et al., 2009, MNRAS, 393, L11; Zurita Heras & Chaty, 2009, A&A, 493, L1), suggesting an eccentric orbit.

Further Swift observations are ongoing, and we encourage multiwavelength observations of this source.

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

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R. E. Rutledge , Editor
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