Swift/BAT observes a new outburst from the SFXT SAX J1818.6-1703

ATel #2279; P. Romano (INAF-IASF Palermo), S. Barthelmy (GSFC), L. Sidoli (INAF-IASF Milano), D. N. Burrows (PSU), M. M. Chester (PSU), G. Cusumano (INAF-IASF Palermo), P. Esposito (INAF-IASF Milano), N. Gehrels (GSFC), J. A. Kennea (PSU), H. A. Krimm (CRESST/GSFC/USRA), V. La Parola (INAF-IASF Palermo), D. M. Palmer (LANL), S. Vercellone (INAF-IASF Palermo)
on 4 Nov 2009; 16:53 UT
Distributed as an Instant Email Notice Request For Observations
Credential Certification: Pat Romano (romano@ifc.inaf.it)

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The Swift/BAT triggered on a new outburst of the Supergiant Fast X-ray Transient SAX J1818.6-1703 on 2009 November 4 at 07:24:10 UT. Swift did not perform a slew to the target, hence no narrow-field instrument observations are available.

Using the data set from T-239 to T+780 s, the mask-weighted light curve shows that the source was active when the location came into the BAT FOV at T-239 s. It maintained a roughly constant emission of 0.03 ph/cm2/s out to T+780 s (the end of the mask-weighted data).

The BAT time-averaged spectrum from T+0.0 to T+320.0 s is best fit by a simple power-law model. The photon index of the time-averaged spectrum is 2.60 +- 0.35. The fluence in the 15-150 keV band is (7.2 +- 1.5)E-7 erg/cm2. The 1-s peak photon flux measured from T+0.00 s in the 15-150 keV band is 0.047 +- 0.007 ph/cm2/s. All the quoted errors are at the 90% confidence level.

The results of the batgrbproduct analysis are available at http://gcn.gsfc.nasa.gov/notices_s/374869/BA/
The SAX J1818.6-1703 light curve from the BAT hard X-ray transient monitor (15-50 keV) can be found at
http://swift.gsfc.nasa.gov/docs/swift/results/transients/weak/SAXJ1818.6-1703/

The orbital period of this SFXT is known to be 30 days (Bird et al. 2009, MNRAS, 393, L11; Zurita Heras & Chaty 2009, A&A, 493, L1), based on the analysis of available Swift/BAT and INTEGRAL data; these data suggest an eccentric orbit (e=0.3-0.4) and an outburst duration of around 4 to 6 days. Swift observed flares from this source on 2009 September 5 (Romano et al., ATel #2191), 2009 May 6 (Romano et al., ATel #2044, Sidoli et al. 2009, MNRAS, in press, arXiv:0907.4041), 2008 March 15 (Barthelmy et al., GCN #7419), 2008 February 14, 2007 October 16, 2007 September 17, 2006 September 22, and 2006 August 23. The bright flare of SAX J1818.6-1703 observed today occurred at the same orbital phase as the two previous outbursts observed by Swift, and within 4 days of all previous outbursts.