The first broad-band X-ray study of the Supergiant Fast X-ray Transient SAX J1818.6-1703 in outburst

The Supergiant Fast X-ray Transient (SFXT) SAX J1818.6-1703 underwent an outburst on 2009 May 6 and was observed with Swift. We report on these observations which, for the first time, allow us to study the broad-band spectrum from soft to hard X-rays of this source. No X-ray spectral information was available on this source before the Swift monitoring. The spectrum can be deconvolved well with models usually adopted to describe the emission from HMXB X-ray pulsars, and is characterized by a very high absorption, a flat power law (photon index ∼0.1-0.5) and a cut-off at about 7-12 keV. Alternatively, the SAX J1818.6-1703 emission can be described with a Comptonized emission from a cold and optically thick corona, with an electron temperature kT_e = 5-7 keV, a hot seed photon temperature, kT₀, of 1.3-1.4 keV and an optical depth for the Comptonizing plasma, τ, of about 10. The 1-100 keV luminosity at the peak of the flare is 3 × 10³⁶ erg s ^-1 (assuming the optical counterpart distance of 2.5 kpc). These properties of SAX J1818.6-1703 resemble those of the prototype of the SFXT class, XTE J1739-302. The monitoring with Swift/XRT reveals an outburst duration of about 5 d, similar to other members of the class of SFXTs, confirming SAX J1818.6-1703 as a member of this class.