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 High Mass X–ray Binary X–ray pulsars , and is characterized by a very high absorption , a flat power law ( photon index \sim 0.1–0.5 ) and a cutoff 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 , kT _ { 0 } , of 1.3–1.4 keV , and an optical depth for the Comptonizing plasma , \tau , of about 10 .
The 1–100 keV luminosity at the peak of the flare is 3 \times 10 ^ { 36 } 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 days , similarly to other members of the class of SFXTs , confirming SAX J1818.6 - 1703 as a member of this class .