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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,  kT  0 , 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 × 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 d, similar to other members of the class of SFXTs, confirming SAX J1818.6−1703 as a member of this class.

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