The first broad-band X-ray study of the Supergiant Fast X-ray Transient SAXJ1818.6-1703 in outburst

The Supergiant Fast X-ray Transient (SFXT) SAXJ1818.6-1703 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 ~0.1-0.5) and a cutoff at about 7-12 keV. Alternatively, the SAXJ1818.6-1703 emission can be described with a Comptonized emission from a cold and optically thick corona, with an electron temperature kTe=5-7 keV, a hot seed photon temperature, kT0, of 1.3-1.4 keV, and an optical depth for the Comptonizing plasma of about 10. The 1-100 keV luminosity at the peak of the flare is 3E36 erg/s (assuming the optical counterpart distance of 2.5 kpc). These properties of SAXJ1818.6-1703 resemble those of the prototype of the SFXT class, XTEJ1739-302. The monitoring with Swift/XRT reveals an outburst duration of about 5 days, similarly to other members of the class of SFXTs, confirming SAXJ1818.6-1703 as a member of this class.