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

Abstract

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 sof t 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 SAX J1818.6 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 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 days, similarly to ot her members of the class of SFXTs, confirming SAX J1818.6 1703 as a member of this class.