The Supergiant Fast X-ray Transient with the shortest orbital period: Suzaku observes one orbit in IGR J16479-4514

Abstract

The eclipsing hard X–ray source IGR J16479–4514 is the Supergiant Fast X-ray Transient (SFXT) with the shortest orbital period (Pₒᵣb∼3.32 days). This allowed us to perform a 250 ks long X–ray observation with Suzaku in 2012 February, covering most of its orbit, including the eclipse egress. Outside the eclipse, the source luminosity is around a few 10³⁴ erg s⁻¹ . The X–ray spectrum can be fit with an absorbed power law together with a neutral iron emission line at 6.4 keV. The column density, NH, is constant at ∼10²³ cm⁻² outside the X–ray eclipse. During the eclipse it is lower, consistent with a scattering origin for the low X–ray emission during the eclipse by the supergiant companion. The scattered X–ray emission during the X–ray eclipse is used to directly probe the density, ρw, of the companion wind at the orbital separation, resulting in ρ w=7×10⁻¹⁴ g cm⁻³ , which translates into a ratio M˙w/v∞ = 7×10⁻¹⁷ M⊙/km of the wind mass loss rate to the wind terminal velocity. This ratio, assuming reasonable terminal velocities in the range 500–3000 km s⁻¹ , translates into an accretion luminosity two orders of magnitude higher than that observed. We conclude that a mechanism reducing the accretion rate onto the compact object is at work, likely due to the neutron star magnetosphere.