The Supergiant Fast X-ray Transient with the shortest orbital period: Suzaku observes one orbit in IGRJ16479-4514

TL;DR

This study uses Suzaku observations to analyze the shortest orbital period supergiant fast X-ray transient IGR J16479-4514, revealing insights into its wind density, accretion processes, and the influence of the neutron star magnetosphere.

Contribution

First detailed X-ray analysis of IGR J16479-4514 covering most of its orbit, providing new measurements of wind density and insights into accretion suppression mechanisms.

Findings

Measured wind density at orbital separation: 7E-14 g/cm3.

Estimated wind mass loss rate to terminal velocity ratio: 7E-17 solar masses/km.

Identified a reduction mechanism in accretion rate likely due to neutron star magnetosphere.

Abstract

The eclipsing hard X-ray source IGR J16479-4514 is the Supergiant Fast X-ray Transient (SFXT) with the shortest orbital period (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 1E34erg/s. 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 is constant at 1E23 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 wind. The scattered X-ray emission during the X-ray eclipse is used to directly probe the density of the companion wind at the orbital separation, resulting in 7E-14 g/cm3, which translates into a ratio Mdot_w/v_terminal = 7E-17 solar masses/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, 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.