A Suzaku X-ray observation of one orbit of the supergiant fast X-ray transient IGR J16479-4514

TL;DR

This study presents a detailed Suzaku X-ray observation of the supergiant fast X-ray transient IGR J16479-4514, revealing variability, eclipse phenomena, and wind properties that inform accretion processes in such systems.

Contribution

First continuous 250 ks Suzaku observation covering most of the orbital period, revealing eclipse, wind structure, and accretion dynamics in IGR J16479-4514.

Findings

Eclipse causes low emission state lasting 46 ks.

Presence of phase-locked wind structures affecting accretion.

Wind density measured at 7E-14 g/cm3, implying reduced accretion efficiency.

Abstract

We report on a 250 ks long X-ray observation of the supergiant fast X-ray transient (SFXT) IGR J16479-4514 performed with Suzaku in 2012 February. About 80% of the short orbital period (Porb=3.32 days) was covered as continuously as possible for the first time. The source light curve displays variability of more than two orders of magnitude, starting with a very low emission state lasting the first 46 ks (1E-13 erg/cm2/s, 1-10 keV), consistent with being due to the X-ray eclipse by the supergiant companion. The transition to the uneclipsed X-ray emission is energy dependent. Outside the eclipse, the source spends most of the time at a level of (6-7)x10^-12 erg/cm2/s punctuated by two structured faint flares with a duration of about 10 and 15 ks. Remarkably, the first faint flare occurs at a similar orbital phase of the bright flares previously observed in the system. This indicates the presence of a phase-locked large scale structure in the supergiant wind, driving a higher accretion rate onto the compact object. The scattered component visible during the X-ray eclipse allowed us to directly probe the wind density at the orbital separation, resulting in rho=7E-14 g/cm3. Assuming a spherical geometry for the supergiant wind, the derived wind density translates into a ratio Mdot_w/v_terminal = 7E-17 solar masses/km which, assuming terminal velocities in a large range 500-3000 km/s, implies an accretion luminosity two orders of magnitude higher than that observed. As a consequence, a mechanism is at work reducing the mass accretion rate. Different possibilities are discussed.