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

Date

2013-01-07

Department

Program

Citation of Original Publication

L. Sidoli and others, A Suzaku X-ray observation of one orbit of the supergiant fast X-ray transient IGR J16479−4514, Monthly Notices of the Royal Astronomical Society, Volume 429, Issue 3, 1 March 2013, Pages 2763–2771, https://doi.org/10.1093/mnras/sts559

Rights

This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2013 L. Sidoli, P. Esposito, V. Sguera, A. Bodaghee, J. A. Tomsick, Katja Pottschmidt, J. Rodriguez, P. Romano, J. Wilms. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved

Subjects

Abstract

We report on a 250 ks long X-ray observation of the supergiant fast X-ray transient IGR J16479−4514 performed with Suzaku in 2012 February. During this observation, about 80 per cent of the short orbital period (Porb ∼ 3.32 d) 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 (10⁻¹³ erg cm⁻² s⁻¹; 1–10 keV) lasting the first 46 ks, 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 × 10⁻¹² erg cm⁻² s⁻¹ punctuated by two structured faint flares with a duration of about 10 and 15 ks, respectively, reaching a peak flux of 3–4 × 10−11 erg cm⁻² s⁻¹, separated by about 0.2 in orbital phase. 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 on to the compact object. The average X-ray spectrum is hard and highly absorbed, with a column density, NH, of 1023 cm⁻², clearly in excess of the interstellar absorption. There is no evidence for variability of the absorbing column density, except that during the eclipse, where a less absorbed X-ray spectrum is observed. A narrow Fe Kα emission line at 6.4 keV is viewed along the whole orbit, with an intensity which correlates with the continuum emission above 7 keV. The scattered component visible during the X-ray eclipse allowed us to directly probe the wind density at the orbital separation, resulting in ρw = 7 × 10⁻¹⁴ g cm⁻³. Assuming a spherical geometry for the supergiant wind, the derived wind density translates into a ratio M˙w/v∞ =7×10⁻¹⁷ M⊙ km⁻¹ which, assuming terminal velocities in a large range 500–3000 km s⁻¹, implies an accretion luminosity two orders of magnitude higher than that observed. As a consequence, a mechanism should be at work reducing the mass accretion rate. Different possibilities are discussed.