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.
Highlights
IGR J16479–4514 is a hard X–ray transient discovered by IN T EGRAL on 2003, August 8–10 (Molkov et al 2003) in the energy range 18–50 keV
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
After the eclipse an intermediate level of emission is observed, lasting about 38 ks, during which the soft X–ray intensity below 3 keV is indistinguishable from the emission during eclipse (Fig. 2), while at harder energies the source appears uneclipsed, with a reduced intensity with respect to the uneclipsed emission displayed in the intervals n. 5 and n. 8
Summary
IGR J16479–4514 is a hard X–ray transient discovered by IN T EGRAL on 2003, August 8–10 (Molkov et al 2003) in the energy range 18–50 keV. Several X–ray flares were caught by IN T EGRAL/IBIS, displaying variable durations (from 0.5 to 50 hr) and peak fluxes (from 20 to about 600 mCrab, 20–60 keV; Sguera et al 2005, 2006, 2008, Walter & Zurita Heras 2007, Ducci et al 2010). The optical identification confirmed the initial classification of IGR J16479–4514 as a member of the new sub-class of high mass X–ray binaries, the supergiant fast X–ray transients (SFXTs), at first suggested only based on the short duration of its hard X–ray activity (Sguera et al 2006).
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