On the radial onset of clumping in the wind of the B0I massive star QV NOR

We present an analysis of a 78 ks Chandra high-energy transmission gratings observation of the B0I star QV Nor, the massive donor of the wind-accreting pulsar 4U1538−52. The neutron star (NS) orbits its companion in a very close orbit ($r<1.4R_{\ast }$, in units of the stellar radii), thereby allowing probing of the innermost wind regions. The flux of the Fe Kα line during eclipse reduces to only ∼30% of the flux measured out of eclipse. This indicates that the majority of Fe fluorescence must be produced in regions close to the NS, at distances smaller than $1R_{\ast }$ from its surface. The fact that the flux of the continuum decreases to only ∼3% during eclipse allows for a high contrast of the Fe Kα line fluorescence during eclipse. The line is not resolved and centered at $\lambda ={1.9368}_{-0.0018}^{+0.0032}$ Å. From the inferred plasma speed limit of $v<c\frac{\Delta \lambda }{\lambda }<800$ km s⁻¹ and range of ionization parameters of $log\xi =[-1,2]$, together with the stellar density profile, we constrain the location of the cold, dense material in the stellar wind of QV Nor using simple geometrical considerations. We then use the Fe Kα line fluorescence as a tracer of wind clumps and determine that these clumps in the stellar wind of QV Nor (B0I) must already be present at radii $r<1.25R_{\ast }$, close to the photosphere of the star.