A NICER viewing angle on the accretion stream and disk of Vela X-1

Vela X-1 is the archetypical eclipsing high-mass X-ray binary, composed of a neutron star accreting the B-star wind. It was observed by nearly all X-ray observatories, often multiple times, and yet many questions still surround this binary system. Specifically, the origin of the rich fluorescence lines is hard to ascertain. We perform a systematic, phase dependent analysis of the reflected Fe Kα fluorescence line using over 100 NICER observations. We resolve the line variability into 500s time bins and find that it is predominantly due to variation in the ionizing flux, with a subtle underlying phase dependence over the 9 day orbital period. Our analysis reveals a significant reflection component that cannot originate from the host B-star alone. We also find that a significant portion of the host B-star is obscured by the accretion stream. We argue that the Vela X-1 accretion stream is responsible for both the additional fluorescence emission component and for obscuring the host star.