Measuring HMXB Winds with NICER Observations of Cyg X-1 Near Orbital Phase 0

Cyg X-1 is a High Mass X-ray Binary with a black hole and a supergiant donor star in a 5.6 day orbit. The supergiant star donates mass to the black hole via “focused wind accretion”. Near orbital phase 0, our line of sight to the black hole passes through this wind, and allows X-rays from the inner accretion flow to probe the wind's structure. Historically, we see “dips” associated with both highly ionized absorption and colder, denser near neutral absorption. NICER, with its large effective area, superb soft X-ray response, and low background is uniquely suited to study this dipping behavior on time scales potentially as fast as 0.1 s. We discuss recent NICER observations that targeted orbital phase 0 during the spectrally hard state. We use modeling of light curves, time-dependent color-color diagrams, and spectra at different color/flux levels to describe the absorbing structures detected in the wind. One challenge is that NICER lightcurves can be fairly short (only hundreds of seconds long), and thus we do not always cover ingress/egress to/from longer and deeper dips. However, we discuss how we use comparisons to additional data taken near orbital phase 0.5 to distinguish these events even in short lightcurves.