NICER observation list: Target ID 459001

Over the past few years, successful campaigns on a handful of galactic Black Hole X-ray Binaries have revealed remarkable sub-second variability and significant optical-vs-X-ray correlations. These promise new constraints on various physical components in the hearts of their accretion flows. But there is also puzzling complexity from source-to-source, and the exact driver of these variations remains unclear, mostly because of the scarcity of high-quality coordinated observations. Here, we propose up to 10 individual anticipated ToO observations on ~1-2 hard state outbursts with NICER, strictly simultaneous with ground-based optical/infrared timing. We will probe rapid, sub-second photometric variations as outbursts evolve, probing these systems on unprecedented, theory-critical scales.