NICER observation list: Target ID 366801

Cir X-1 is one of the few galactic X-ray binaries that has entered ULX territory (Lx>1e39 erg/s). It provides a unique bridge between the near-/super-Eddington Z sources and ULXs. Archival data suggest that on its way to becoming a ULX Cir X-1 develops a strong outflow that affects the measured temperatures of the continuum components. After a long relatively quiet period, Cir X-1 has become persistently active again, allowing us to investigate this behavior with modern instrumentation. We request 3 NuSTAR/NICER observations (75/45 ks total) near peaks of the source's X-ray cycles, to probe its mildly super-Eddington regime. Our goal is to better understand inflows/outflows in X-ray binaries as they become super-Eddington and to establish a connection to the phenomenology of ULX spectra.