Constraining the evolution of the unstable accretion disk in SMC X-1 with NICER

Neutron star high mass X-ray binaries that exhibit superorbital variability offer an opportunity to study the geometry and stability of warped accretion disks. The high mass X-ray binary SMC X-1 is an ideal system in which to investigate these questions because the supeorbital period has epochs of instability known as excursions likely caused by disk instability. Using the high resolution spectral and timing capabilities of the Neutron Star Interior Composition Explorer (NICER) we examined the high state of four different superorbital cycles of SMC X-1 to search for short term changes in spectral shape and any connection to the unstable accretion disk geometry. We performed phase averaged and phase resolved spectroscopy, as well as principal component analysis to closely compare the spectral characteristics and any cycle-to-cycle variations. While soft, disk-related spectral components showed variations with time, the accretion column related parameters (i.e. photon index) remained mostly constant, indicating that the disk instability does not significantly change SMC X-1's accretion process.