Long-term Study of the First Galactic Ultraluminous X-Ray Source Swift J0243.6+6124 Using NICER

We present the results obtained from detailed X-ray timing and spectral studies of X-ray pulsar Swift J0243.6+6124 during its giant and normal X-ray outbursts between 2017 and 2023 observed by the Neutron star Interior Composition Explorer (NICER). We focused on a timing analysis of the normal outbursts. A distinct break is found in the power density spectra of the source. The corresponding break frequency and slopes of the power laws around the break vary with luminosity, indicating a change in the accretion dynamics with the mass accretion rate. Interestingly, we detected quasiperiodic oscillations within a specific luminosity range, providing further insights into the underlying physical processes. We also studied the neutron star spin period evolution and a luminosity variation in the pulse profile during the recent 2023 outburst. The spectral analysis was conducted comprehensively for the giant and all other normal outbursts. We identified a double transition at luminosities of ≈7.5 × 10³⁷ and 2.1 × 10³⁸ erg s⁻¹ in the evolution of continuum parameters like the photon index and cutoff energy with luminosity. This indicates three distinct accretion modes experienced by the source, mainly during the giant X-ray outburst. A soft blackbody component with a temperature of 0.08-0.7 keV is also detected in the spectra. The observed temperature undergoes a discontinuous transition when the pulsar evolves from a sub- to super-Eddington state. Notably, in addition to an evolving 6-7 keV iron line complex, a 1 keV emission line was observed during the super-Eddington state of the source, implying X-ray reflection from the accretion disk or outflow material.