Tracing the Spin Period Evolution and Probing the Polarization of Super-Eddington Accreting pulsar SMC X-1.

📖 Project Description

SMC X-1 is a high-mass X-ray binary containing a neutron star and a supergiant companion. It is an intriguing source for its superorbital modulation with a period varying between 35 and 70 days. While this modulation was long been explained by the neutron star being obscured by a warped accretion disk, recent observations suggest a more complex scenario. In particular, pulsations detected during the low state, the absence of spectral hardening during transitions, and a possible decrease in the spin-up rate hint that the mass accretion rate may also change with superorbital phase. These challenge the warped-disk model and make SMC X-1 more like ultraluminous X-ray pulsars with super-Eddington accretion rate. To investigate this further, we will track the system’s long-term spin evolution using data from several X-ray observatories, including XMM-Newton, NICER, and the all-sky monitor MAXI. By combining these observations, we aim to determine how the spin-up rate of SMC X-1 evolves with time. Moreover, we will examine possible changes in polarization across the spin, orbital, and superorbital phases using IXPE data, and explore the physical mechanisms of the superorbital modulation.