PhD. Thesis Dissertation: Ndiogou Niang
ROTATIONAL PHASES AND LONG-TERM EVOLUTION OF WEAKLY
MAGNETIZED LOW MASS X-RAY BINARIES
Ndiogou Niang
Physics, PhD Dissertation, 2025
Thesis Jury
Prof. Dr. Ünal Ertan (Thesis Advisor)
Prof. Dr. Ersin Göğüş
Prof. Dr. Kazım Yavuz Ekşi
Prof. Dr. Emrah Kalemci
Asst. Prof. Dr. Ayşe Ulubay
Date & Time: July 18th, 2025 – 11:00 AM
Place: FENS 2019
Zoom Link: https://sabanciuniv.zoom.us/
Keywords : Accretion, Accretion Discs, Neutron Stars, Millisecond Pulsar
Abstract
Low-mass X-ray binaries (LMXBs) are the progenitors of millisecond pulsars (MSPs). Neutron stars (NSs) in these systems are spun up to millisecond periods by the accretion torques during their long-term evolution. Discovery of accreting millisecond X-ray pulsars (AMXPs) and transitional millisecond pulsars (tMSPs) has provided direct observational evidence for the evolutionary connection between MSPs and LMXBs. In this thesis, we investigate the long-term rotational evolution of LMXBs using the analytical model that can account for torque reversals and rotational phases of LMXBs. In the model, the inner disc radius depends on the spin period, the magnetic dipole moment, and the mass inflow rate of the accretion disc, Min. We have studied the long-lasting problem which is the absence of X-ray pulsations from most LMXBs. We investigated whether the extension of the inner disc down to the NS surface could be the mechanism that impedes the pulsed X-ray emission. We have shown that the inner discs of a large majority of LMXBs are likely to be at the surface of the NS, which can account for the lack of pulsations from these systems consistently with the pulsating AMXPs. Furthermore, we analyzed the long-term rotational evolution of NSs in LMXBs to the AMXP phase using the same model. We obtained the long-term Min. evolution used in this analysis from the binary evolution simulations with the MESA code. For the binary evolution, we obtained the initial conditions that can produce the typical binary evolution tracks in agreement with the observed orbital period, the mass-transfer rate of the donor, the donor mass and donor type of AMXPs. The Min histories estimated from these binary evolution can reproduce, in the analytical model, the rotational properties of the NSs in AMXPs as well.