Long-term X-ray Luminosity and Rotational Evolution of Accreting Millisecond X-ray Pulsars
Fatmanur Ertuğrul
Physics, MSc. Thesis, 2025
Thesis Jury
Prof. Dr. Ünal Ertan (Thesis Advisor)
Prof. Dr. Emrah Kalemci
Asst. Prof. Dr. Ayşe Ulubay
Date & Time: 18th July, 2025 – 1:00 PM
Place: FENS 2019
Zoom Link: https://sabanciuniv.zoom.us/j/95305079175
Keywords : Accretion, Accretion Discs, Neutron Stars, Accreting Millisecond X-ray Pulsars
Abstract
Accreting millisecond X-ray pulsars (AMXPs) are neutron stars in low mass X-ray binaries (LMXBs) spun-up through mass accretion from their companion. To date, 26 AMXPs including three transitional millisecond pulsars (tMSPs) have been observed with spin periods shorter than 10![]()
ms. All known AMXPs are transient sources likely to be in the final stages of the LMXB evolution, showing periodic X-ray pulses during their outbursts. It is important to study the rotational evolution of AMXPs since there are uncertainties in the disc-field interaction and torque mechanisms leading these sources to their descendants namely the radio millisecond pulsars (RMSPs). Observations show that the average torques acting on these systems during their long-term evolution are spin-down torques. In this thesis, we investigate the torques acting on the AMXPs during their outburst and quiescent states. We use observational data of five AMXPs which have been observed during more than one outburst and have spin period derivative measurements for both outburst and quiescent states. We have found that the spin-up torques during the shorter outbursts are dominated by the spin-down torques during the much longer quiescent states, resulting in a net spin-down in their long-term transient evolution. Our model results are consistent with the observed X-ray luminosities and rotational properties of these five sources, which indicate that long-term rotational evolution of AMXPs can be reproduced by the accretion torque, disk (magnetic) torque, and magnetic dipole torque, without invoking any additional external torque mechanisms.
