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MSc. Thesis Defense: Fatmanur ERTUĞRUL, Long-term X-ray Luminosity and Rotational Evolution of Accreting Millisecond X-ray Pulsars

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 10ms. 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.