Long-Term X-Ray Luminosıty And Rotational Evolution Of Accreting Milisecond X-Ray Pulsars Evolution Of Accreting Millisecond X-Ray Pulsars

Accreting millisecond X-ray pulsars (AMXPs) are neutron stars in low mass X-raybinaries (LMXBs). They spun-up through mass accretion from their companion. Todate, 26 AMXPs including three transitional millisecond pulsars have been observedwith spin periods shorter than 10 ms. All known AMXPs are transient sources likelyto be in the final stages of the LMXB evolution, showing periodic X-ray pulsesduring their outbursts. It is important to study the rotational evolution of AMXPssince there are uncertainties in the disc-field interaction and torque mechanismsleading these sources to their descendants namely the radio millisecond pulsars.Observations show that the average torques acting on these systems during theirlong-term evolution are spin-down torques. In some conventional models, observedlong-term spin-down behaviour of AMXPs require external spin-down torques, likegravitational radiation, in addition to disc and magnetic dipole torques. In thisthesis, we investigate the torques acting on the AMXPs during their outburst andquiescent states. We use observational data of five AMXPs which have been observedduring more than one outburst and have spin period derivative measurements forboth outburst and quiescent states. We have found that the spin-up torques duringthe shorter outbursts are dominated by the spin-down torques during the muchlonger quiescent states, resulting in a net spin-down in their long-term transientevolution. Our model results are consistent with the observed X-ray luminositiesand rotational properties of these five sources, which indicate that the long-termrotational evolution of AMXPs can be reproduced by the accretion torque, disc(magnetic) torque, and magnetic dipole torque, without invoking any additionalexternal torque mechanisms.