Kütle aktarımı yapan pulsarlarda torkun gözlemsel olarak kısıtlanması

X-ray pulsars are neutron stars in binary systems accreting from a companion. The source of luminosity of these objects, reaching million solar luminosity, is the gravitational energy of the accreting matter. Accretion onto a compact object is a well known astrophysical process studied since early seventies. If the companion is massive the mass transfer occurs via the capture of the stellar wind by the neutron star. If the companion is of low mass the stellar wind is not significant. In this case mass transfer occurs by the Roche lobe overflow. In this case the companion star fills its Roche lobe either by swelling at an evolutionary stage or as a result of the shrinking binary separation as a consequence of energy loss from the system. The overflowing matter is transferred to the lobe of the neutron star through the first Lagrange point. The inflowing matter, as it carries angular momentum, can not directly hit onto the neutron star; instead it forms a disk. Mass transfer takes place by the matter loading onto the magnetosphere and then onto the polar caps of the neutron star. The interaction between the magnetosphere of a star with a surrounding accretion disk is a common theme in astrophysics. Disk accretion onto magnetic central objects occurs in a variety of astrophysical context involving accretion-powered X-ray pulsars. Numerous theoretical studies have been developed to understand the interaction between accretion disks and the magnetosphere of stars. One of them, proposed by Ghosh and Lamb in 1979 is the magnetically threaded disk (MTD) model which also forms the basis of this work. An important ingredient of the disk-magnetosphere interaction is the torque acting on the star depending on the rotation rate of the star. In this thesis, this relation is determined from observational data of an accreting X-ray pulsar, 4U 1626􀀀67. In the first chapter, disk magnetosphere interaction is summarized with general equations. The method that is used is explained in the second chapter. The method developed so as to eliminate all the parameters having uncertainty like mass of the accreting object and the distance. In the third chapter, observational data of 4U 1626􀀀67 is analyzed. This pulsar has a high magnetic field (approximately 1012 G) and is accreting from a low mass companion. As such it is identified as the most suitable source for determining the torque-rotation rate relation. This source underwent two torque reversals in June 1990 and February 2008. Also, application of the method is given in this chapter. Observational data belong to the 2008 reversal used in method. The results are summarized in the fourth chapter. It's found that, the torque exerted on the star by the disk depends on the cube of the rotation rate near the torque equilibrium. Away from the equilibrium the torque-rotation rate relation is erratic. Also, a comparison of our model with the some of the existing models are given in this section. In the last part of the thesis, the astrophysical significance of the results are discussed.