LEE- Fizik Mühendisliği Lisansüstü Programı

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http://hdl.handle.net/11527/19275

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Konu "Birikim diski" ile LEE- Fizik Mühendisliği Lisansüstü Programı'a göz atma
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    Interaction between magnetized stars and disks
    (Lisansüstü Eğitim Enstitüsü, 2021) Türkoğlu, Murat Metehan ; Ekşi, Kazım Yavuz ; 709913 ; Fizik Mühendisliği
    X-ray binary systems consist of a compact object, such as a neutron star, white dwarf or black hole, and a normal star that transfers mass to this compact object. X-ray binary systems are split into two groups depending on the mass of the donor star. If the mass of the donor star is Md ≤ 1M⊙, these kinds of systems are called LMXB and if the donor star mass is Md > 10M⊙, these systems are known as HMXB. The other component of the X-ray binary systems are compact stars: white dwarfs, neutron stars or black holes. The observed X-ray power of these systems originate from the gravitational potential energy released by the accretion of matter onto the compact star and depends on the compactness, M∗/R. In LMXB, matter from the outer envelope of the donor star may may be transferred to the compact star by Roche lobe overflow. In HMXB, matter from the outer envelope of the donor star may be transferred to compact star by stellar wind. In both cases because the matter transferred from the donor star has angular momentum, the matter can not accrete on to the compact object directly; instead an accretion disk forms. The physical parameters that define the interaction between a neutron star and a surrounding disk are the magnetic field and angular velocity of the of the compact star, and the mass flow rate in the disk. The interactions occur in three different stages: i-) Mass accretion stage: If the inner radius of the disk, Rm, is smaller than the corotation radius, Rco, the matter follows the magnetic field lines and flow to the polar caps of the neutron stars. ii-) Propeller stage: In this stage, Rm > Rco, the matter at the inner region of the disk meets with more rapidly rotating field lines attached to the star. A decline may occur in the observed X-ray flux because the mass accretion is centrifugally inhibited. iii-) Radio pulsar stage: If the inner radius of the disk is larger than the light cylinder radius, RL, an interaction can not occur between the neutron star and the disk. In this stage, the cause of the observed X-ray flux is the slowing down of the rotation of the neutron star. The QPO are thought to be generated in regions close to the neutron star and the inner part of the disk. Therefore, special types of QPOs provide direct evidence for disk-magnetosphere interaction. In this study, models were created by using both observational physical parameters (period, period derivative, luminosity, etc) and QPOs. The observation that the X-ray luminosity does not change significantly during transitions to the spin-down stage led to MTD of Ghosh & Lamb in 1979. In this model, magnetic field can thread the disk by instabilities between disk and magnetosphere and the presence of turbulence in the disk. The magnetic field lines slip around the disk due to the differential rotation between disk and the neutron star. According to the Ghosh & Lamb model, there is a stable region in which the twisted magnetic field balances the spread magnetic field around the disk. In this way a toroidal magnetic field is generated. However, as long as the magnetic field gets twisted around the disk, arbitrarily strong toroidal magnetic field is generated and such strong magnetic fields can destroy the disk. Because of the problems mentioned above, Ghosh & Lamb model have important inadequacies. The magnetic field lines that penetrate the disk beyond the corotation radius slow down the neutron star. The net torque acting on the neutron star is the sum of the material torque which spins up the star and the magnetic torque which slows down the star. Toroidal magnetic field is an important factor that determines the net torque. In order to understand the long-term evolution of the neutron star, it is important to specify how the torque depends on the fastness parameter, ω∗. As LMXB have weak magnetic fields, it is hard to observe the spin change of the system. Also HMXB have stellar winds that affect the torque and observed luminosity, the relation between the fastness and the torque can not be specified, sensitively. For these reasons, we choose 4U–1626 67 which has high magnetic field and accretes from a low mass donor star. 4U–1626 67 underwent two torque reversals in June 1990 and February 2008. We used the torque reversal data and explored the coherence between observational data and some torque models in the literature. It is discovered that each nearby galaxy host one or two "ultraluminous X-ray sources" (ULXs) whose luminosity exceed the Eddington limit for a solar mass object. It was initially assumed that the ULX host IMBH but later with the discovery of X-ray pulsations from some of these objects (e.g M82 X-2, ULX NGC 5907, ULX NGC 7793 P-13, NGC 300 ULX1, M51 ULX-7, NGC 1313 X-2 and Swift J0243.6+6124) showed that they at least some of them are neutron stars. Population studies indicate that the accreting neutron stars are common sources in the ULX population. In this thesis, we investigate the surface magnetic field dipole strength, beaming fraction and fastness parameter of the, PULX, taking into account the accretion flow in the super-critical regime, beaming of X-ray emission and the reduction of the scattering cross section in the presence of a strong magnetic field. We used three different methods for determining the magnetic fields of the PULX: i-) We assume the system to be near torque equilibrium. ii-) We rely on the spin-up rate and solve the torque equation. iii-) We assume the systems to be accreting at the critical rate. This critical rate depends on the electron scattering cross-section determined by the super-critical magnetic fields. The plan of the thesis is as follows: In Chapter 1, the main ideas of the star-disk interactions are given. In Chapter 2, the flux, the period and the period derivative data of 4U-1626 67 embracing the torque reversal events in June 1990 and February 2008 are analysed and compared with Ghosh-Lamb model and some other models in the literature. In Chapter 3, the magnetic fields of the pulsating X-ray sources are calculated using three different assumptions. Also, as the beaming fraction depends on the inner radius of the disc which in turn depends on the mass accretion rate, we find that the isotropic-equivalent luminosity of the source does not depend linearly on the mass accretion rate. In Chapter 4 all of the results are discussed