LEE- Fizik Mühendisliği-Doktora
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Yazar "Ekşi, Kazım Yavuz" ile LEE- Fizik Mühendisliği-Doktora'a göz atma
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ÖgeInteraction between magnetized stars and disks(Lisansüstü Eğitim Enstitüsü, 2021) Türkoğlu, Murat Metehan ; Ekşi, Kazım Yavuz ; 709913 ; Fizik MühendisliğiX-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
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ÖgeModeling the magnetosphere of neutron stars with numerical simulations(Graduate School, 2022-06-27) Çıkıntoğlu, Sercan ; Ekşi, Kazım Yavuz ; 509152107 ; Physics EngineeringIn this thesis, I study the magnetosphere of a neutron star in two different contexts. Firstly, I investigate the interaction between an accretion disc and the magnetosphere of a neutron star. I perform a number of two spatial dimensional general relativistic magnetohydrodynamics simulations within the ideal magnetohydrodynamics limit by employing Black Hole Accretion Code. I vary the strength of the dipole magnetic field of the star while keeping the other parameters fixed. I initialise a thick torus around the star and trigger a magnetorotational instability to drive the disc towards the star. I determine the magnetospheric radius numerically and then investigate how it depends on the magnetic dipole moment and the mass accretion rate. I find that the magnetospheric radius is proportional to the magnetic dipole moment as in the Newtonian case, i.e., r_{msph}\propto \mu^{4/7}, but also that it depends weakly on the mass-accretion rate. Also, I calculate the mass accretion rate and the angular momentum transfer rate. I investigate the correlation between the mass accretion rate and the matter part of the angular momentum transfer rate and find that they are almost linearly correlated. On the other hand, I observe that the total angular momentum transfer rate fluctuates vividly even though the system reaches a steady-state. The amplitudes of the fluctuations are so large that the angular momentum transfer rate sometimes takes negative values. These could be associated with the spin fluctuations observed in X-ray pulsars. I observe that the discs driven by the magnetorotational instability are quite different than the constant alpha-viscosity discs. The disc quantities within the disc such as the pitch factor and the alpha-parameter exhibit fluctuations larger than their time averages. Secondly, I investigate newly born magnetars by modelling X-ray afterglow lightcurves following gamma-ray bursts. I employ the magnetic dipole torque of the plasma-filled magnetosphere and a decaying magnetic field. I find approximate analytic solutions for the torque equations. By modelling the X-ray afterglows within this model, I determine the initial period, the inclination angle, magnetic dipole moment as well as the time scale of the decay of the magnetic moment and its asymptotic value. Finally, I study fallback discs with low-angular momentum, hence short lifetime, around newly born neutron stars in the context of X-ray afterglow lightcurves following gamma-ray bursts. Some models of gamma-ray burst afterglows invoke fallback discs interacting with the magnetospheres of nascent millisecond magnetars. Initially, the accretion rate in such a disc is very high, well exceeding the rate required for the Eddington limit. Inner parts of such a disc get spherical due to the radiation pressure and the mass accretion rate within the spherization radius is regulated so that the Eddington luminosity is exceeded only logarithmically. This restrains the achievable luminosity produced by the disc-magnetosphere interaction to very low levels compared to the typical luminosities observed in the X-ray afterglow light curves. Due to the high magnetic field and the spin frequency of the magnetar, the disc cannot penetrate the light cylinder and cannot interact with the magnetosphere until the star slows down sufficiently by magnetic dipole radiation. Accordingly, the interaction of the fallback disc with the star during the first few days in the life of the star is very unlikely. Even if they interact, it would be hard to observe since the required drop in the spin frequency would lead to an abrupt drop in the X-ray luminosity which is larger than the sensitivity range of Swift's XRT telescope. We conclude that a fallback disc model can only address sources with unusually low luminosities.
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ÖgePulsar magnetospheres and intra - pulse variability by plasmoid formatio(Graduate School, 2024-12-06) Andaç, İbrahim Ceyhun ; Ekşi, Kazım Yavuz ; 509152103 ; Physics EngineeringPulsars are remnants of massive stars that have endured supernova explosions. They are rapidly rotating neutron stars with extraordinary magnetic fields. These celestial objects serve as natural laboratories for studying physics under extreme conditions that includes physics of dense matter, strong-field gravity, and relativistic plasma processes. Their magnetospheres are dominated by strong magnetic fields and populated by an electron-positron plasma. It exhibits complex dynamics that give rise to a broad spectrum of electromagnetic emissions which spans from radio waves to gamma-rays. Among the various phenomena observed, pulse-to-pulse variability offers critical insights into the underlying plasma processes and emission mechanisms. It also includes intra-pulse structures such as subpulses. The precise origins of this variability remain incompletely understood despite extensive research. It is posing a significant challenge in pulsar astrophysics. What is investigated in this thesis is the role of plasmoid formation induced by magnetic reconnection in pulsar magnetospheres and its impact on intra-pulse variability. Magnetic reconnection in the current sheet beyond the light-cylinder radius ($\rlc$) is a fundamental plasma process. It can lead to the formation of plasmoids—coherent structures consisting of plasma and magnetic fields. The intermittent nature of reconnection and the hierarchical merging of plasmoids can introduce significant fluctuations in the emitted radiation. It is potentially accounting for the observed pulse-to-pulse variability and subpulse phenomena. To examine this hypothesis, we employ two-dimensional particle-in-cell (PIC) simulations using the \texttt{ZELTRON} code, modeling an orthogonal pulsar magnetosphere restricted to the equatorial plane. The simulations begin with a split magnetic monopole configuration and evolve into a quasi-steady-state force-free magnetosphere filled with electron-positron plasma. Plasma is continuously injected from the neutron star surface, ensuring a sustained supply of charged particles. The wind current sheet forms as two thin Archimedean spirals extending beyond $\rlc$, where magnetic reconnection triggers the tearing instability. This instability leads to the fragmentation of the current sheet into a dynamic chain of plasmoids, which evolve through processes of growth, merging, and outward propagation. Our simulations expose that plasmoids are predominantly formed near the light cylinder and subsequently grow and merge hierarchically as they move out at relativistic speeds. The size distribution of plasmoids ranges from small-scale structures on the order of $0.02 \rlc$ to large entities comparable to the neutron star radius ($\sim 0.3 \rlc$). Statistical analysis indicates that the plasmoid size distribution follows an inverse relationship with their width. This is consistent with theoretical predictions for hierarchical merging in relativistic reconnection processes. This distribution suggests that a multitude of small plasmoids coexist with fewer large ones, contributing differently to the emission characteristics. We compute synthetic synchrotron emission profiles by tracking particles and electromagnetic fields within the simulation. Our computation focuses on the incoherent high-energy emission originating from the current sheet. The results demonstrate that plasmoid formation leads to significant intra-pulse variability. This variability is manifested as bright subpulses superimposed on the main pulse profile. The subpulses predominantly appear on the leading edge of each pulse and exhibit a broad range of fluxes and durations. A key finding is the proportionality between subpulse flux and width in pulsar phase. This correlation indicates that the larger plasmoids produce brighter and broader subpulses. This relationship arises because larger plasmoids contain more energetic particles and occupy a greater angular extent. It enhances their contribution to the observed emission. The statistical properties of the subpulses align with the plasmoid size distribution with the number of subpulses following a power-law dependence on their flux and width. This correlation suggests that the observed intra-pulse variability is directly linked to the dynamics of plasmoid formation and evolution within the magnetosphere. The power-law behavior indicates that there is a significant probability of observing exceptionally bright and wide subpulses. This corresponds to the fact that the largest plasmoids formed through merging events while most subpulses are weak and narrow. Advanced analysis shows that the intermittent nature of magnetic reconnection leads to episodes where the current sheet thickens near the light cylinder. It temporarily suppresses plasmoid formation. These quiet periods result in reduced emission and are followed by recovery phases. The intense subpulses reappear in the recovery phases and this corresponds to the delayed fragmentation of the stretched current sheet. This behavior contributes to the overall variability observed in the emission profiles. It may explain observed phenomena such as nulling and mode changes in some pulsars. Our study offers several testable predictions.