Modeling the magnetosphere of neutron stars with numerical simulations

Çıkıntoğlu, Sercan
Süreli Yayın başlığı
Süreli Yayın ISSN
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Graduate School
In 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.
Thesis(Ph.D.) -- Istanbul Technical University, Graduate School, 2022
Anahtar kelimeler
astrophysics, astrofizik, physics, fizik, neutron stars, nötron yıldızlar