Yengeç atarcasının azalan eylemsizlik momentiyle açılan eğiklik açısı

A star with a mass between $8-22$ $M_{\odot}$ becomes a neutron star when it has used up all of its nuclear energy sources. This transition occurs with a gigantic supernova explosion. Today there are many types of neutron stars classified according to their energy source. The first discovered neutron stars are the rotationally powered pulsars. These are highly magnetized rapidly rotating objects which are slowly spinning down, and the rate of rotational kinetic energy lost forms their luminosity. These objects, presumably the younger ones, radiate in all bands of the electromagnetic spectrum yet they are generally detected in the radio wavelength. The first models envisaged the pulsars as a magnetic dipole rotating in vacuum converting the kinetic energy to magnetic dipole radiation (MDR). It was soon understood that the rotating dipole generates electric fields large enough to rip off charged particles from the surface of the neutron star which then form a co-rotating plasma. It is harder to solve the magnetospheric structure in the presence of this plasma, but recent numerical simulations provide insight into this problem. Even in that case, the basic dependence of the spin-down torque on the pulsars in the presence of the plasma is similar to the MDR model. Interestingly both models predict that the inclination angle between the magnetic moment and the spin axis would be aligned in time. The Crab pulsar is the best known and studied rotationally powered pulsar among the more than 2000 pulsars discovered to date. It is about 960 years old according to the ancient Chinese records, which call the supernova explosion as a `guest star'. Lyne et al. discovered in 2013 that the inclination angle of this object is not decreasing, as expected from the MDR or the plasma model, but increasing at a rate of 0.62 degrees per century. We argue that this observation can be explained if the moment of inertia of this object decreases in time. In standard estimations of pulsar properties the moment of inertia and magnetic dipole moment are assumed to be constant. In this thesis, I investigate the implications of a decrease in the moment of inertia on the spin evolution of this object. The Crab pulsar, just like many other young pulsars, occasionally show sudden spin-up events called ``glitches''. The glitch events of the Crab pulsar are explained by the starquake model. According to this model the solid crust of the Crab pulsar, which is formed when the object cooled down while rotating much more rapidly, can not follow the equilibrium shape corresponding to the rotation rate which slowed down. The stress increases as the pulsar slows down further and finally the crust of the neutron star fractures. Recently, we had shown that the rate of decrease of the moment of inertia as a result of these crustal fractures is sufficient to explain the rate of increase of the inclination angle.