Physics in the neutron star crust and glitch phenomena
Akbal, Onur (2016) Physics in the neutron star crust and glitch phenomena. [Thesis]
Glitches are sudden changes in rotation frequency and spin-down rate, observed from pulsars of all ages. Standard glitches are characterized by a positive step in angular velocity ( > 0) and a negative step in the spin-down rate of the pulsar. There are no glitch-associated changes in the electromagnetic signature of rotationpowered pulsars most cases. For the first time, in the last glitch of PSR J1119-6127, there is clear evidence for changing emission properties coincident with the glitch. This glitch is also unusual in its signature. Further, the absolute value of the spin-down rate actually decreases in the long term. This is in contrast to usual glitch behaviour. In the first Chapter the vortex creep model is extended in order to take into account these peculiarities. It is proposed that a starquake with crustal plate movement towards the rotational poles of the star induces inward vortex motion which causes the unusual glitch signature. The component of the magnetic field perpendicular to the rotation axis will decrease, giving rise to a permanent change in the pulsar external torque. The vortex creep model explains the postglitch behaviour of Vela pulsar well, while it has the difficulties to estimate the interglitch time intervals. In the second Chapter it is hypothesized that for each Vela glitch there might be a persistent shift, which will not relax back, in the post-glitch “triangle” fashion of. This step would not be distinguished observationally at the time of the glitch. The modified expression for the time between glitches by using this consideration is applied for 14 Vela glitches by minimizing rms deviations between the model and observed glitch times. The estimates are in better agreement with the observed values with the persistent shift of 3 for all Vela glitches. Different _ p values for each Vela glitch are also calculated by inserting the observed interglitch times in the modified expression. Glitches are triggered by an initial crust breaking event. The size of the crust breaking is determined by the critical strain angle, cr. The broken crust plate size in turn determines the number of vortices involved in the unpinning avalanche that effects the size of the amplified glitch. The event of minimum glitch size of the Crab pulsar observed by Espinoza et al. (2014) is investigated in the third Chapter. Modelling the “pure” crustquake as a trigger mechanism with some breaking geometries, some physical quantities in neutron star crust, like the size of the broken plate, the critical strain angle at which fracture occurs, and the number of triggered vortices involved in larger glitches are estimated. In the final Chapter the critical strain angle in the Coulomb crystal in the neutron star crust is estimated on the assumption that this dimensionless number is of the order of the ratio of the Coulomb potential energy to the kinetic energy of the relativistic electrons.
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