Asymmetries In Surface Temperature Distribution Of Neutron Stars

Abstract

Neutron stars are the final stage of the evolution of massive stars. They are formed when the core collapse of progenitor stars is halted by the degeneracy pressure of neutrons. As such, neutron stars have densities comparable to atomic nuclei, making them the densest objects with a surface known in the Universe. Neutron stars are born strongly magnetized (∼ 108 − 1015 G) and extremely hot(∼ 1011 K). Predominantly identified by their pulsed radio emission, thermal emission from the stellar surface of a cooling neutron star is detected in soft x-rays from several sources. The thermal emission is directly affected by various physical properties of the star, and hence, carries a wealth of information bearing directly on the stellar age, radius, mass, composition, and magnetic field structure. The shape of the pulse profiles is principally dependent on the thermal map of the stellar surface and the viewing geometry. Observations of asymmetric and multi-peaked pulse profiles from several sources hint to the presence of an anisotropic surface thermal map. The thermal map is determined by the stellar magnetic field topology. A study of asymmetric pulse profiles helps understand any deviations from the generally accepted dipolar magnetic field structure of neutron stars. This thesis investigates the model of neutron stars with a non-dipolar magnetic field topology where the second pole is shifted from the antipodal position (distorted-dipole). Using a general relativistic ray tracing software, populations of pulse profiles are generated with random viewing geometries and different temperature maps corresponding to the distorted- dipole field structure. Distributions of specific parameters that describe the anisotropies introduced in the thermal maps are compared with those of the observed sample of thermally emitting, isolated neutron stars from which x-ray pulsations have been detected (34 sources). Similar population studies have been performed for magnetars, the results of which are recreated in the thesis. Multiple studies that consider complex magnetic field structures to recreate asymmetric pulse profiles from thermally emitting sources have been done in the past for specific sources. These studies provide very broad conclusions as the shape of the pulse profile is affected by a variety of parameters. The novelty and relevance of this thesis is that it performs a general population study of specific parameters for a variety of randomized initial conditions which allows us to make more general and robust conclusions.