X-ray Spectra of Magnetars: Models and Applications

Anomalous X-ray Pulsars (AXP) and Soft Gamma Repeaters (SGR) are young and isolated neutron stars that are believed to harbour the strongest magnetic fields (1014-15 Gauss) in the universe, as indicated by their energetic bursts, relatively slow spin and their rapid spindowns. However, a direct measurement of their surface field strengths has not been made todate. It is expected that strong surface magnetic fields have important effects on both the surface emission of neutron stars and their magnetospheres, which creates strong deviations from a zero field strenght assumption. Although the effects of strong surface magnetic field strenghts on the surface emission of neutron stars and on the structure of their magnetospheres have been studied, both effects have not been investigated together todate and X-ray spectra of these neutron stars have not been analysed within the framework of a model with physical assumptions. In this thesis, with an aim of analysing the X-ray spectra of these sources with physical models, Surface Thermal Emisson and Magnetospheric Scattering (STEMS) model have been developed, beased on two physical models calculated by Özel (2001, 2003) and Lyutikov and Gavriil (2006). Using the model, 54 X-ray spectra of 9 magnetars, neutron stars with strong magnetic fields, that are obtained in different dates with different satellites, have been analysed and the surface magnetic field strength, surface temperature as well as the optical depth and the velocity of charged particles in the magnetosphere of these sources have been inferred. In the light of the analysis, nature of these sources have been studied. In addition, by an analysis of all the archival X-ray observations, the changes of these parameters have also been investigated. As a result, STEMS model have been applied to all the archival observations, obtained by recent X-ray observatories (Chandra and XMM-Newton), of magnetars and the STEMS model could very well fit the X-ray spectra. Inferred results further show that the surface magnetic field strengths of these sources are in the order of ~2 - 6 x 10^14 Gauss as predicted by the theory. (Thesis is written in Turkish, copies can be obtained from the author.)