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Measurements of the Mass and Radius of a Neutron Star: consequences for the Equation of State of dense nuclear matter

Vos, T. (2015) Measurements of the Mass and Radius of a Neutron Star: consequences for the Equation of State of dense nuclear matter. Bachelor's Thesis, Astronomy.

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The neutron star low-mass X-ray binary EXO 0748-676 started a transition from outburst to quiescence in 2008, after more than 24 years of continuous accretion. Here, I use the spectral fitting package XSPEC to analyse the X-ray spectra of four XMM-Newton observations, elapsing a period of over four years, that started in March 2009. I confirm that cooling NS models, modified by interstellar absorption, provide the best fits to the spectra. Both hydrogen and helium NS atmosphere models can account for the observed flux levels. I detect a gaussian line feature that accounts ~10% of the total flux. From the spectral fits, I infer limits to the mass and radius of the neutron star. I find that the recent outburst cannot by itself account for the mass reached by the neutron star. By comparing the best-fit masses and radii to theoretical predictions, I find that none of the proposed equations of state for dense, cold nuclear matter can account for the mass and radius predicted by the helium atmosphere model. Therefore, I conclude that the NS atmosphere of EXO 0748-676 is mainly composed of hydrogen. Furthermore, I deduce that the internal composition of neutron stars is dominated by neutrons and protons, rather than hyperons, kaons or quark matter. Finally, I find that the line feature is most likely due to imperfections in the abundance and cross-section tables used to describe the interstellar medium. If it is a real emission line, it is presumably produced by highly ionised carbon in the NS atmosphere.

Item Type: Thesis (Bachelor's Thesis)
Degree programme: Astronomy
Thesis type: Bachelor's Thesis
Language: English
Date Deposited: 15 Feb 2018 08:05
Last Modified: 15 Feb 2018 08:05

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