Characteristics of Neutron Stars from X-Rays Observations

• Author / Creator

  Elshamouty, Khaled G.

• Neutron stars (NSs) are some of the densest objects in the universe. In this thesis, I focus on studying X-ray thermal emission from neutron stars, using X-ray observations of different varieties of NSs. X-ray observations can put constraints on the mass and radius of NSs, and thus on density and pressure of neutron star interiors, with consequences for the dense matter equation of state. X-ray observations can also constrain the temperature of NSs, with implications for the superfluidity of the core, and the composition and superfluid state of the crust. In the first part of this thesis, I constrain the cooling of the NS surface temperature in the young NS in the Cas A supernova remnant, using X-ray flux measurements over 10 years using several detectors on the Chandra X-ray Observatory. Although measurements using Chandra’s ACIS-S detector show fast cooling, measurements using Chandra’s HRC-S detector find a significantly slower rate of cooling for this NS. Fitting the thermal X-ray spectra of neutron stars in quiescent X-ray binaries can be used to measure the NS radii. However, the effect of undetected hot spots on the spectrum, and thus on the inferred NS mass and radius, has not yet been explored for appropriate atmospheres and spectra. A hot spot would harden the observed spectrum, so that spectral modeling tends to infer radii that are too small. However, a hot spot may also produce detectable pulsations. I simulated the effects of a hot spot on the pulsed fraction and spectrum of the quiescent NSs X5 and X7 in the globular cluster 47 Tucanae using appropriate spectra and beaming for hydrogen atmosphere models, incorporating special and general relativistic effects, and sampling a range of system angles. A search for pulsations in archival Chandra HRC-S observations of two quiescent NS low-mass X-ray binaries in 47 Tuc, X5 and X7, places a 90% confidence upper limits on their pulsed fractions below 16%. These pulsation limits constrain the temperature differential of any hot spots. I then constrain the effects of the possible hot spots on the X-ray spectrum and the inferred radius from spectral fitting. Hot spots below our pulsation limit could bias the spectroscopically inferred radius downward by up to 28%. For Cen X-4 (which has deeper published pulsation searches), an undetected hot spot could bias its inferred radius downward by up to 10%. Improving constraints on pulsations from quiescent LMXBs is essential for progress in constraining their radii. Finally, I investigate the spectrum of a high-magnetic-field NS in a high-mass X-ray binary (HMXB). The propeller effect in accreting rotating neutron stars should cut off accretion in fast-spinning neutron star HMXBs at low mass transfer rates. However, the accretion continues in some HMXBs, as evidenced by continuing pulsations, at low luminosities. Indications of spectral softening in systems in the propeller regime suggest that some HMXBs are undergoing fundamental changes in their accretion regime. I use a 39 ks XMM-Newton observation of the transient HMXB V0332+53 at a very low X-ray luminosity to study the source of the X-ray emission in quiscence and constrain the surface temperature of the NS.

• Subjects / Keywords

  • Neutron Stars
  • X-ray

• Graduation date

  Fall 2016

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3VM4376M

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.