Many techniques for measuring neutron star radii rely on absolute flux measurements in the X-rays. As a result, one of the fundamental uncertainties in these spectroscopic measurements arises from the absolute flux calibrations of the detectors being used. Using the stable X-ray burster, GS 1826-238, and its simultaneous observations by Chandra HETG/ACIS-S and RXTE/PCA as well as by XMM-Newton EPIC-pn and RXTE/PCA, we quantify the degree of uncertainty in the flux calibration by assessing the differences between the measured fluxes during bursts. We find that the RXTE/PCA and the Chandra gratings measurements agree with each other within their formal uncertainties, increasing our confidence in these flux measurements. In contrast, XMM-Newton EPIC-pn measures 14.0 +/- 0.3% less flux than the RXTE/PCA. This is consistent with the previously reported discrepancy with the flux measurements of EPIC-pn, compared with EPIC MOS1, MOS2, and ACIS-S detectors. We also show that any intrinsic time-dependent systematic uncertainty that may exist in the calibration of the satellites has already been implicity taken into account in the neutron star radius measurements.

The hydrogen and helium accreted by X-ray bursting neutron stars is periodically consumed in runaway thermonuclear reactions that cause the entire surface to glow brightly in X-rays for a few seconds. With models of the emission, the mass and radius of the neutron star can be inferred from the observations. By simultaneously probing neutron star masses and radii, X-ray bursts (XRBs) are one of the strongest diagnostics of the nature of matter at extremely high densities. Accurate determinations of these parameters are difficult, however, due to the highly non-ideal nature of the atmospheres where XRBs occur. Observations from X-ray telescopes such as RXTE and NuStar can potentially place strong constraints on nuclear matter once uncertainties in atmosphere models have been reduced. Here we discuss current progress on modeling atmospheres of X-ray bursting neutron stars and some of the challenges still to be overcome.

Nuclear physics tells us a lot about astrophysics, particularly the energy generation in stars. The present work is a thesis in experimental nuclear physics, reporting the results of 30S radioactive beam development for a future experiment directly measuring data to extrapolate the 30S(α,p) stellar reaction rate in Type I X-ray bursts, a phenomena where nuclear explosions occur repeatedly on the surface of accreting neutron stars. On the astrophysics side, the work details basic stellar physics and stellar reaction formalism in Chapter 1, the behaviour of compact stars in Chapter 2, and a full literature review of Type I X-ray bursts in Chapter 3. Nuclear experiments are non-trivial, and the results reported here were not accomplished by the author alone. Stable-beam experiments are technically challenging and involved, but for the case at hand, the halflife of 30S is a mere 1.178 seconds, and in order to measure reaction cross-sections, we must make a beam of the radionuclide 30S in situ and use these rare nuclei immediately in our measurement. Particle accelerator technology and radioactive ion production are treated in Chapter 4, and the experimental facility and nuclear measurement techniques are discussed in some detail in Chapter 5. In order to perform a successful future experiment which allows us to calculate the stellar 30S(α, p) reaction rate, calculations indicate we require a 30S beam of ~ 10^5 particles per second at ~ 32 MeV. Based on our recent beam development experiments in 2006 and 2008, it is believed that such a beam may be fabricated in 2009 according to the results presented in Chapters 6 and 7. We plan to measure the 4He(30S,p) cross-section at astrophysical energies in 2009, and some remarks on the planned (α,p) technique are also elucidated in Chapters 5, 6 and 7. / Thesis / Master of Science (MSc)