X-ray and optical observations of accreting neutron stars and black holes and the construction and testing of the stellar x-ray polarimeter

In the first part of this thesis, I present results from observations of accreting neutron stars and black hole candidates (BHCs). Through measurements of the physical parameters of neutron stars, including the mass, spin period and magnetic field strength, accreting neutron stars provide an opportunity to learn about matter at extremely high densities and in strong magnetic fields. Here, I study the neutron star X-ray binary XTE J2123–058. X-ray observations are used to estimate the neutron star spin period and magnetic field strength, and the measured spin period of 3.9 ms has implications for neutron star evolution. Results of optical observations provide some of the information necessary to eventually determine the neutron star mass.; For BHC X-ray binaries, demonstrating the existence of the event horizon and measuring physical parameters of black holes are of great interest. In order to make progress toward these goals, it is necessary to understand the accretion processes and emission mechanisms that operate in these systems. For some accreting compact objects, significant changes in the X-ray emission properties are observed over time. Here, I study the changes in X-ray emission properties that occurred over three months for an accreting BHC, 4U 1630–47. I also include a study of sharp drops in the 4U 1630–47 X-ray flux (i.e. X-ray dips). X-ray dips provide an opportunity to constrain the sizes of the X-ray emission regions. The 4U 1630–47 X-ray dips provide evidence that one of the two X-ray emission components comes from within ten Schwarzschild radii of the compact object.; In the second part of this thesis, I describe work I have done on the Stellar X-Ray Polarimeter (SXRP). The SXRP will be more than an order of magnitude more sensitive than any previous X-ray polarimeter in the 2–15 keV energy band and is expected to increase the number of sources with X-ray polarization detections from one, the Crab Nebula, to between 20 and 30. X-ray binaries will be primary SXRP targets because they are the brightest class of X-ray sources and because detectable polarization is expected. X-ray polarization measurements will improve our understanding of X-ray binaries since they provide a direct probe of source geometry and a means of distinguishing between different emission processes. Here, I describe results I obtained from the flight model calibration, including measurement of the graphite polarimeter modulation factor, which is 99.75% ± 0.11%, and experimental verification that systematic errors affecting polarization measurements can be reduced below the 1% level.