Development and testing of hard X -ray imaging detectors and spectral evolution in X -ray bursters.

摘要

It has been more than twenty years since the last all-sky hard X-ray (>20 keV) survey was performed by the HEAO 1 A-4 instrument. A new, high-sensitivity survey, such as the EXIST concept, is urgently needed to extend soft X-ray surveys above 100 keV and enable the discovery of obscured AGN and supernova, remnants, study the spectra and variability of accreting black holes and neutron stars throughout the galaxy, and detect and study large samples of distant gamma-ray bursts and soft gamma-ray repeaters. In this thesis I describe three projects aimed both at the development of advanced hard X-ray and gamma-ray instrumentation needed for a new all-sky survey mission and at the study of astronomical objects in the hard X-ray band. The first is the design, construction, and testing of prototype gamma-ray detectors made of cadmium-zinc-telluride (CZT). CZT is a high-Z, wide-bandgap semiconductor that shows great promise as the hard X-ray detector material of the future. I demonstrate techniques for constructing wide-field survey telescopes from this material and show through balloon flight tests that the background in CZT with proper shielding is comparable to existing detector materials. The second project is the development of and analysis of data from the EXITE2 hard X-ray balloon payload, a coded-aperture, imaging phoswich detector. EXITE2 was flown in 1997, and I describe a system for the spectral analysis of flight data, including the generation of a response matrix that incorporates all the unique features of an imaging phoswich telescope. Finally, I present a detailed spectral analysis in the medium hard X-ray band of two ultra-compact (orbital periods of 11 minutes and 50 minutes) low mass X-ray binaries (LMXBs) and X-ray bursters using data from RXTE. I show that, for a variety of spectral models, the spectral parameters are strongly correlated with the accretion rate of the system, as parameterized by position in the color-color diagram (CCD). Hard X-ray emission in these LMXBs is confined to the lowest accretion rates and thus the dimmest states; a large-area survey telescope would therefore also be well-suited to studying the hard X-ray spectra of these and other systems.