Instrumentation and data analysis for hard X-ray astronomy

Zhang, Shuang Nan (1989) Instrumentation and data analysis for hard X-ray astronomy. University of Southampton, Doctoral Thesis.

Abstract

Observations of many astronomical objects at the hard x-ray wavelength will provide new and important information about the physical conditions in the source regions. The current status of hard x-ray astronomy is reviewed and it is concluded that the next generation of hard x-ray telescopes will require higher sensitivity, good spectral resolution and, importantly, an imaging capability so that the sources may be located accurately for study in other wavebands. The applicability of various hard x-ray detectors in such new telescopes are discussed and the current status of hard x-ray telescope design is reviewed. It is concluded that no adequately large area, imaging, hard x-ray telescope exists to fulfill the great need for further hard x-ray astronomy observations. Background production mechanisms in hard x-ray telescopes are discussed and some techniques for background reduction are also discussed. A simple method for estimating the optimum detector thickness is presented and the use of some detector materials which should minimise the neutron background is proposed. Three imaging techniques are briefly described. A simple discussion about their relative merits has suggested that the RMC technique based on an improved detector technique provides the best imaging system. The Pulse Shape Discrimination (P.S.D.) technique is essential for the realization of such new detectors. The basic principles of various P.S.D. methods are introduced and theoretical methods for analysing their performance are developed. Using these methods, numerical comparisons of various kinds of P.S.D. methods are made for hard x-ray phoswich detector applications. A new fast-veto technique is proposed and may be used in a quadruple phoswich detector system to provide good energy resolution and efficient background rejection. A proposed satellite-borne hard x-ray telescope, CHIXSAT, which would use such detectors as its main detection elements, is briefly described. A range of statistical data analysis methods for hard x-ray and γ-ray astronomy are studied. Fisher's exact test is found to give the most reliable significance test. Methods for parameter estimation, including upper limit calculations, are described. The sensitivity of a telescope is defined with clear statistical meaning and a method for estimating the sensitivity is derived. A new method, called _X^3-test is proposed to be unbiased and have a very high power for multiple `on-off' observations especially in searching for transient sources. Based on decision theory, Stein's estimator is found to give higher precision than the normally used maximum likelihood method for estimating the source intensity if multiple `on-off' observation mode is used.

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