I. Development of the energetic X-ray imaging telescope experiment; and, II. Studies of low-mass X-ray binaries

In Part I, the design, development and testing of the Energetic X-ray Imaging Telescope Experiment (EXITE) are described. EXITE should have sensitivity sufficient to carry out many important observations of the high energy X-ray emission of galactic and extra-galactic sources. The combination of moderately large effective area, coded aperture imaging, and improved energy resolution should allow some of the most accurate measurements of high energy X-ray spectra to date.; In Part II, I report on the results of a survey of the temporal and spectral variations of {dollar}sim{dollar}40 low mass x-ray binaries (LMXRB) observed with the EINSTEIN MPC. A variety of methods have been used to search for correlations among measured spectral and temporal parameters with the following results: (1) As has been found in previous studies, the spectra of many LMXRB harden (on relatively short time scales) with increasing flux. Remarkably, the spectra of a number of the lower luminosity LMXRB soften (on time scales {dollar}>{dollar} 1 month) with increasing flux. This behavior appears to be due to Compton cooling in a surrounding corona. (2) The most x-ray luminous LMXRB often have hardness ratios which are bi-valued with luminosity, in contrast with the less luminous sources. These more luminous sources are also preferentially detected as radio emitters. These differences may indicate the presence of an additional energy generation mechanism (which may be the spin-down of a rapidly rotating neutron star) in the brightest sources. (3) A few of the more interesting sources have been followed up at x-ray, radio, optical and infrared (IR) wavelengths, with the following results: (a) Simultaneous observations of GX13 + 1 with the VLA and EXOSAT show uncorrelated x-ray and radio variations in this LMXRB. Infrared observations have revealed a bright counterpart to this LMXRB. (b) Discovery of a burst from 4U2129 + 47 identifies the compact component as a neutron star, which is particularly intriguing in light of previous measurements of its mass (0.6 {dollar}pm{dollar} 0.2 M{dollar}sb{lcub}odot{rcub}{dollar}). This is the only neutron star with a mass measured to be significantly lower than the Chandrasekhar limit.