Circinus X-1: a Laboratory for Studying the Accretion Phenomenon in Compact Binary X-Ray Sources.

The observations of the binary X-ray source Circinus X-1 discussed in this thesis provide samples of a range of spectral and temporal behavior whose variety is thought to reflect a broad continuum of accretion conditions in an eccentric binary system. The data support an identification of three or more X-ray spectral components, probably associated with distinct emission regions: (1) A luminous optically thick component, associated with the cyclic outburst, has a spectrum which is to good approximation a blackbody of kT (TURN) 0.8 - 1.0 keV, with an apparent radius of (TURN) 40 kilometers for an assumed spherical emitter at a distance of 10 kiloparsecs. This component sometimes exhibits large amplitude fluctuations, including quasiperiodic oscillations on a timescale of seconds, corresponding to individual square shots of luminosity > 10('37) erg. The luminosity, size, and variability of this component suggest an origin in the inner region of an optically thick accretion disk around a compact object of mass(, )>(, )three solar masses. If the compact objects in Circinus and other black hole candidates such as Cygnus X-1 and GX339-4, which exhibit a component with similar characteristics, are neutron stars, they form a separate class whose properties are not predicted by any current model of accretion onto neutron stars. (2) A low level residual component, steady on a timescale of days, comes from a region larger than the binary system if the source is not continuously fed. The spectrum is optically thin, with little absorption and strong iron line emission. The latter suggests the presence of thermal gas, though a nonthermal contribution to the flux is also likely, from inverse Compton scattering of quiescent infrared flux by nonthermal electrons associated with the extended quiescent radio source. (3) A post-outburst X-ray flare component, possibly related to the radio and infrared flares, shows a flat, absorbed spectrum. Variability on a timescale of minutes restricts the emission region to << 10('13) cm, smaller than the radio flare region. A likely origin of the X-ray flares is inverse Compton scattering of infrared flux by a low energy extension of the nonthermal electrons producing the radio flares. Comparable luminosity in gamma -ray flares seems probable.