Mass transfer and accretion in the eccentric neutron-star binary Circinus X-1

Abstract

I have carried out a project to study the eccentric neutron-star binary Circinus X-1 through an extensive series of observational studies with the Rossi X-ray Timing Explorer satellite and through theoretical computer models I developed to explore mass transfer and evolution in an eccentric binary. We also organized two multi-frequency campaigns to study correlated variability in different frequency bands. The X-ray observations showed that the intensity of Cir X- 1 currently maintains a bright baseline level, with strong flares occurring after phase zero of each 16.55-day cycle of the source. This behavior is thought to be due to enhanced mass transfer occurring near periastron of a highly eccentric binary orbit. Dips below the baseline intensity level also occur near phase zero. I modeled the evolution of the energy spectrum during dips with a variably absorbed bright component plus a fainter unabsorbed component. I show that variability not attributable to absorption dips is related to the spectral/intensity states of the "Z source" class of low-mass X-ray binaries (LMXBs), namely motion along (or shifts of) the horizontal, normal, and flaring branches of the "Z" track in color-color and hardness-intensity diagrams. I found rapid X-ray variability properties associated with each spectral/intensity state: On the horizontal branch, quasi-periodic oscillations (QPOs) in the X-ray intensity shift in frequency from 1.3 to 35 Hz. On the normal branch, a different QPO occurs at about 4 Hz. On the flaring branch only strong aperiodic variability occurs. I modeled the evolution of the energy spectra associated with each of these branches. To study mass transfer in an eccentric binary, I developed computer codes for transfer via Rochelobe overflow and from a stellar wind. I derive theoretical mass accretion profiles and compare them to the observed profile of the X-ray intensity. In order to explore the possible evolutionary history of Circinus X-1, I developed a binaryevolution computer code for a neutron-star and low-mass companion in an eccentric orbit. I use this code in a population-synthesis study to show that the number of systems in the Galaxy expected to resemble Cir X-1 is of order unity, consistent with its unique status as an LMXB with high eccentricity.