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R. E. Shirey (UC Santa Barbara), H. V. Bradt, A. M. Levine, E. H. Morgan (MIT)
We have carried out a study of the spectral and temporal X-ray variability of Circinus X-1 through extensive RXTE observations. Dramatic spectral evolution associated with intensity dips is well-fit by variable and at times heavy absorption of a bright component, plus an underlying faint component. An iron emission line at 6.4--6.6 keV present during dips maintains a constant absolute flux level outside the dips as well, indicating that the line is associated with the faint component. These results are consistent with a model in which the faint component is due to scattering of the bright component, with the addition of an iron line due to fluorescence.
We 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. 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. Tight correlations exist between the HBO frequency, the cutoff frequency and amplitude of flat-topped noise, and the frequency of a broad high-frequency QPO peak occuring at 20--200 Hz.
We also found that each branch of the spectral track is associated with a specific type of evolution of the energy spectrum, and we have parameterized the evolution of the spectrum in terms of a two-component model consisting of a multi-temperature ``disk blackbody'' and a higher-temperature isothermal blackbody. We also show that an unusual line- or edge-like feature occurs at about 11~keV in energy spectra from the flaring branch and lower normal branch.
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The author(s) of this abstract have provided an email address for comments about the abstract: shirey@orion.physics.ucsb.edu