The Complete “Z” Track of Circinus X‐1

We carried out an extensive RXTE campaign, in 1997 June, to study CircinusX-1 during the active portion of its 16.55-d intensity cycle. The observationsspanned 10 days, including 56% coverage for 7 d, and allowed us to find timesegments which clearly demonstrate continuous evolution along the horizontal,normal, and flaring branches (HB/NB/FB) of a Z-source low-mass X-ray binary.These results confirm and extend the behavior we inferred from earlierobservations. Here we study the continuous evolution of the Fourier powerspectra and the energy spectra around the complete hardness-intensity track. Anarrow quasi-periodic oscillation (QPO) peak, previously observed in the powerspectra at 1.3-32 Hz, increases in frequency from 12 Hz to 25 Hz moving down avertical extension of the HB in the hardness-intensity diagram. Thesehorizontal branch QPOs (HBOs) occur near 30 Hz and fade in strength on thehorizontal portion of the HB, while a broad peak in the power spectrum arisesnear 4 Hz. This peak becomes much more prominent along the NB and remains near4 Hz (the normal branch QPOs, or NBOs). On the FB, neither QPO is present andthe power spectrum is dominated by very low frequency noise. We also found thateach branch of the spectral track is associated with a specific type ofevolution of the energy spectrum. We explored various models for the energyspectrum and parameterized the evolution of the spectrum in terms of atwo-component model consisting of a multi-temperature "disk blackbody" and ahigher-temperature (~2 keV) blackbody. We also show that an unusual line- oredge-like feature occurs at ~10 keV in energy spectra from the FB and lower NB.This unusual feature is very similar to one seen on the FB and lower NB of theZ source GX 5-1.Comment: To be published in the May 20, 1999 issue of ApJ (Vol. 517). (19 pages, including 13 figures