Quasi-periodic Oscillations Associated with Spectral Branches in Rossi X-Ray Timing Explorer Observations of Circinus X-1
Abstract
We present Rossi X-Ray Timing Explorer (RXTE) All-Sky Monitor observations of the X-ray binary Circinus X-1 that illustrate the variety of intensity profiles associated with the 16.55 day flaring cycle of the source. We also present eight observations of Cir X-1 made with the RXTE Proportional Counter Array over the course of a cycle wherein the average intensity of the flaring state decreased gradually over ~12 days. Fourier power density spectra for these observations show a narrow quasi-periodic oscillation (QPO) peak that shifts in frequency between 6.8 and 32 Hz, as well as a broad QPO peak that remains roughly stationary at ~4 Hz. We identify these as Z-source horizontal and normal branch oscillations (HBOs/NBOs), respectively. Color-color and hardness-intensity diagrams (CDs/HIDs) show curvilinear tracks for each of the observations. The properties of the QPOs and very low frequency noise allow us to identify segments of these tracks with Z-source horizontal, normal, and flaring branches that shift location in the CDs and HIDs over the course of the 16.55 day cycle. These results contradict a previous prediction, based on the hypothesis that Cir X-1 is a high-Ṁ atoll source, that HBOs should never occur in this source.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- October 1998
- DOI:
- 10.1086/306247
- arXiv:
- arXiv:astro-ph/9803278
- Bibcode:
- 1998ApJ...506..374S
- Keywords:
-
- STARS: INDIVIDUAL: CONSTELLATION NAME: CIRCINUS X-1;
- STARS: NEUTRON;
- STARS: OSCILLATION;
- X-RAYS: STARS;
- Stars: Individual: Constellation Name: Circinus X-1;
- Stars: Neutron;
- stars: oscillation;
- X-Rays: Stars;
- Astrophysics
- E-Print:
- To be published in the October 10, 1998 issue of The Astrophysical Journal (Vol 506)