Timing and Spectral Studies of the Peculiar X-ray Binary Circinus X-1
Circinus X-1 (Cir X-1) is an X-ray binary displaying an array of phenomena which makes it unique in our Galaxy. Despite several decades of observation, controversy surrounds even the most basic facts about this system. It is generally classified as a Neutron Star (NS) Low Mass X-ray Binary (LMXB),though this classification is based primarily on the observation of Type I X-ray Bursts by EXOSAT in 1985. It is believed to be in a very eccentric {approx} 16.5 day orbit, displaying periodic outbursts in the radio and other frequency bands (including optical and IR) which reinforce the notion that this is in fact the orbital period. Cir X-1 lies in the plane of the Galaxy, where optical identification of the companion is made difficult due to dust obscuration. The companion is thought to be a low mass star, though a high mass companion has not currently been ruled out. In this work, the author analyzes recent observations of Cir X-1 made with the Unconventional Stellar Aspect (USA) experiment, as well as archival observations of Cir X-1 made by a variety of instruments, from as early as 1969. The fast (< 1 s) timing properties of Cir X-1 are studied by performing FFT analyses of the USA data. Quasi-Periodic Oscillations (QPOs) in the 1-50 Hz range are found and discussed in the context of recent correlations which question the leading models invoked for their generation. The energy dependence of the QPOs (rms increasing with energy) argues against them being generated in the disk and favors models in which the QPOs are related to a higher energy Comptonizing component. The power spectrum of Cir X-1 in its soft state is compared to that of Cygnus X-1 (Cyg X-1), the prototypical black hole candidate. Using scaling arguments the author argues that the mass of Cir X-1 could exceed significantly the canonical 1.4 M{circle_dot} mass of a neutron star, possibly partly explaining why this object appears so different to other neutron stars. The spectral evolution of Cir X-1 is studied by constructing both instrument-independent and model-independent color-color diagrams. Spectral fits are performed on all USA observations and physical parameters are derived. Cir X-1 periodically evolves from a high/soft state to a low/hard state within each orbital cycle. The spectral fits obtained indicate that a two-component model provides an appropriate description of the Cir X-1 spectrum with one component representing the emission from a multicolor blackbody accretion disk while the other component represents the higher energy Comptonized emission probably coming from a boundary layer on the surface of the neutron star. To study the long-term X-ray variability of Cir X-1 the author analyzes archival data from instruments going back to 1969 (Vela 5 satellite), up to more recent RXTE ASM data (for the last 6.5 years). The author uses various period finding techniques such as Lomb-Scargle periodograms and Phase Dispersion Minimization. Cir X-1 shows large variations in overall flux over the course of the last 30 years, with the brightest epoch corresponding to the recent RXTE/USA era. The author derives an X-ray ephemeris based on these long term observations showing that the period of Cir X-1 is rapidly decreasing (P/P < 3000 yrs), possibly implying a very young age for the system.
- Research Organization:
- SLAC National Accelerator Lab., Menlo Park, CA (United States)
- Sponsoring Organization:
- USDOE Office of Science (US)
- DOE Contract Number:
- AC03-76SF00515
- OSTI ID:
- 815635
- Report Number(s):
- SLAC-R-642; TRN: US0304745
- Resource Relation:
- Other Information: PBD: 26 Aug 2003
- Country of Publication:
- United States
- Language:
- English
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