Timing and Spectral Studies of the Peculiar X-ray Binary Circinus X-1

Abstract

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 ∼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, I analyze 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 I argue that the mass of Cir X-1 could exceed significantly the canonical 1.4 M mass of a vii

X-Ray Timing and Spectral Observations of Galactic Black Hole Candidate XTE J1550--564 During Outburst

“…Photons in the energy range ∼ 1-16 keV will be detected according to the overall energy dependent efficiency of the chamber. Figure 3.6 shows the USA detector efficiency as a function of energy, for the complete details of the USA energy calibration see Saz Parkinson 2003. Photons with energies much greater than ∼ 16 keV have a very small probability of begin stopped in the chamber and, thus, go undetected.…”

“…3 The USA orbit may be broken into a day side and night side which corresponds to the ascending node and descending node, respectively. 4 The details of the USA energy calibration can be found in Saz Parkinson 2003 USA's use of offset pointing has also been mentioned. The reason for using offset pointing was to meet the needs of other experiments on ARGOS.…”

“…The labels AN and SP refer to observations taken on the ascending node and over the south pole, respectively. thermal cycling in the instrument as it progresses through its orbit (Saz Parkinson 2003). This means that there is a small shift in the energy of a PHA channel as USA moves through its orbit (Saz Parkinson 2003).…”

“…thermal cycling in the instrument as it progresses through its orbit (Saz Parkinson 2003). This means that there is a small shift in the energy of a PHA channel as USA moves through its orbit (Saz Parkinson 2003). The XTE J1550-564 data were taken in two different orbital nodes, so it was necessary to make sure that gain changes were not the cause of the cyclic structure in Figure 5.2 and to make sure that these gain changes did not have a significant impact on the hardness ratios shown in Figure 5.1.…”

“…This allows mktimeusa to make background cuts based on the difference and ratio, between the top and bottom layer, of PHA channel 15, for modes 1 and 2. The efficiencies for channel 15 in each of the layers is known to be 0.022 and 0.026, for the top and bottom layer, respectively(Saz Parkinson 2003). Mktimeusa calculates rates for the top and bottom layer in channel 15 and then calculates a channel 15 difference and ratio for the detector, see…”