Robert Shirey

First Results from the All-Sky Monitor on the Rossi X-Ray Timing Explorer

The all-sky monitor on the Rossi X-Ray Timing Explorer has been monitoring the sky in the 1.5-12 keV band since late February. The instrument consists of three coded-aperture cameras that can be rotated to view different regions by a motorized drive assembly. Intensities of ~100 known sources are obtained via least-squares fits of shadow patterns to the data and compiled to form X-ray light curves. Six orbital periodicities and four long-term periodicities, all previously known, have been detected in these light curves. Searches for additional sources have also been conducted. X-ray light curves for the Crab Nebula, Cyg X-1, 4U 1705-44, GRO J1655-40, and SMC X-1 are reported. They illustrate the quality of the results and the range of observed phenomena.

X-Ray Timing and Spectral Evolution of Circinus X-1 versus Orbital Phase with the Rossi X-Ray Timing Explorer

We have carried out a study of Cir X-1 through (1) detailed sampling over a single 16.5 day intensity cycle with the Rossi X-Ray Timing Explorer (RXTE) proportional counter array and (2) complementary monitoring over six such cycles with the RXTE all-sky monitor (ASM). We report here the temporal and spectral evolution at nonflaring phases, during which Cir X-1 remained unusually bright (~1.0 crab) and relatively steady. The Fourier power density spectrum of the source was observed to vary with strong correlations among low-frequency flat-topped power (1-10 Hz), a quasi-periodic oscillation (QPO) peak centered at 1.3-12 Hz, and a broad QPO peak centered at ~20 Hz up to ~100 Hz. As orbital phase increased within the cycle, the rms amplitude of the flat-topped power generally (but not monotonically) decreased while the QPO features generally evolved toward higher frequency. The 1.5-12 keV spectrum was observed to generally harden during the nonflaring phases of each of the six 16.5 day periods monitored by the ASM.

Quasi-periodic Oscillations Associated with Spectral Branches in Rossi X-Ray Timing Explorer Observations of Circinus X-1

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- atoll source, that HBOs should never occur in this source.

Observations of circinus X-1 with RXTE

Abstract Data accumulated with RXTE during the active state of Cir X-1 as well as during an unusually long transition from the active state to the quiescent state are reported. The long decline from the active state allowed the source characteristics to be studied systematically as a function of intensity. The following results are presented: (1) spectral fits during entry into a dip clearly show absorption with partial covering as previously reported (Brandt et al. 1996), and (2) correlations between position in the hardness-intensity plane and the character of the power density spectrum as the source entered the quiescent state are suggestive of Z source behavior seen in LMXB sources.