X-ray periodicities in sources observed by the RXTE ASM

Abstract

The X-ray intensities measured from 230 X-ray sources observed by the RXTE All-Sky Monitor (ASM) were analyzed for periodic behavior. The ASM has been observing sources for nine years in the 1.5-12 keV energy range. In this search, we investigated a frequency range of 5 d⁻¹ to 144 d⁻¹ for periodic modulation. The search was carried out by applying the Fast Fourier Transform on the ASM data and whitening the resulting power density spectrum using a local averaging technique. A power threshold was set to determine the statistical significance of a peak in the power density spectrum. The search verified the orbital frequencies of four known X-ray binaries and also identified four previously unreported periodicities. The 5.7232 d⁻¹ orbital frequency of GX 9+9 was observed for the first time in ASM data, although this modulation had been observed before. The 6.0999 d⁻¹ peak found for XTE J1723-376 has not been reported before and may be an orbital frequency.