Rapid X-ray variability of the gamma-ray binary LS I+61 303

TL;DR

This study analyzed archival RXTE data of LS I+61 303, revealing a ~26.6-day periodicity and characterizing its X-ray variability, but found no significant high-frequency signals or quasi-periodic oscillations.

Contribution

The paper provides the first detailed timing analysis of LS I+61 303's rapid X-ray variability using extensive archival data, identifying a consistent periodicity and noise characteristics.

Findings

Detected a ~26.6-day periodicity in X-ray data.

Red noise dominates the power density spectra below 0.1 Hz.

No significant high-frequency or quasi-periodic signals found.

Abstract

The gamma-ray binary LS I+61 303 has been widely monitored at different wavelengths since its discovery more than sixty years ago. However, the nature of the compact object and the peculiar behavior of the system are still largely debated. Aimed at investigating the rapid X-ray variability of LS I+61 303, we have analysed all the archival RXTE/PCA data of the source, taken between 1996 and 2011. The timing analysis yields a periodicity of $P\sim 26.6\pm 0.3$ days, which is statistically compatible with several periodicities reported in the literature for LS I+61 303. Using this period, we performed a data phase-resolved analysis to produce a set of phase-bin-averaged energy spectra and power density spectra. These power density spectra are dominated by weak red noise below 0.1 Hz, and show no signal above this frequency. The amplitude of the red noise varies mildly with the phase, and shows a maximum that coincides with a dip of the X-ray flux and a softer photon index. Aside from low-frequency noise, this analysis does not provide any statistically significant periodic or quasi-periodic timing feature in the RXTE/PCA data of LS I+61 303.