Long-term X-ray monitoring of LS I +61 303: analysis of spectral variability and flares

TL;DR

This study presents an extensive analysis of X-ray observations of LS I +61 303, revealing spectral variability, flares, and the absence of clear super-orbital modulation, advancing understanding of its high-energy behavior.

Contribution

It provides the largest X-ray dataset for LS I +61 303, analyzing spectral variability, flare characteristics, and searching for super-orbital modulation, which were previously less well characterized.

Findings

Spectral index anti-correlates with flux.

Detected two ks-timescale flares with short-term variability.

No coherent QPOs or super-orbital modulation detected.

Abstract

We report on the full analysis of a Rossi X-ray Timing Explorer (RXTE) Proportional Counter Array (PCA) monitoring of the {\gamma}-ray binary system LS I +61 303. The data set covers 42 contiguous cycles of the system orbital motion. Analyzing this X-ray monitoring dataset, the largest to date for this source, we report on the variability of the orbital profile and the spectral distribution, and provide strong evidence for an anti-correlation between flux and spectral index (the higher the flux, the harder the spectral index). Furthermore, we present the analysis of two newly discovered ks-timescale flares, which present significant variability also on shorter timescales, and tend to occur at orbital phases between 0.6-0.9. However, a detailed timing analysis of the flares does not show any coherent or quasi-coherent (QPO) structure in their power spectra. We also investigated the possible appearance of the radio super-orbital modulation at X-rays energies, but we could not unambiguously detect such modulation in the system flux history, nor in the evolution of its orbital modulation fraction.