Energy dependence of Normal Branch Oscillation in Scorpius X-1

TL;DR

This study investigates how the properties of normal branch oscillations in Scorpius X-1 vary with photon energy, revealing complex energy-dependent behaviors that suggest a transition layer origin.

Contribution

It provides the first detailed analysis of energy dependence of NBOs in Scorpius X-1, highlighting the relationship between oscillation properties and photon energy.

Findings

NBO centroid frequency decreases with energy below 6-8 keV and increases above that.

RMS amplitude increases with energy below 13 keV and decreases afterward.

Quality factor shows no clear energy dependence.

Abstract

We report the energy dependence of normal branch oscillations (NBOs) in Scorpius X-1, a low-mass X-ray binary Z-source. Three characteristic quantities (centroid frequency, quality factor, and fractional root-mean-squared (rms) amplitude) of a quasi-periodic oscillation signal as functions of photon energy are investigated. We found that, although it is not yet statistically well established, there is a signature indicating that the NBO centroid frequency decreases with increasing photon energy when it is below 6-8 keV, which turns out to be positively correlated with the photon energy at the higher energy side. In addition, the rms amplitude increases significantly with the photon energy below 13 keV and then decreases in the energy band of 13-20 keV. There is no clear dependence on photon energy for the quality factor. Based on these results, we suggest that the NBO originates mainly in the transition layer.