The energy dependence of the centroid frequency and phase lag of the QPOs in GRS 1915+105

TL;DR

This study investigates how the centroid frequencies and phase lags of QPOs in GRS 1915+105 depend on photon energy, revealing energy-dependent behaviors and an anti-correlation that challenge existing models.

Contribution

It provides new observational insights into the energy dependence of QPO properties and highlights limitations of current theoretical models.

Findings

QPO centroid frequencies depend on photon energy.

Phase lags of QPOs are energy dependent.

An anti-correlation exists between QPO frequency and phase lag.

Abstract

We present a study of the centroid frequencies and phase lags of the quasi-periodic oscillations (QPOs) as functions of photon energy for GRS 1915+105. It is found that the centroid frequencies of the 0.5-10 Hz QPOs and their phase lags are both energy dependent, and there exists an anti-correlation between the QPO frequency and phase lag. These new results challenge the popular QPO models, because none of them can fully explain the observed properties. We suggest that the observed QPO phase lags are partially due to the variation of the QPO frequency with energy, especially for those with frequency higher than 3.5 Hz.