Phase-resolved spectroscopy of low frequency quasi-periodic oscillations in GRS 1915+105

TL;DR

This study analyzes phase-resolved spectra of low-frequency QPOs in GRS 1915+105, revealing spectral pivoting and reflection changes that suggest a geometric origin of the QPOs, with implications for understanding black hole accretion physics.

Contribution

First to reconstruct QPO waveforms across energy channels and analyze phase-resolved spectra, providing new insights into the physical mechanisms behind QPOs in black hole binaries.

Findings

Detected spectral pivoting during QPO cycle

Observed modulation in iron line equivalent width

Indications of a geometric QPO origin

Abstract

X-ray radiation from black hole binary (BHB) systems regularly displays quasi-periodic oscillations (QPOs). In principle, a number of suggested physical mechanisms can reproduce their power spectral properties, thus more powerful diagnostics which preserve phase are required to discern between different models. In this paper, we first find for two Rossi X-ray Timing Explorer (RXTE) observations of the BHB GRS 1915+105 that the QPO has a well defined average waveform. That is, the phase difference and amplitude ratios between the first two harmonics vary tightly around a well defined mean. This enables us to reconstruct QPO waveforms in each energy channel, in order to constrain QPO phase-resolved spectra. We fit these phase resolved spectra across 16 phases with a model including Comptonisation and reflection (Gaussian and smeared edge components) to find strong spectral pivoting and a modulation in the iron line equivalent width. The latter indicates the observed reflection fraction is changing throughout the QPO cycle. This points to a geometric QPO origin, although we note that the data presented here do not entirely rule out an alternative interpretation of variable disc ionisation state. We also see tentative hints of modulations in the iron line centroid and width which, although not statistically significant, could result from a non-azimuthally symmetric QPO mechanism.