Applying wavelet analysis to the X-ray light curves of active galactic nuclei and quasi-periodic eruptions

TL;DR

This study demonstrates the effectiveness of wavelet analysis in detecting and characterizing transient and periodic features in X-ray light curves of active galactic nuclei and quasi-periodic eruptions, revealing significant signals and transient phenomena.

Contribution

The paper applies wavelet transform techniques to AGN and QPE X-ray data, showing its capability to identify transient and periodic features, including QPOs, in complex astrophysical signals.

Findings

Wavelet analysis detects significant QPOs in RE J1034+396.

Transient features during flares in IRAS 13224-3809 are identified.

Persistent signals in GSN 069 suggest high-frequency QPOs.

Abstract

In this work, we examine the application of the wavelet transform to the X-ray timing analyses of active galactic nuclei (AGN) and quasi-periodic eruption sources (QPEs). Several scenarios are simulated to test the effectiveness of the wavelet analysis to stationary and non-stationary data. We find that the power spectral density (PSD) slope and the nature of the periodic signal can influence the ability to identify important features in the wavelet power spectrum. In general, weak and transient features can be discerned, which make the wavelet spectrum an important tool in examining AGN light curves. We carried out a wavelet analysis to four unique objects: Ark 120, IRAS 13224-3809, RE J1034+396, and the QPE GSN 069. The well-known quasi-periodic oscillation (QPO) in RE J1034+396 is significantly detected in the wavelet power spectrum. In IRAS 13224-3809, significant transient features appear during a flare at frequencies coincident with previously detected reverberation signals. Finally, the wavelet power spectrum of the QPE GSN 069 significantly reveals four persistent signals that exhibit a 3:2 ratio in oscillation frequencies, consistent with high-frequency QPOs in stellar mass X-ray binaries, but we cannot rule out the possibility this is an artefact of the calculation.