Detection of a quasi-periodic oscillation at $\sim$40 mHz in Cen X-3 with Insight-HXMT

TL;DR

This paper reports the detection and analysis of a 40 mHz quasi-periodic oscillation in the X-ray pulsar Cen X-3 observed by Insight-HXMT, revealing energy-dependent timing features and potential disk instability origins.

Contribution

It presents the first detailed characterization of mHz QPOs in Cen X-3, including their energy dependence and phase variation, suggesting an accretion disk instability near the corotation radius.

Findings

QPO at ~40 mHz with ~9% rms amplitude detected in Cen X-3.

QPO frequency varies with orbital phase but not X-ray intensity.

Energy-dependent QPO rms amplitude decreases from 2 to 20 keV.

Abstract

We investigated the quasi-periodic oscillation (QPO) features in the accretion-powered X-ray pulsar Cen X-3 observed by Insight-HXMT. For two observations in 2020 when Cen X-3 was in an extremely soft state, the power density spectrum revealed the presence of obvious QPO features at $\sim$40 mHz with an averaged fractional rms amplitude of $\sim9\%$. We study the mHz QPO frequency and rms amplitude over orbital phases, and find that the QPO frequency is $\sim$33-39 mHz at the orbital phase of 0.1-0.4, increasing to $\sim$37-43 mHz in the orbital phase of 0.4-0.8, but has no strong dependence on X-ray intensity. We also carried out an energy-dependent QPO analysis, the rms amplitude of the mHz QPOs have a decreasing trend as the energy increases from 2 to 20 keV. In addition, the QPO time-lag analysis shows that the time delay is $\sim 20$ ms (a hard lag) in the range of $\sim$5-10 keV, and becomes negative (time lag of $-(20-70)$ ms) above $\sim 10$ keV. The different QPO theoretical models are summarized and discussed. In the end, we suggest that these energy-dependent timing features as well as the origin of mHz QPOs in Cen X-3 may be ascribed to an instability when the accretion disk is truncated near the corotation radius.