PREPRINT
E810F65F-D04F-4705-9718-998E0E317000

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

Q. Liu, W. Wang, X. Chen, W. Yang, F. J. Lu, L. M. Song, J. L. Qu, S. Zhang, S. N. Zhang

Submitted on 14 September 2022

## 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 $\sim 9\mathrm{%}$. 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 $-\left(20-70\right)$ 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.

## Preprint

Comment: 9 pages, MNRAS in press

Subjects: Astrophysics - High Energy Astrophysical Phenomena; Astrophysics - Solar and Stellar Astrophysics