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

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 similar to 40 mHz with an averaged fractional rms amplitude of similar to 9 per cent. We study the mHz QPO frequency and rms amplitude over orbital phases, and find that the QPO frequency is similar to 33-39 mHz at the orbital phase of 0.1-0.4, increasing to similar to 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 similar to 20 ms (a hard lag) in the range of similar to 5-10 keV, and becomes negative [time-lag of -(20-70) ms] above similar to 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 disc is truncated near the corotation radius.