The nature of an imaginary quasi-periodic oscillation in the soft-to-hard transition of MAXI J1820+070

作者：Candela Bellavita, Mariano Méndez, Federico García, Ruican Ma, Ole König · 发表于：Astronomy and Astrophysics · 年份：2025 · DOI：10.1051/0004-6361/202453092 · 被引用次数：13 · 研究领域：Astrophysical Phenomena and Observations、Pulsars and Gravitational Waves Research、Stellar, planetary, and galactic studies

A recent study shows that if the power spectra (PS) of accreting compact objects consist of a combination of Lorentzian functions that are coherent in different energy bands but incoherent with each other, the same is true for the real and imaginary parts of the cross spectrum (CS). Using this idea, we discovered imaginary quasi-periodic oscillations (QPOs) in NICER observations of the black hole candidate MAXI J1820+070. The imaginary QPOs appear as narrow features with a small real and large imaginary part in the CS but are not significantly detected in the PS when they overlap in frequency with other variability components. The coherence function drops and the phase lags increase abruptly at the frequency of the imaginary QPO. We show that the multi-Lorentzian model that fits the PS and CS of the source in two energy bands correctly reproduces the lags and the coherence, and that the narrow drop in the coherence is caused by the interaction of the imaginary QPO with other variability components. The imaginary QPO appears only in the decay of the outburst, during the transition from the high-soft to the low-hard state of MAXI J1820+070, and its frequency decreases from ∼5 Hz to ∼1 Hz as the source spectrum hardens. We also analysed the earlier observations of the transition, where no narrow features were seen, and we identified a QPO in the PS that appears to evolve into the imaginary QPO as the source hardens. As for the type-B and C QPOs in this source, the rms spectrum o...