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Detection of free precession is very important for analysing the internal structure of neutron stars. So far, because of the interference of the accretion process, no feasible method has been proposed to detect the precession of accreting neutron stars in both theory and observations. Based on the analysis of archive data from Chandra and the Neutron Star Interior Composition ExploreR (NICER) of the ultra-compact X-ray binary system 4U 1820–30, we find that the energy spectra have a stable sinusoidal bi-directional oscillation period near 1000 s, no matter which state the X-ray binary system is in. After we fit the energy spectra and carry out Fourier decomposition of the fitting parameters, we find that this period is more steadily emitted from the neutron star. We discuss the possible origin of this period and conclude that the period is most likely the free precession period of the neutron star in 4U 1820–30. Its nutation can result in the seesaw-like periodical bi-directional oscillation we find in this paper. [ABSTRACT FROM AUTHOR]

Fast transitions between different types of power density spectra (PDS) happening over time-scales of several tens of seconds are rare phenomena in black hole X-ray binaries. In this paper, we report a broad-band spectral-timing analysis of the fast transitions observed in the 2021 outburst of GX 339−4 using NICER and Insight -HXMT observations. We observe transitions between band-limited noise-dominated PDS and type-B quasi-periodic oscillations (QPOs), and their rapid appearance or disappearance. We also make a detailed comparison between the fast transitions in GX 339−4 with those seen in MAXI J1820+070 and MAXI J1348−630. By comparing the spectra of the periods with and without type-B QPOs, we find that the spectral ratios above 10 keV are nearly constant or slightly decreasing, and the values are different between sources. Below 10 keV, the flux change of the Comptonization component is inversely proportional to the flux change of the thermal component, suggesting that the appearance of type-B QPOs is associated with a redistribution of the accretion power between the disc and the Comptonizing emission region. The spectral ratios between the periods with type-B QPO and those with broad-band noise are significantly different from that with type-B QPO and without type-B QPO, where the ratios (type-B QPO/broad-band noise) show a maximum at around 4 keV and then decrease gradually towards high energies. Finally, we discuss the possible change of the geometry of the inner accretion flow and/or jet during the transitions. [ABSTRACT FROM AUTHOR]

We present the results of the neutron star X-ray binary system Cen X-3 performed by Insight -HXMT with two observations during 2017 and 2018. During these two observations, the source reached a X-ray luminosity of ∼1038 erg s−1 from 2–105 keV. The analysis of the broadband X-ray spectrum reports the presence of two cyclotron resonance scattering features (CRSFs) with the fundamental line at ∼ 28 keV and the harmonic line at ∼47 keV. The multiple lines exist by fittings with different continuum models, like the absorbed negative and positive power-law with an exponential cutoff (NPEX) model and a power-law with high energy exponential cutoff model. This is the first time that both fundamental and harmonic lines are detected in Cen X-3. We also show evidence of two cyclotron lines in the phase-resolved spectrum of Cen X-3. The CRSF and continuum spectral parameters show evolution with the pulse profile, and the two line centroid energy ratio does not change significantly and locates in a narrow value range of 1.6−1.7 over the pulse phase. The implications of the discovering two cyclotron absorption features and phase-resolved spectral properties are discussed. [ABSTRACT FROM AUTHOR]

Using wavelet analysis and power density spectrum, we investigate two transient quasi-periodic oscillations (QPOs) observed in MAXI J1535−571 observed with Insight - HXMT. The transient QPOs have a centroid frequency of ∼10 Hz with an FWHM ∼0.6 Hz, and an rms amplitude |$\sim 14{{\ \rm per\ cent}}$|⁠. Energy spectra of QPO and non-QPO regimes are also separated and analysed, and the spectra become softer with higher E cut in the non-QPO regime compared to the QPO regime. Our results suggest that the transient QPOs detected in MJD 58016 and 58017 are still the type-C QPO, and the source remains in its HIMS. The duration of all type-C QPO signals based on wavelet is positively correlated with the mean count rate above ∼10 keV, implying appearance of QPOs in different time-scales should be coupled with the corona. The transient QPO properties could be related to the jet or flares, perhaps the partial ejection of the corona is responsible for the disappearance of the type-C QPO. [ABSTRACT FROM AUTHOR]

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

We present a detailed study of the high-mass X-ray binary Vela X-1, using observations performed by Insight-HXMT in 2019 and 2020, concentrating on timing analysis and spectral studies including pulse phase-resolved spectroscopy. The cyclotron line energy is found to be ∼21–27 and 43–50 keV for the fundamental and first harmonic, respectively. We present the evolution of spectral parameters and find that the two line centroid energy ratio E 2/ E 1 evolved from ∼2 before MJD 58900 to ∼1.7 after that. The harmonic cyclotron line energy has no relation to the luminosity but the fundamental line energy shows a positive correlation with X-ray luminosity, suggesting that Vela X-1 is located in the sub-critical accreting regime. In addition, the pulse phase-resolved spectroscopy in Vela X-1 is performed. Both the cyclotron resonant scattering feature and continuum parameters show strong variability over the pulse phase with the ratio of two line energies about 2 near the peak phases and down to ∼1.6 around off-peak phases. Long-term significant variations of the absorption column density and its evolution over the pulse phase may imply the existence of the clumpy wind structure near the neutron star. [ABSTRACT FROM AUTHOR]

In this paper, wavelet analysis is used to study the spectral-timing properties of MAXI J1535–571 observed by the Hard X-ray Modulation Telescope (Insight -HXMT). Low-frequency quasi-periodic oscillations (QPOs) are detected in nine observations. Based on wavelet analysis, the time intervals with QPOs and non-QPOs are isolated, and the corresponding spectra with QPOs and non-QPOs are analysed. We find that the spectra with QPOs (hereafter QPO spectra) are softer than those without QPOs (hereafter non-QPO spectra) in the hard intermediate state (HIMS), while in the soft intermediate state (SIMS), the QPO spectra are slightly harder. The disc temperature of the QPO regime is slightly lower during the HIMS, but becomes higher during the SIMS. The cutoff energies of QPO spectra and non-QPO spectra do not show significant differences. The flux ratio of the disc to total flux is higher for the time intervals with non-QPOs than that in the QPO regime. We propose that these differences in the spectral properties between QPO and non-QPO regimes could be explained through the scenario of Lense–Thirring precession, and the reversal of the QPO/non-QPO behaviour between the HIMS and SIMS may be associated with the appearance/disappearance of a type-B QPO, which might originate in the precession of the jet. [ABSTRACT FROM AUTHOR]

After 2010, three outbursts of H1743−322 were detected by RXTE. We have carried out timing and spectral analysis of the data, emphasizing on the two with relatively complete evolution history presented in the RXTE/Proportional Counter Array observations. We then constitute an enlarged outburst sample for H1743−322 which allows us to investigate the spectral transitions in more details. We find that the spectral transitions to high-soft state constrain a region for four outbursts in hardness–intensity diagram. An extension of the region locates in the vicinity of the failed outburst in 2008, and excludes it from a successful group. We therefore suggest that the failed outburst in 2008 may present the first almost successful outburst sample, which is important for modelling the outburst, especially upon the conditions required for transition to high-soft state. [ABSTRACT FROM AUTHOR]