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Context. Thermonuclear bursts, also known as type I X-ray bursts, result from unstable nuclear burning of H/He accreted to the surface of neutron stars, lasting from tens to hundreds of seconds. Thermonuclear bursts have an important impact on accretion environments around the neutron stars, such as their disks and coronas, and are therefore a subject of extensive research. Thermonuclear bursts can be used as probes to gain a deeper understanding of the properties of their disks and coronas. Aims. By analyzing the data from Insight-HXMT and NICER, we can determine the evolution of the significance of the hard shortage in 4U 1636–536 with its spectral state, as well as the evolution of the fraction of deficit with energy. Additionally, we investigate the possible geometry and evolution of the corona in 4U 1636–536 by combining our findings with the results of spectral analysis. Methods. We extracted the light curves from the Insight-HXMT low-energy, medium-energy, and high-energy data and subtracted their pre-burst emission, which allowed us to estimate the significance of the hard shortages during the bursts. By fitting the spectra, the correlation between the persistent spectral parameters and the significance of the hard shortages could be established. The bursts were then grouped according to the spectral state in which they occurred, and the significance of the hard shortages was estimated. These in turn helped to investigate the evolution of the fraction of deficit with energy. Results. We find that during the soft state the significance of possible hard X-ray shortage in bursts is almost zero. However, in the hard state, some bursts exhibit significant shortages (> 3σ), while others do not. We attempt to establish a correlation between the significance of the hard X-ray shortage and the spectral parameters, but the data quality and the limited number of bursts prevent us from finding a strong correlation. For bursts with insignificant shortages in the soft state, the fraction of the deficit remains small. However, in the hard state the fraction of deficit for all bursts increases with energy, regardless of the significance of the shortage of individual bursts. For bursts during the hard state, we investigated the evolution of the fraction of deficit during the bursts by stacking the peaks and decays of the bursts, and find that as the flux of the bursts decreases, the energy corresponding to the maximum of the fraction of deficit becomes progressively higher. Conclusions. We explore the possible geometry and evolution of the corona suggested by the evolution of the fraction of deficit, which is obtained from the spectral and temporal analysis. [ABSTRACT FROM AUTHOR]

Context. 4U 1543–47 is one of a handful of known black hole candidates located in the Milky Way galaxy. It underwent a very luminous outburst in 2021, reaching a peak intensity of ~9 Crab, as observed by the Monitor of All-sky Image (MAXI), and exceeding twice its Eddington luminosity. Aims. The unprecedented bright outburst of 4U 1543–47 provides a unique opportunity to test the behavior of accretion disk models at high luminosities and accretion rates. In addition, we explore the possibility of constraining the spin of the source at high accretion rates, given that the previous spin measurements of 4U 1543–47 are largely inconsistent with each other. Methods. We measure the spectral evolution of the source throughout its outburst as observed by Insight-HXMT, and compare the behavior of both the thin-disk model kerrbb2 and the slim disk model slimbh up to the Eddington limit for two different values of disk α-viscosity. In addition, given the behavior of these two models, we identify two "golden" epochs in which it is most suitable to measure the spin with the continuum fitting method. Results. We find evidence of a disk state transition from a thicker slim disk to a thin disk occurring around 1 LEdd from fits to the luminosity-temperature relation. We obtain consistent and constant spin measurements from both slimbh and kerrbb2 as the luminosity varies towards the Eddington limit, implying the recovery of thin-disk solutions above the traditional thin-disk criterion of 30% LEdd. We constrain the spin to a* = 0.65−0.24+0.14, assuming an α-viscosity = 0.01 from both Insight-HXMT and NICER observations from the above-mentioned "golden" epochs where the condition of the disk being truncated at the innermost stable circular orbit is most closely met. [ABSTRACT FROM AUTHOR]

We investigate the dynamical properties of low-frequency quasi-periodic oscillations (QPOs) observed from the black hole X-ray binary MAXI J1820+070 during the early part of its 2018 outburst, when the system was in a bright hard state. To do this, we used a series of observations from the Hard X-ray Modulation Telescope Insight-HXMT and applied a wavelet decomposition (weighted wavelet Z-transforms) to the X-ray light curve. We find that the QPO phenomenon is intermittent within each individual observation, with some subintervals in which the oscillation is strongly detected (high root-mean-square amplitude) and others in which it is weak or absent. The average lifetime of individual QPO segments is about 5 oscillation cycles, with a 3σ tail of up to about 20 cycles. There is no substantial difference between the energy spectra during intervals with strong and weak or absent QPOs. Alternatively, we suggest that the changes in oscillation amplitude can be caused by changes in the jet speed. We discuss a possible reason for the intermittent QPO strength within the precessing-jet model previously proposed for MAXI J1820+070: we suggest that the changes in oscillation amplitude are caused by changes in the jet speed. We argue that a misaligned precessing-jet scenario is also consistent with other recent observational findings that suggest that the oscillation of the Compton reflection component is in phase with the QPOs. [ABSTRACT FROM AUTHOR]