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During thermonuclear bursts, it is suspected that cooling of the corona by the burst emission may be the cause of hard X-ray deficits. Although such a deficit has been observed in nine sources, it has not been observed from 4U 1608–52, a nearby prolific burster. Therefore, the authenticity and universality of the hard X-ray deficit may be in question. To investigate this suspicion, Insight–HXMT performed cadence observations during the low/hard state of 4U 1608–52 in 2022 September and detected 10 thermonuclear X-ray bursts. Two of these bursts show a double-peaked structure in the soft X-ray band, which could be caused by the high temperature of the burst emission and a marginal photospheric radius expansion (PRE) around the burst peak time. This is indicated by their peak fluxes being up to the Eddington limit and having a large colour factor at the peak of the bursts. A hard X-ray deficit is observed at a significant level during bursts at > 30 keV. Furthermore, the fraction of this deficit shows saturation at 50 per cent for the first eight bursts. This saturation may indicate that the corona is layered and only part of the corona is cooled by the bursts: for example, the part close to the neutron star surface is cooled while the rest remains intact during bursts. This result provides a clue to the geometry of the corona. For example, a possible scenario is that the corona has two forms: a quasi-spheric corona between the neutron star and the disc and a disc corona on both surfaces of the disc. [ABSTRACT FROM AUTHOR]

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]

We report the phase-resolved spectral results of the first Galactic pulsating ultraluminous X-ray source (PULX) Swift J0243.6+6124, modeling its 2017–2018 outburst peak using data collected by the Hard X-ray Modulation Telescope (Insight-HXMT). The broad energy coverage of Insight-HXMT allows us to obtain a more accurate spectral continuum to reduce the coupling of broad iron line profiles with other components. We use three different continuum spectrum models but obtain similar iron line results. For the first time, we detect the pulse characteristics of the broad iron line in a PULX. The variation in the width and intensity of this iron line with σ ∼ 1.2–1.5 keV has a phase offset of about 0.25 from the pulse phase. We suggest that the uneven irradiation of the thick inner disk by the accretion column produces the modulated variation of the broad iron line. In addition, the nonpulsed narrow line is suggested to come from the outer disk region. [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]

We study the spectral and temporal properties of the black hole X-ray transient binary MAXI J1820+070 during the 2018 outburst with Insight -HXMT observations. The outburst of MAXI J1820+070 can be divided into three intervals. For the two intervals of the outburst, we find that low-energy (below 140 keV) photons lag high-energy (140–170 keV) ones, while in the decay of the outburst, high-energy photons lag low-energy photons, both with a time-scale of the order of days. Based on these results, the canonical hysteresis effect of the 'q' shape in the hardness–intensity diagram can be reformed into a roughly linear shape by taking into account the lag corrections between different energy bands. Time analysis shows that the high-frequency break of hard X-rays, derived from the power-density spectrum of the first interval of the outburst, is in general larger and more variable than that of soft X-rays. The spectral fitting shows that the coverage fraction of the hard X-rays drops sharply at the beginning of the outburst to about 0.5, and then increases slightly. The coverage fraction drops to roughly zero once the source steps into soft state and increases gradually to unity when the source returns to low–hard state. We discuss the possible overall evolution scenario of corona hinted from these discoveries. [ABSTRACT FROM AUTHOR]

We report the timing and spectral analyses of the type-II X-ray bursts from the rapid burster (MXB 1730–335) observed by the Hard X-ray Modulation Telescope (Insight-HXMT) and Swift/X-Ray Telescope (XRT). By stacking the long-duration bursts, we find for the first time that the hard X-rays are lagging behind the soft X-rays by 3 s. However, such a lag is not visible for the short-duration bursts, probably because of the poor statistics. For all bursts the energy spectrum is found to be nonthermal, thanks to the broadband coverage of Insight-HXMT. These findings provide new insights into the type-II bursts and require a temporally visible corona for possible interpretation. [ABSTRACT FROM AUTHOR]