Outburst Evolution Research Articles

Burst-recurrence properties revealed with Insight-HXMT and NICER for the newly discovered accreting millisecond pulsar MAXI J1816--195

We report the results of our analysis of 83 type-I bursts during the 2022 outburst of the newly discovered accreting millisecond pulsar MAXI J1816--195 based on Insight-HXMT and NICER observations. We focus on the burst-recurrence time and its correlation with persistent flux and outburst evolution. The extensive observations of Insight-HXMT and NICER confirm the presence of quasi-periodic thermonuclear bursts during this outburst, with a recurrence time in the range of 1.15 to 2 hours, which varies with the source persistent flux. The burst recurrence times are, in general, longer at comparable flux levels in the outburst rising phase than those in the fading phase, forming an apparent hysteresis phenomenon. These burst properties make MAXI J1816--195 a unique target for investigating the underlying burst-accretion mechanisms. We discuss the plausible explanations for the hysteresis phenomenon, which appears to be related to changes in the ignition condition or accretion geometry during the outburst.

The current state of disk wind observations in BHLMXBs through X-ray absorption lines in the iron band

The presence of blueshifted absorption lines in the X-ray spectra of black hole low-mass X-ray binaries is the telltale mark of massive outflows called winds. These signatures are found almost exclusively in soft states of high-inclined systems, hinting at equatorial ejections originating from the accretion disk and deeply intertwined with the evolution of the outburst patterns displayed by these systems. In the wake of the launch of the new generation of X-ray spectrometers, studies of wind signatures remain mostly restricted to single sources and outbursts, with some of the recent detections departing from the commonly expected behaviors. We thus give an update to the current state of iron band absorption line detections through the analysis of all publicly available XMM-Newton-pn and Chandra-HETG exposures of known black hole low-mass X-ray binary candidates. Our results agree with previous studies, as our wind detections are exclusively found in dipping, high-inclined sources and almost exclusively in bright (LX > 0.01LEdd) soft (HR < 0.8) states with blueshift values generally restricted to a few 100 km s−1. The line parameters indicate similar properties between objects and outbursts of single sources, and despite more than 20 yr of data, very few sources have the HID sampling necessary to properly study the evolution of the wind during a single outburst. We provide an online tool with details of the wind signatures and outburst evolution data for all sources in our sample.

Ask the machine: systematic detection of wind-type outflows in low-mass X-ray binaries

ABSTRACT The systematic discovery of outflows in the optical spectra of low-mass X-ray binaries opened a new avenue for the study of the outburst evolution in these extreme systems. However, the efficient detection of such features in a continuously growing data base requires the development of new analysis techniques with a particular focus on scalability, adaptability, and automatization. In this pilot study, we explore the use of machine learning algorithms to perform the identification of outflows in spectral line profiles observed in the optical range. We train and test the classifier on a simulated data base constructed through a combination of disc emission line profiles and outflow signatures, emulating typical observations of low-mass X-ray binaries. The final, trained classifier is applied to two sets of spectra taken during two bright outbursts that were particularly well covered, those of V404 Cyg (2015) and MAXI J1820+070 (2018). The resulting classification gained by this novel approach is overall consistent with that obtained through traditional techniques, while simultaneously providing a number of key advantages over the latter, including the access to low-velocity outflows. This study sets the foundations for future studies on large samples of spectra from low-mass X-ray binaries and other compact binaries.

Development and application of gas adsorption model for coal based on particle flow code

This study investigated the deformation characteristics and engineering properties of gas-bearing coal. Based on the particle flow discrete element method, the gas was regarded as a flexible solid particle, and the relationship between the radius, elastic modulus and gas pressure of the flexible gas particle was derived through the ideal gas equation. Then, a gas adsorption model for coal based on the Langmuir equation was developed by coupling gas particles with rigid coal particles and applied to the coal and gas outburst evolution process. The results revealed that the flexible gas particles can effectively simulate the flow and deformation of gas, and the gas adsorption model exhibits adsorption deformation characteristics consistent with those of real coal specimens. Moreover, the adsorption model can indicate the adsorption saturation of coal particles at any moment, and the distribution characteristics of gas pressure within the microscopic pores, which is helpful to study the microscopic mechanism of gas adsorption process. Finally, the gas adsorption model for coal was applied to the coal and gas outburst and good simulation results were obtained, which further validated the reasonableness of the gas adsorption model. These studies promote the application of the particle method in the direction of gas-solid coupling research, and also provide a theoretical and numerical basis for the prevention and control of gas disasters.

A possible overall scenario for the outburst evolution of MAXI J1820+070 revealed by Insight-HXMT

ABSTRACT 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.

Insight-HXMT Study of the Inner Accretion Disk in the Black Hole Candidate EXO 1846–031

We study the spectral evolution of the black hole candidate EXO 1846−031 during its 2019 outburst, in the 1–150 keV band, with the Hard X-ray Modulation Telescope. The continuum spectrum is well modeled with an absorbed disk-blackbody plus cutoff power law, in the hard, intermediate, and soft states. In addition, we detect an ≈6.6 keV Fe emission line in the hard intermediate state. Throughout the soft intermediate and soft states, the fitted inner disk radius remains almost constant; we suggest that it has settled at the innermost stable circular orbit (ISCO). However, in the hard and hard intermediate states, the apparent inner radius was unphysically small (smaller than the ISCO), even after accounting for the Compton scattering of some of the disk photons by the corona in the fit. We argue that this is the result of a high hardening factor, f col ≈ 2.0–2.7, in the early phases of the outburst evolution, well above the canonical value of 1.7 suitable for a steady disk. We suggest that the inner disk radius was already close to the ISCO in the low/hard state. Furthermore, we propose that this high value of the hardening factor in the relatively hard state was probably caused by the additional illuminating of the coronal irradiation onto the disk. Additionally, we estimate the spin parameter using the continuum-fitting method, over a range of plausible black hole masses and distances. We compare our results with the spin measured using the reflection-fitting method and find that the inconsistency of the two results is partly caused by different choices of f col.

Investigating the nature and properties of MAXI J1810−222 with radio and X-ray observations

ABSTRACT We present results from radio and X-ray observations of the X-ray transient MAXI J1810−222. The nature of the accretor in this source has not been identified. In this paper, we show results from a quasi-simultaneous radio and X-ray monitoring campaign taken with the Australia Telescope Compact Array, the Neil Gehrels Swift Observatory X-ray Telescope (XRT), and the Swift Burst Alert Telescope. We also analyse the X-ray temporal behaviour using observations from the Neutron star Interior Composition Explorer. Results show a seemingly peculiar X-ray spectral evolution of MAXI J1810−222 during this outburst, where the source was initially only detected in the soft X-ray band for the early part of the outburst. Then, ∼200 d after MAXI J1810−222 was first detected the hard X-ray emission increased and the source transitioned to a long-lived (∼1.5 yr) bright, harder X-ray state. After this hard state, MAXI J1810−222 returned back to a softer state, before fading and transitioning again to a harder state and then appearing to follow a more typical outburst decay. From the X-ray spectral and timing properties, and the source’s radio behaviour, we argue that the results from this study are most consistent with MAXI J1810−222 being a relatively distant (≳6 kpc) black hole X-ray binary. A sufficiently large distance to source can simply explain the seemingly odd outburst evolution that was observed, where only the brightest portion of the outburst was detectable by the all-sky XRTs.

Peculiar Disk Behaviors of the Black Hole Candidate MAXI J1348–630 in the Hard State Observed by Insight-HXMT and Swift

Abstract We present a spectral study of the black hole candidate MAXI J1348−630 during its 2019 outburst, based on monitoring observations with Insight-HXMT and Swift. Throughout the outburst, the spectra are well fitted with power-law plus disk-blackbody components. In the soft-intermediate and soft states, we observed the canonical relation L ∝ T in 4 between disk luminosity L and peak color temperature T in, with a constant inner radius R in (traditionally identified with the innermost stable circular orbit). At other stages of the outburst cycle, the behavior is more unusual, inconsistent with the canonical outburst evolution of black hole transients. In particular, during the hard rise, the apparent inner radius is smaller than in the soft state (and increasing), and the peak color temperature is higher (and decreasing). This anomalous behavior is found even when we model the spectra with self-consistent Comptonization models, which take into account the upscattering of photons from the disk component into the power-law component. To explain both anomalous trends at the same time, we suggest that the hardening factor for the inner-disk emission was larger than the canonical value of ≈1.7 at the beginning of the outburst. A more physical trend of radii and temperature evolution requires a hardening factor evolving from ≈3.5 at the beginning of the hard state to ≈1.7 in the hard-intermediate state. This could be evidence that the inner disk was in the process of condensing from the hot, optically thin medium and had not yet reached a sufficiently high optical depth for its emission spectrum to be described by the standard optically thick disk solution.

Quasi-periodic whispers from a transient ULX in M 101: signatures of a fast-spinning neutron star?

ABSTRACT We have studied the unusual time variability of an ultraluminous X-ray source in M 101, 4XMM J140314.2 + 541806 (henceforth, J1403), using Chandra and XMM-Newton data. Over the last two decades, J1403 has shown short-duration outbursts with an X-ray luminosity ∼1–3 × 1039 erg s−1, and longer intervals at luminosities ∼0.5–1 × 1038 erg s−1. The bimodal behaviour and fast outburst evolution (sometimes only a few days) are more consistent with an accretor/propeller scenario for a neutron star than with the canonical outburst cycles of stellar-mass black holes. If this scenario is correct, the luminosities in the accretor and propeller states suggest a fast spin (P ≈ 5 ms) and a low surface magnetic field (B ∼ 1010 G), despite our identification of J1403 as a high-mass X-ray binary. The most striking property of J1403 is the presence of strong ∼600-s quasi-periodic oscillations (QPOs), mostly around frequencies of ≈1.3–1.8 mHz, found at several epochs during the ultraluminous regime. We illustrate the properties of such QPOs, in particular their frequency and amplitude changes between and within observations, with a variety of techniques (Fast Fourier Transforms, Lomb–Scargle periodograms, weighted wavelet Z-transform analysis). The QPO frequency range <10 mHz is an almost unexplored regime in X-ray binaries and ultraluminous X-ray sources. We compare our findings with the (few) examples of very low frequency variability found in other accreting sources, and discuss possible explanations (Lense–Thirring precession of the inner flow or outflow; radiation pressure limit-cycle instability; marginally stable He burning on the neutron star surface).

Detailed analysis on the reflection component for the black hole candidate MAXI J1348-630

ABSTRACT The black hole candidate MAXI J1348-630 was discovered in 2019 January 26, with the Gas Slit Camera (GSC) on-board MAXI. We report a detailed spectral analysis of this source by using the archived data of NuSTAR. A total of nine observations covered the complete outburst evolution of MAXI J1348-630 from the hard state to the soft state and finally back to the hard state. Additionally, the intermediate state is found in the transition from the hard state to the soft state. We use the state-of-the-art reflection model relxill family to fit all the nine spectra, and the spectra from two focal plane module detectors of NuSTAR are jointly fitted for each observation. In particular, we concentrate on the results of the black hole spin parameter and the inclination of the accretion disc. Based on the analysis of the inner radius of the accretion disc, we obtain the spin parameter $a_* =0.78_{-0.04}^{+0.04}$, and the inclination angle of the inner disc $i = 29.2_{-0.5}^{+0.3}$°. Furthermore, we also find that when the black hole is in the hard state, the accretion disc would show a significant truncation. The high iron abundance and ionization of the accretion disc obtained in the fitting results can be possibly explained by the high density of the accretion disc.

Hard-state Optical Wind during the Discovery Outburst of the Black Hole X-Ray Dipper MAXI J1803–298

Abstract We present 12 epochs of optical spectroscopy taken across the discovery outburst of the black hole (BH) candidate MAXI J1803−298 with the Gran Telescopio Canarias and Very Large Telescope. The source followed a standard outburst evolution with hard and soft states. The system displays a triangular shape in the hardness intensity diagram, consistent with that seen in high-inclination BH transients and the previously reported detection of X-ray dips. The two epochs observed during the initial hard state exhibited asymmetric emission-line profiles, including a P-Cygni profile simultaneously detected in Hα and He i 6678, which indicates the presence of an optical wind in the system. The remaining spectra, obtained during the transition to the soft state and the subsequent decay, are instead characterized by narrower, double-peaked emission lines embedded into broad absorption components. One epoch (intermediate state) also includes near-infrared (NIR) coverage, revealing complex line profiles in the Paschen and Bracket series, which suggests that the outflow is still present during the outburst decay through the soft state. The growing list of low-mass X-ray binaries with optical and NIR outflow signatures indicates that these are common features. Furthermore, the lowest luminosity spectrum exhibits an Hα FWHM of 1570 ± 100 km s−1. This, together with previous constraints on the binary parameters, allows us to favor a compact object mass of ∼3–10 M ⊙, further supporting its BH nature.

Numerical Investigation of Coal and Gas Outbursts under Different In Situ Stresses and Gas Pressures and the Physical Characteristics of Coal.

The coal and gas outbursts are recognized as a worldwide difficulty, and there is still ample research space in this field, especially on the mechanical mechanism of outbursts. The main purpose of this paper is to attempt to reveal the outburst mechanism as fully as possible from the point of view of mechanics. In this paper, a mechanical model on coal and gas outbursts including the governing equations of gas desorption–seepage, the stress state in the coal sample, and the criteria of coal sample failure and outburst evolution is put forward according to the porous media seepage theory and elastic theory. Based on the proposed model, the variation and distribution of the gas state and stress state in the coal sample in the outburst are analyzed quantitatively and a series of detailed discussions are conducted in terms of the in-situ stress, gas pressure, and the physical characteristics of coal in outbursts. The results of theoretical analysis and numerical simulation show that the stress concentration in the front of the outburst cavity is the main reason for the failure of the coal sample in this area, and then, the drag force caused by gas flow provides energy for the movement of the crushed coal sample, which leads to the outburst cavity expansion and the increase of stress concentration factor. The end of the outburst is because the gas velocity is less than the threshold friction velocity of the crushed coal sample. Additionally, the outburst strength increases with the increase of the vertical in-situ stress and initial gas pressure and decreases with the increase of the internal friction angle and cohesion of coal.

Experimental Study on Dynamic Evolution Mechanism during Coal and Gas Outburst

Coal and gas outburst is one of the most serious disasters in coal mining. To research the outburst evolution mechanism, we used the multi-field coupled coal mine dynamic disaster large-scale test system for simulating coal and gas outburst. In the present study, the evolution characteristics of gas pressure, temperature, and coal body stress during the outburst process were explored. Experiment results show that (1) the coal seam gas pressure decreased faster where the position is closer to the outburst, and the spherical shell-shaped outburst wave front formed in the coal seam propagated rapidly from the outburst outlet to the inside; (2) during the process of coal and gas outburst, the elastic energy of coal body released instantly, which slightly increased the temperature and then the gas desorption absorbed the heat, thereby slowly dropping the temperature; (3) after the outburst, coal and gas released the pressure, and the stress inside of the coal body decreased. The reduction of stress around the outburst hole was the most obvious.

NICER observations reveal that the X-ray transient MAXI J1348−630 is a black hole X-ray binary

ABSTRACT We studied the outburst evolution and timing properties of the recently discovered X-ray transient MAXI J1348−630 as observed with NICER. We produced the fundamental diagrams commonly used to trace the spectral evolution, and power density spectra to study the fast X-ray variability. The main outburst evolution of MAXI J1348−630 is similar to that commonly observed in black hole transients. The source evolved from the hard state (HS), through hard- and soft-intermediate states, into the soft state in the outburst rise, and back to the HS in reverse during the outburst decay. At the end of the outburst, MAXI J1348−630 underwent two reflares with peak fluxes approximately one and two orders of magnitude fainter than the main outburst, respectively. During the reflares, the source remained in the HS only, without undergoing any state transitions, which is similar to the so-called ‘failed outbursts’. Different types of quasi-periodic oscillations (QPOs) are observed at different phases of the outburst. Based on our spectral-timing results, we conclude that MAXI J1348−630 is a black hole candidate.

Influence of Geo-stress on Dynamic Response Characteristics of Coal and Gas Outburst

The influence of tectonic, geostatic, and mining stresses on coal and gas outburst (CGO) becomes more obvious with increasing mining depth. The limitations of general experimental methods and test systems hinder the direct observation and characterization of the dynamic responses of the multi-physical parameters associated with CGO. In this study, a custom-made large-scale physical simulation test system was used to study the characteristics of outburst evolution under different geo-stress levels, to overcome dynamic disasters in coal mines. The intensity and initial velocity of the outburst coal flow increased with increasing geo-stress. The geo-stress changes and gas pressure released during the outburst process mainly manifested in the relief and abutment stress zones, while the release of the elastic strain energy manifested in the abutment stress zone. This showed that the sphere of influence of the change on the physical parameters, such as the geo-stress, gas pressure, and elastic strain energy, is important for setting the drilling depth in measures to eliminate outburst. Interestingly, there seems to be a mechanism that suppresses the excess energy released during the outburst process, such that the solid–gas ratio of the two-phase outburst flow does not increase with increasing initial geo-stress. The dynamic outburst phenomenon is not completely positively correlated with increasing geo-stress. The mechanism causes cyclical fluctuations in the geo-stress, elastic strain energy, and gas pressure during the CGO process and pulse characteristics of the outburst coal flow simultaneously.

Evolution of MAXI J1631–479 during the January 2019 outburst observed by INTEGRAL/IBIS

ABSTRACT We report on a recent bright outburst from the new X-ray binary transient MAXI J1631–479, observed in January 2019. In particular, we present the 30–200 keV analysis of spectral transitions observed with INTEGRAL/IBIS during its Galactic plane monitoring program. In the MAXI and BAT monitoring period, we observed two different spectral transitions between the high/soft and low/hard states. The INTEGRAL spectrum from data taken soon before the second transition is best described by a Comptonized thermal component with a temperature of kTe ∼ 30 keV and a high-luminosity value of $L_{2-200\, \mathrm{keV}}\sim 3\times 10^{38}$ erg−1 (assuming a distance of 8 kpc). During the second transition, the source shows a hard, power-law spectrum. The lack of high energy cut-off indicates that the hard X-ray spectrum from MAXI J1631–479 is due to a non-thermal emission. Inverse Compton scattering of soft X-ray photons from a non-thermal or hybrid thermal/non-thermal electron distribution can explain the observed X-ray spectrum although a contribution to the hard X-ray emission from a jet cannot be determined at this stage. The outburst evolution in the hardness-intensity diagram, the spectral characteristics, and the rise and decay times of the outburst are suggesting that this system is a black hole candidate.

Experimental investigations of stress‐gas pressure evolution rules of coal and gas outburst: A case study in Dingji coal mine, China

AbstractCoal and gas outburst is a potentially fatal risk during the mining of gassy coal seams, which seriously threatens the safe mining of collieries. To understand the outburst mechanism and evolution rules, a new apparatus (LSTT) was developed to conduct simulated experiment. In the context of an outburst accident in Dingji coal mine, the authors launched an authentic outburst experiment to replay the outburst accident. Experimental apparatus, similar criterion, coal‐like materials and gas sources, and experimental design were discussed systematically in this paper. Experimentally, the study analyzed the geo‐stress has significant influence on the outburst evolution. At the driving face, the stress concentration possibly caused gas outburst, under the influence of mining‐induced stress. After the outburst occurred, the stress balance of the coal changed, resulting in the instability of the coal. Furthermore, the elastic energy, gas enthalpy, and gravitational potential energy were released rapidly. The experimental result stated that outburst coal has the sorting characteristics, in line with the field outburst law. The intensity prediction model has been built based on the energy model. Moreover, the factors that impact outburst intensity were analyzed. In the process of coal and gas outburst, the gas enthalpy of gas and the elastic potential of coal are the main energy sources. This study provides guidance for the development of the outburst mechanism and outburst mine management.

An experimental investigation of the roles of water content and gas decompression rate for outburst in coal briquettes

The coal and gas outburst has become a worldwide challenge and is still not fully understood. In this study, an experimental investigation was carried out for outburst evolution. Coal briquettes were fabricated based on less than 0.6 mm pulverized coal particles of Tunliu Coal Mine. CO2 was used to simulate outbursts by saturating coal briquettes in varying gas pressures (0.2, 0.4 and 0.6 MPa). The outbursts of coal briquettes were induced when rapid gas decompression was performed. The results indicate the outbursts depend on the gas pressure, water content and the rate of gas decompression. The differential pressure between the gas inside and outside the coal briquette is essential to outbursts and varies for different water contents while other parameters affect the initiation and intensity of outbursts as well. The higher gas pressure the more intense outburst occurs. It is worth noting that low rate of gas decompression cannot activate any outburst in the experiments even for a high gas pressure. The water content affects the outburst strongly. The critical minimum gas pressure in the coal briquettes with high water content becomes much higher than that in the coal briquettes with low water content. The major energy of the outbursts in the experiments is the expansion energy of the free state gas inside the coal briquettes. This study clearly demonstrates the progressive fragmentation of outbursts due to gas decompression.

Ice-Dammed Lake Drainage Evolution at Russell Glacier, West Greenland

Glaciological and hydraulic factors that control the timing and mechanisms of glacier lake outburst floods (GLOFs) remain poorly understood. This study used measurements of lake level at 15 min intervals and known lake bathymetry to calculate lake outflow during two GLOF events from the northern margin of Russell Glacier, west Greenland. We used measured ice surface elevation, interpolated subglacial topography and likely conduit geometry to inform a melt enlargement model of the outburst evolution. The model was tuned to best-fit the hydrograph rising limb and timing of peak discharge in both events; it achieved Mean Absolute Errors of <5%. About one third of the way through the rising limb, conduit melt enlargement became the dominant drainage mechanism. Lake water temperature, which strongly governed the enlargement rate, preconditioned the high peak discharge and short duration of these floods. We hypothesize that both GLOFs were triggered by ice dam flotation, and localized hydraulic jacking sustained most of their early-stage outflow, explaining the particularly rapid water egress in comparison to that recorded at other ice-marginal lakes. As ice overburden pressure relative to lake water hydraulic head diminished, flow became confined to a subglacial conduit. This study has emphasized the inter-play between ice dam thickness and lake level, drainage timing, lake water temperature and consequently rising stage lake outflow and flood evolution.

Swift observations of V404 Cyg during the 2015 outburst: X-ray outflows from super-Eddington accretion

The black-hole binary V404 Cyg entered the outburst phase in June 2015 after 26 years of X-ray quiescence, and with its behaviour broke the outburst evolution pattern typical of most black-hole binaries. We observed the entire outburst with the Swift satellite and performed time-resolved spectroscopy of its most active phase, obtaining over a thousand spectra with exposures from tens to hundreds of seconds. All the spectra can be fitted with an absorbed power law model, which most of the time required the presence of a partial covering. A blue-shifted iron-Kalpha line appears in 10% of the spectra together with the signature of high column densities, and about 20% of the spectra seem to show signatures of reflection. None of the spectra showed the unambiguous presence of soft disk-blackbody emission, while the observed bolometric flux exceeded the Eddington value in 3% of the spectra. Our results can be explained assuming that the inner part of the accretion flow is inflated into a slim disk that both hides the innermost (and brightest) regions of the flow, and produces a cold, clumpy, high-density outflow that introduces the high-absorption and fast spectral variability observed. We argue that the black hole in V404 Cyg might have been accreting erratically or even continuously at Eddington/Super-Eddington rates - thus sustaining a surrounding slim disk - while being partly or completely obscured by the inflated disk and its outflow. Hence, the largest flares produced by the source might not be accretion-driven events, but instead the effects of the unveiling of the extremely bright source hidden within the system.