The observed properties of Gamma-Ray Bursts such as rapid variability of X-ray light curve and large energies strongly signature the compact binary, disk accreting system. Our work particularly highlights the extremel...The observed properties of Gamma-Ray Bursts such as rapid variability of X-ray light curve and large energies strongly signature the compact binary, disk accreting system. Our work particularly highlights the extremely rotating, disk accreting black holes as physical source of the flares variability and X-ray afterglow plateaus of GRBs. We investigate the compact binary mergers (neutron star - neutron star and neutron star onto black hole) and gravitational core collapse of super massive star, where in both cases hyper-accreting Kerr hole is formed. The core collapse in a powerful gravitational wave explained as a potential source for the radiated flux of hard X-rays spectrum. We described the evolution of rapidly rotating, accreting BH in general relativity and the relativistic accretion flow in resistive MHD for viscous radiation. We compute the structure of accretion disk, the accretion luminosity of the dynamical evolution of inner accretion disk and precisely determine their radiation spectra, and compare to observational data of X-ray satellites. Finally, we obtained the resulting disk radiation basically explained as the X-ray luminosity of the central source, such as LMC X-1 and GRO J1655-40. These results are interestingly consistent with observational data of galactic X-ray source binary systems such as X-ray luminosities of Cygnus X-1 and Seyfert galaxies (NGC 3783, NGC 4151, NGC 4486 (Messier 87)) which are powerful emitters in X-ray and gamma-ray wavebands of the observed X-ray variability with typical luminosity.展开更多
In this paper,we have investigated accreting millisecond X-ray pulsars,which are rapidly rotating neutron stars in low-mass X-ray binaries.These systems exhibit coherent X-ray pulsations that arise when the accretion ...In this paper,we have investigated accreting millisecond X-ray pulsars,which are rapidly rotating neutron stars in low-mass X-ray binaries.These systems exhibit coherent X-ray pulsations that arise when the accretion flow is magnetically channeled to the stellar surface.Here,we have developed the fundamental equations for an accretion disk around accreting millisecond X-ray pulsars in the presence of a dynamo generated magnetic field in the inner part of the disk.We have also formulated the numerical method for the structure equations in the inner region of the disk and the highest accretion rate is enough to form the inner region of the disk,which is overpowered by radiation pressure and electron scattering.Finally,we have examined our results with the effects of dynamo magnetic fields on accreting millisecond X-ray pulsars.展开更多
One ultraluminous X-ray source in M82 has recently been identified as an accreting neutron star （named NuSTAR J095551＋6940.8）. It has a super-Eddington luminosity and is spinning up. An aged magnetar is more likely...One ultraluminous X-ray source in M82 has recently been identified as an accreting neutron star （named NuSTAR J095551＋6940.8）. It has a super-Eddington luminosity and is spinning up. An aged magnetar is more likely to be a low magnetic field magnetar. An accreting low magnetic field magnetar may explain both the super- Eddington luminosity and the rotational behavior of this source. Considering the effect of beaming, the spin-up rate is understandable using the traditional form of accretion torque. The transient nature and spectral properties of M82 X-2 are discussed. The theoretical range of periods for accreting magnetars is provided. Three observational appearances of accreting magnetars are summarized.展开更多
Using the Modules for Experiments in Stellar Astrophysics(MESA) code, we investigate Type I X-ray bursts(XRBs) produced by neutron stars(NSs) accreting pure helium, which are called intermediate XRBs in observations. ...Using the Modules for Experiments in Stellar Astrophysics(MESA) code, we investigate Type I X-ray bursts(XRBs) produced by neutron stars(NSs) accreting pure helium, which are called intermediate XRBs in observations. We simulate 21 models for intermediate XRBs with various mass-accretion rates(M) from 2.5 × 10^-8 to 5 × 10^-10 M⊙yr^-1. Compared with normal XRBs, in which the NS accretes matter with solar metallicity, intermediate XRBs have higher luminosity and longer recurrence time,which are essentially consistent with observations. We find that the recurrence time of intermediate XRBs is proportional to M^-2.0.展开更多
We report an indication （3.22 or） of ≈ 1860 Hz quasi-periodic oscillations from a neutron star low-mass X-ray binary 4U 1636-536. If confirmed, this will be by far the highest frequency feature observed from an acc...We report an indication （3.22 or） of ≈ 1860 Hz quasi-periodic oscillations from a neutron star low-mass X-ray binary 4U 1636-536. If confirmed, this will be by far the highest frequency feature observed from an accreting neutron star system, and hence could be very useful in understanding such systems. This plausible timing feature was observed simultaneously with lower （≈ 585 Hz） and upper （≈ 904 Hz） kilohertz quasi-periodic oscillations. The two kilohertz quasi-periodic oscillation frequencies had a ratio of ≈ 1.5, and the frequency of the alleged ≈ 1860 Hz feature was close to the triple and the double values of these frequencies. This can be useful for constraining the models of all the three features. In particular, the ≈ 1860Hz feature could be （1） from a new and heretofore unknown class of quasi-periodic oscillations, or （2） the first observed overtone of lower or upper kilohertz quasi-periodic oscillations. Finally, we note that, although the relatively low significance of the ≈ 1860 Hz feature argues for caution, even a 3.22 σ feature at such a uniquely high frequency should be interesting enough to spur a systematic search in the archival data, as well as to scientifically motivate sufficiently large timing instruments for the next generation X-ray missions.展开更多
X-ray pulsars(XRPs)consist of a magnetized neutron star(NS)and an optical donor star.The NS accretes matter from the donor star,producing pulsed X-ray emission.In most cases,the donor stars are Be stars,and accretion ...X-ray pulsars(XRPs)consist of a magnetized neutron star(NS)and an optical donor star.The NS accretes matter from the donor star,producing pulsed X-ray emission.In most cases,the donor stars are Be stars,and accretion is episodic,that is,the NSs are generally X-ray dim but occasionally experience outbursts.Here,we carry out a statistical study with the X-ray monitoring data and obtain strong correlations between the spin periods of the NSs and the outburst parameters for the first time.We show that XRPs containing faster rotating NSs tend to display more violent eruptions.In addition,pulsating ultraluminous X-ray sources in nearby galaxies follow a similar relationship.We demonstrate that most of these systems are close to the spin equilibrium,and that brighter pulsars have acquired more angular momentum by accreting matter from their companion stars,resulting in faster rotating NSs.展开更多
In this review a detailed treatment of the dynamics of an accreting raindrop falling through stationary mist under the action of gravity and air resistance,is presented.There are two contributions to the air resistanc...In this review a detailed treatment of the dynamics of an accreting raindrop falling through stationary mist under the action of gravity and air resistance,is presented.There are two contributions to the air resistance:one term pro-portional to the velocity and one proportional to the square of the velocity.It is discussed when it is a good approximation to neglect one of these terms.Also,a new form of the solution of the equation of motion is presented when accretion is neglected,but both the linear and quadratic contributions to the air resistance are present.The paper is written in a style making it useful in the teaching of classical dynamics,including in between calculations and providing several examples where the equation of motion of the drop,and the equation for the change of the droplet’s mass due to accretion,can be solved analytically in terms of elementary functions.展开更多
Clouds play an important role in global atmospheric energy and water vapor budgets, and the low cloud simulations suffer from large biases in many atmospheric general circulation models. In this study, cloud microphys...Clouds play an important role in global atmospheric energy and water vapor budgets, and the low cloud simulations suffer from large biases in many atmospheric general circulation models. In this study, cloud microphysical processes such as raindrop evaporation and cloud water accretion in a double-moment six-class cloud microphysics scheme were revised to enhance the simulation of low clouds using the Global-Regional Integrated Forecast System(GRIST)model. The validation of the revised scheme using a single-column version of the GRIST demonstrated a reasonable reduction in liquid water biases. The revised parameterization simulated medium-and low-level cloud fractions that were in better agreement with the observations than the original scheme. Long-term global simulations indicate the mitigation of the originally overestimated low-level cloud fraction and cloud-water mixing ratio in mid-to high-latitude regions,primarily owing to enhanced accretion processes and weakened raindrop evaporation. The reduced low clouds with the revised scheme showed better consistency with satellite observations, particularly at mid-and high-latitudes. Further improvements can be observed in the simulated cloud shortwave radiative forcing and vertical distribution of total cloud cover. Annual precipitation in mid-latitude regions has also improved, particularly over the oceans, with significantly increased large-scale and decreased convective precipitation.展开更多
Experiments with interacting high-velocity flows in a laser plasma can help answer fundamental questions in plasma physics and improve understanding of the mechanisms behind some astrophysical phenomena,such as the fo...Experiments with interacting high-velocity flows in a laser plasma can help answer fundamental questions in plasma physics and improve understanding of the mechanisms behind some astrophysical phenomena,such as the formation of collisionless shock waves,deceleration of accretion flows,and evolution of solar and stellar flares.This work presents the first direct experimental observations of stagnation and redirection of counterstreaming flows(jets)of laser plasma induced by intense laser pulses with intensity I~2×10^(18) W/cm^(2).Hybrid particlein-cell-fluid modeling,which takes into account the kinetic effects of ion motion and the evolution of the pressure tensor for electrons,demonstrates the compression of counterdirected toroidal self-generated magnetic fields embedded in counterstreaming plasma flows.The enhancement of the toroidal magnetic field in the interaction region results in plasma flow stagnation and redirection of the jets across the line of their initial propagation.展开更多
An unsteady numerical simulation is conducted to examine the dynamic runback characteristics of a water film flow driven by a boundary layer airflow over a solid surface pertinent to the dynamic glaze ice accretion pr...An unsteady numerical simulation is conducted to examine the dynamic runback characteristics of a water film flow driven by a boundary layer airflow over a solid surface pertinent to the dynamic glaze ice accretion process over aircraft wing surfaces.The multiphase flow simulation results of the wind-driven water runback(WDWR)flow are compared quantitatively with the experimental results in terms of the time-dependent variations of the water film thickness profiles and evolution of the front contact point of the runback water film flow.The underlying mechanism of the intermittent water runback behavior is elucidated by analyzing the time evolution of the airflow velocity and vorticity fields above the runback water film flow over the solid surface.To the best knowledge of the authors,the work presented here is the first successful attempt to numerically examine the transient runback characteristics of WDWR flows.It serves as an excellent benchmark case for the development of best practices to model the important micro-physical processes responsible for the transient water transport over aircraft wing surfaces.展开更多
LB-1 is a binary system that has drawn great attention since its discovery in 2019.The two components of LB-1 are suggested to be a B-type star plus a black hole (BH).In this paper,we first calculate the wind mass-los...LB-1 is a binary system that has drawn great attention since its discovery in 2019.The two components of LB-1 are suggested to be a B-type star plus a black hole (BH).In this paper,we first calculate the wind mass-loss rate of the B-type star.We then calculate the mass capture rate by the BH,with which as the initial mass accretion rate,we calculate the truncation radius of the accretion disk and the corresponding emergent spectra of the accretion flow(comprising an inner advection-dominated accretion flow (ADAF)+an outer truncated accretion disk) within the framework of the disk evaporation model.It is found that the predicted truncation radius of the accretion disk with appropriate model parameters is consistent with observations inferred from the observed broad H_(α) emission line.The predicted X-ray luminosity is definitely below the estimated upper limits with the sensitivity of the Chandra X-ray Observatory of the X-ray luminosity≈2×10^(31)ergs^(-1),which is also consistent with observations.Finally,we argue that the disk evaporation model can indeed reflect the intrinsic physics of the accretion flow in LB-1,and has potential to be applied to other quiescent BHs in the future.展开更多
Stars can form and evolve within gaseous disks around active galactic nuclei(AGNs).In the sub-parsec region of disks around~10~8M_(☉)black holes,stars accrete rapidly,reaching■200 M_(☉)and settling into a quasi-ste...Stars can form and evolve within gaseous disks around active galactic nuclei(AGNs).In the sub-parsec region of disks around~10~8M_(☉)black holes,stars accrete rapidly,reaching■200 M_(☉)and settling into a quasi-steady state in which accretion balances wind-driven mass loss.Within this environment,their ultimate fate depends critically on the radiative-zone diffusion coefficient(Dmix),which encapsulates various mixing processes and governs chemical transport between surface and core.Using the MESA stellar evolution code,we simulate AGN stars across a range of mixing efficiencies.We find a critical threshold floor value D_(mix,min)≈1010 cm~2 s^(-1)that separates two distinct fates:1."Immortal stars"—when mixing is over-efficient(D_(mix,min)■10^(10)cm~2 s^(-1)),rapid hydrogen replenishment sustains core hydrogen burning,maintains main-sequence equilibrium,rendering the star effectively“immortal.”2."Metamorphic stars"—when mixing is merely efficient(D_(mix,min)<1010 cm~2 s^(-1)),stars exhaust core hydrogen,evolve off-main-sequence,shed mass to≈15 M_(☉),and produce super-solar a-abundances consistent with AGN observations.We conclude that maintaining a mixing floor below this threshold is sufficient to avoid immortality,as flux-induced extra mixing can be effectively modeled via constant floor values.Our estimates provide a foundation for future work on disk enrichment and stellar evolution.展开更多
Changes in the coastline are characterized by accretion and erosion. The aim of this study is to contribute to a better understanding of the dynamics of the coastline and the study areas with a view to mitigating and ...Changes in the coastline are characterized by accretion and erosion. The aim of this study is to contribute to a better understanding of the dynamics of the coastline and the study areas with a view to mitigating and preventing the risk of coastal erosion in order to propose a coastal occupation model with planned development policies in the future. These phenomena lead to changes in the position of the coastline. After extraction, the satellite images are compiled, then superimposed and processed using Geographic Information Systems (GIS) for statistical calculation of coastline change rates. A morphosedimentary study is carried out using topography and sedimentology. The topographic method is used to calculate sediment volumes using monthly profiles. The sedimentological method is used to determine the granulometric variations in the morphological units by calculating sedimentological indices. With erosion rates of −2.13 m/yr and −2.17 m/yr respectively at Djiffère (Palmarin and Sangomar breccia) and Joal (Joal Fadhiouth and Ngazobil), the EPR index revealed a sediment deficit. Palmarin Ngallou and the island of Fadhiouth are undergoing accretion at rates of +1.43 m/yr and +1.14 m/yr respectively. From a topographical point of view, the respective accumulations of −13.74 m3/m of beach and −8.65 m3/m of beach at Djiffère and Joal respectively point to significant erosion on all the aerial beach units, while for the underwater beaches, accretion was noted with accumulations of +4.00 m3/m of beach and +5.94 m3/m at Djiffère and Joal respectively. As for the sedimentological results, the Mz index shows a decrease in grain size from the high beach to the surf zone. Some points show bimodal deposits, showing the impact of the dune on beach activity, confirmed by the dispersion on the Mz-sigma diagram. The three methods used in this work show that the Djiffère sector in Joal is dominated by erosion, even though accretion points can be noted.展开更多
This paper investigates the star formation efficiency(SFE)of blue compact galaxies(BCGs)by analyzing various indicators,including(O II),(H_(α)),infrared emissions,and the 1.4 GHz radio continuum.The assessment of SFE...This paper investigates the star formation efficiency(SFE)of blue compact galaxies(BCGs)by analyzing various indicators,including(O II),(H_(α)),infrared emissions,and the 1.4 GHz radio continuum.The assessment of SFE was conducted using publicly accessible data that concentrated on star formation rates(SFRs)and depletion timescales.By employing the fundamental equation,I determined the SFE in relation to its freefall time,which highlighted variations in efficiencies associated with SFR masses derived from different spectra.The results demonstrated that the highest efficiency value was observed in the analysis of the 1.4 GHz radio continuum,emphasizing the importance of studying star-forming clouds across a range of electromagnetic wavelengths to achieve a comprehensive understanding of star formation mechanisms.The study's findings indicated a peak efficiency of star formation value of approximately 0.9%,while the relative minimum obtained from(O II)was around 0.42%,aligning with the range of SFEs documented in the existing literature.Additionally,investigating the factors that influence the variability in efficiency when examining clouds in BCGs across different wave bands is crucial for deepening our comprehension of the underlying processes of star formation.展开更多
Based on the RXTE observations of Sco X-1,we identified a clear dichotomy in the behavior of its X-ray crosscorrelation functions(CCFs)which is possibly associated with two different radio jet ejection events.Twenty s...Based on the RXTE observations of Sco X-1,we identified a clear dichotomy in the behavior of its X-ray crosscorrelation functions(CCFs)which is possibly associated with two different radio jet ejection events.Twenty six observations display asymmetric CCFs of low correlation coefficients with soft or hard delays of a few hundred seconds along with no oscillation feature in the power density spectrum associated with horizontal branch/hard apex.All the observations belonging to NB/FB have symmetric CCFs with NBO/NBO+HBO/FBO in the PDS.Based on radio observations,two different types of jets were associated with Sco X-1 i.e.,ballistic radio jets and an ultra-relativistic flow(URF).We noted that asymmetric CCFs with delays were connected with ballistic or lobe radio jets and symmetric CCFs with URFs in Sco X-1.We interpret these findings as evidence of two different states of the inner accretion disk.We propose that the launch of a ballistic jet triggers significant instability within the inner accretion region,likely the boundary layer and corona.This disruption explains the observed delays in the CCF and simultaneously quenches the conditions required for coherent oscillations,leaving only flat-topped noise in the PDS.We constrain the size of this unstable region in the inner region of accretion to be~10-40 km.In contrast,the symmetric CCFs and persistent oscillations(NBO/NBO+HBO)during URF events suggest a more stable,steady accretion flow.Although connecting the URF to a specific oscillation is difficult,its strong association with the NB phase of variation suggests a common underlying physical mechanism.Therefore,the CCF's asymmetry acts as a powerful diagnostic,linking ballistic jets to disk instability and URFs to a stable accretion state.Various physical scenarios are discussed to understand the connection among X-ray CCFs,PDS,and radio ejections in Sco X-1.展开更多
We conducted an analysis of the continuum during the onset and initial decline phases of the 2023 outburst in transient neutron star low-mass X-ray binary Aql X–1 using broadband observations from the Hard X-ray Modu...We conducted an analysis of the continuum during the onset and initial decline phases of the 2023 outburst in transient neutron star low-mass X-ray binary Aql X–1 using broadband observations from the Hard X-ray Modulation Telescope(Insight-HXMT)instrument.To determine the most appropriate model for the continuum of this outburst,we employed three models to explore the evolution of the spectral component.These observations revealed that the source transitions from the hard state to the soft state.The disk-corona and sphere-corona models both adequately described the spectra of the hard state,while the double blackbody model became preferable after the hard X-ray emission(>25 keV)disappeared during the state transition.In the soft state,the total emission is dominated by the accretion disk and two blackbody components.The combination of the sphere-corona model and the double blackbody model is the most suitable model for this outburst.The results suggest that as the source transitioned into the soft state,the emission from the boundary layer was enhanced,and a hot spot occurred.Notably,we identified two type-ⅠX-ray bursts,one of which exhibited a significant hard X-ray deficit(significance~4.82σ),which indicates that Insight-HXMT has the capability to capture the evolution of the corona in a single burst.展开更多
Coastal zones are dynamic interfaces responding to complex natural processes and anthropogenic pressures.Monitoring shoreline evolution is essential for sustainable coastal management,particularly given climate change...Coastal zones are dynamic interfaces responding to complex natural processes and anthropogenic pressures.Monitoring shoreline evolution is essential for sustainable coastal management,particularly given climate change,urban expansion,and sediment flux disruption.This study investigates shoreline changes along Morocco’s northern Atlantic coast from 1990 to 2023,an area of strategic economic importance and environmental vulnerability.Landsat satellite imagery and geospatial techniques,including the Digital Shoreline Analysis System(DSAS v5.1)and the Normalized Difference Water Index(NDWI),provided a high-resolution,diachronic assessment.Shoreline extraction and image enhancement were conducted with ENVI software,and change detection utilized Linear Regression Rate(LRR)and End Point Rate(EPR)indicators.Results revealed significant spatial variability:sectors like northern Moulay Bousselham and Chlihat showed pronounced accretion(+3.2 to+4.7 m/year),while areas such as Tahaddart and southern Mehdia experienced severe erosion(up to−3.4 m/year).The total net eroded area exceeds 58,000 m².Trends correlate strongly with hydrodynamic forces,upstream damming,sediment extraction,and extreme weather events,notably storms in 2014 and 2017.Findings align with studies highlighting compounded effects of sediment starvation and sea-level rise.By integrating remote sensing,time-series analysis,and uncertainty quantification,this research provides insights into the primary drivers of shoreline dynamics,emphasizing the urgent need for adaptive,evidence-based coastal management strategies,including regulation of sand mining,sediment buffer restoration,and soft-engineering solutions.展开更多
Changing-look(CL)active galactic nuclei(AGNs)are an extremely rare type of AGNs,characterized by the appearance(turn-on)or disappearance(turn-off)of broad emission lines within months to years.We construct a sample of...Changing-look(CL)active galactic nuclei(AGNs)are an extremely rare type of AGNs,characterized by the appearance(turn-on)or disappearance(turn-off)of broad emission lines within months to years.We construct a sample of 211 CL AGN candidates by systematically searching the Sloan Digital Sky Survey spectra classified as GALAXY,but with the Zwicky Transient Facility and Near-Earth Object Wide-Field Infrared Explorer midinfrared light curves showing significant variability,at the redshift range from 0.1 to 0.75.To examine the success rate of this method,we cross-match the candidate sample with the Large Sky Area Multi-Object Fiber Spectroscopic Telescope and the Dark Energy Spectroscopic Instrument for repeat spectra,finding that the success rate of this method is 50%.By incorporating previously reported CL AGNs,a total of 48 CL AGNs have been identified within our sample,including two recurrent CL AGNs newly confirmed in this work.Our results suggest that the typical rest-frame timescale of CL behavior is from 1.7 to 18.1 yr,which is inconsistent with the obscuration model,and the Eddington ratios of CL AGNs are about 0.01 in the bright state.Additionally,our CL AGN sample follows the M_(BH)–σ_(*)relation of quiescent galaxies.In the future,we expect to detect more galaxies turning into Type 1 AGNs in this CL AGN candidate sample.展开更多
We present simulation results examining the presence and behavior of standing shocks in zero-energy low angular momentum advective accretion flows and explore their(in)stability properties,taking into account various ...We present simulation results examining the presence and behavior of standing shocks in zero-energy low angular momentum advective accretion flows and explore their(in)stability properties,taking into account various values of specific angular momentum,λ_(0).Within the range 10-50R_(g)(where R_(g)denotes the Schwarzschild radius),shocks are discernible forλ_(0)≥1.75.In the special relativistic hydrodynamic simulation whenλ_(0)=1.80,we find the merger of two shocks resulted in a dramatic increase in luminosity.We present the impact of external and internal flow collisions from the funnel region on luminosity.Notably,oscillatory behavior characterizes shocks within 1.70≤λ_(0)≤1.80.Using free–free emission as a proxy for analysis,we show that the luminosity oscillations between frequencies of 0.1-10 Hz forλ_(0)range as 1.7≤λ_(0)≤1.80.These findings offer insights into quasiperiodic oscillation emissions from certain black hole X-ray binaries,exemplified by GX 339-4.Furthermore,for the supermassive black hole at the Milky Way's center,Sgr A*,oscillation frequencies between 10^(-6)and 10^(-5)Hz were observed.This frequency range,translating to one cycle every few days,aligns with observational data from X-ray telescopes such as Chandra,Swift,and XMM-Newton.展开更多
We present the results of a multi-epoch,detailed spectral and temporal analysis of the ultraluminous X-ray source(ULX),ULX-1,in the galaxy NGC 4088 using XMM-Newton,Chandra,and Swift observations.The presence of a har...We present the results of a multi-epoch,detailed spectral and temporal analysis of the ultraluminous X-ray source(ULX),ULX-1,in the galaxy NGC 4088 using XMM-Newton,Chandra,and Swift observations.The presence of a hard powerlaw spectral slope supports the interpretation of ULX-1 as a hard ULX.The observed inner disk temperature of kT_(in)> 1.5 keV is inconsistent with the presence of an intermediate mass black hole,but favors the super-Eddington accretion state.Moreover,the physically acceptable value of the parameter controlling the radial temperature profile of the disk(p) derived from fitting the first XMM-Newton observation with the slim disk model further points towards the possible presence of a broadened disk,indicating the super-Eddington accretion nature.Slight overall long-term flux variability is evident for this ULX,and a hint towards a positive correlation between flux and the powerlaw photon index is also observed when the relatively better data of XMM-Newton and Chandra are considered.The L-T relationship is observed to follow a positive trend,with the L-T profile consistent with either relation(L ∝ T^(4) or L ∝ T^(2)) in both cases.The source exhibits no significant short-term variability at different time binnings of the light curve as indicated by the chi-square probability of constancy and fractional RMS variability values.The power density spectrum created shows no evidence of intrinsic variability of the source above the white noise.Further,no sign of pulsation was detected for this source.Assuming this ULX to be powered by an accreting black hole and using the slim disk geometry,the upper limit of the black hole mass was estimated and found to be less than 100M_(⊙).展开更多
文摘The observed properties of Gamma-Ray Bursts such as rapid variability of X-ray light curve and large energies strongly signature the compact binary, disk accreting system. Our work particularly highlights the extremely rotating, disk accreting black holes as physical source of the flares variability and X-ray afterglow plateaus of GRBs. We investigate the compact binary mergers (neutron star - neutron star and neutron star onto black hole) and gravitational core collapse of super massive star, where in both cases hyper-accreting Kerr hole is formed. The core collapse in a powerful gravitational wave explained as a potential source for the radiated flux of hard X-rays spectrum. We described the evolution of rapidly rotating, accreting BH in general relativity and the relativistic accretion flow in resistive MHD for viscous radiation. We compute the structure of accretion disk, the accretion luminosity of the dynamical evolution of inner accretion disk and precisely determine their radiation spectra, and compare to observational data of X-ray satellites. Finally, we obtained the resulting disk radiation basically explained as the X-ray luminosity of the central source, such as LMC X-1 and GRO J1655-40. These results are interestingly consistent with observational data of galactic X-ray source binary systems such as X-ray luminosities of Cygnus X-1 and Seyfert galaxies (NGC 3783, NGC 4151, NGC 4486 (Messier 87)) which are powerful emitters in X-ray and gamma-ray wavebands of the observed X-ray variability with typical luminosity.
基金the Ethiopian Space Science and Technology InstituteEntoto Observatory and Research CenterAstronomy and Astrophysics Research and Development Department for supporting this research。
文摘In this paper,we have investigated accreting millisecond X-ray pulsars,which are rapidly rotating neutron stars in low-mass X-ray binaries.These systems exhibit coherent X-ray pulsations that arise when the accretion flow is magnetically channeled to the stellar surface.Here,we have developed the fundamental equations for an accretion disk around accreting millisecond X-ray pulsars in the presence of a dynamo generated magnetic field in the inner part of the disk.We have also formulated the numerical method for the structure equations in the inner region of the disk and the highest accretion rate is enough to form the inner region of the disk,which is overpowered by radiation pressure and electron scattering.Finally,we have examined our results with the effects of dynamo magnetic fields on accreting millisecond X-ray pulsars.
基金Supported by the National Natural Science Foundation of China
文摘One ultraluminous X-ray source in M82 has recently been identified as an accreting neutron star （named NuSTAR J095551＋6940.8）. It has a super-Eddington luminosity and is spinning up. An aged magnetar is more likely to be a low magnetic field magnetar. An accreting low magnetic field magnetar may explain both the super- Eddington luminosity and the rotational behavior of this source. Considering the effect of beaming, the spin-up rate is understandable using the traditional form of accretion torque. The transient nature and spectral properties of M82 X-2 are discussed. The theoretical range of periods for accreting magnetars is provided. Three observational appearances of accreting magnetars are summarized.
基金National Natural Science Foundation of China(Grant Nos.11763007,11473024,11463005,11863005,11803026 and 11503008)Tianshan Youth Project of Xinjiang(Grant No.2017Q014)。
文摘Using the Modules for Experiments in Stellar Astrophysics(MESA) code, we investigate Type I X-ray bursts(XRBs) produced by neutron stars(NSs) accreting pure helium, which are called intermediate XRBs in observations. We simulate 21 models for intermediate XRBs with various mass-accretion rates(M) from 2.5 × 10^-8 to 5 × 10^-10 M⊙yr^-1. Compared with normal XRBs, in which the NS accretes matter with solar metallicity, intermediate XRBs have higher luminosity and longer recurrence time,which are essentially consistent with observations. We find that the recurrence time of intermediate XRBs is proportional to M^-2.0.
文摘We report an indication （3.22 or） of ≈ 1860 Hz quasi-periodic oscillations from a neutron star low-mass X-ray binary 4U 1636-536. If confirmed, this will be by far the highest frequency feature observed from an accreting neutron star system, and hence could be very useful in understanding such systems. This plausible timing feature was observed simultaneously with lower （≈ 585 Hz） and upper （≈ 904 Hz） kilohertz quasi-periodic oscillations. The two kilohertz quasi-periodic oscillation frequencies had a ratio of ≈ 1.5, and the frequency of the alleged ≈ 1860 Hz feature was close to the triple and the double values of these frequencies. This can be useful for constraining the models of all the three features. In particular, the ≈ 1860Hz feature could be （1） from a new and heretofore unknown class of quasi-periodic oscillations, or （2） the first observed overtone of lower or upper kilohertz quasi-periodic oscillations. Finally, we note that, although the relatively low significance of the ≈ 1860 Hz feature argues for caution, even a 3.22 σ feature at such a uniquely high frequency should be interesting enough to spur a systematic search in the archival data, as well as to scientifically motivate sufficiently large timing instruments for the next generation X-ray missions.
基金supported by the China Manned Space Program (Grant No. CMS-CSST-2025-A13)the support from the National Key R&D Program of China (Grant Nos. 2021YFA0718500, and 2023YFA1608100)the National Natural Science Foundation of China (Grants Nos. 12473041, 12473042, 12393812, 12233002, U2031205, 12041301, and 12121003)
文摘X-ray pulsars(XRPs)consist of a magnetized neutron star(NS)and an optical donor star.The NS accretes matter from the donor star,producing pulsed X-ray emission.In most cases,the donor stars are Be stars,and accretion is episodic,that is,the NSs are generally X-ray dim but occasionally experience outbursts.Here,we carry out a statistical study with the X-ray monitoring data and obtain strong correlations between the spin periods of the NSs and the outburst parameters for the first time.We show that XRPs containing faster rotating NSs tend to display more violent eruptions.In addition,pulsating ultraluminous X-ray sources in nearby galaxies follow a similar relationship.We demonstrate that most of these systems are close to the spin equilibrium,and that brighter pulsars have acquired more angular momentum by accreting matter from their companion stars,resulting in faster rotating NSs.
文摘In this review a detailed treatment of the dynamics of an accreting raindrop falling through stationary mist under the action of gravity and air resistance,is presented.There are two contributions to the air resistance:one term pro-portional to the velocity and one proportional to the square of the velocity.It is discussed when it is a good approximation to neglect one of these terms.Also,a new form of the solution of the equation of motion is presented when accretion is neglected,but both the linear and quadratic contributions to the air resistance are present.The paper is written in a style making it useful in the teaching of classical dynamics,including in between calculations and providing several examples where the equation of motion of the drop,and the equation for the change of the droplet’s mass due to accretion,can be solved analytically in terms of elementary functions.
基金National Natural Science Foundation of China(42375153,42105153,42205157)Development of Science and Technology at Chinese Academy of Meteorological Sciences(2023KJ038)。
文摘Clouds play an important role in global atmospheric energy and water vapor budgets, and the low cloud simulations suffer from large biases in many atmospheric general circulation models. In this study, cloud microphysical processes such as raindrop evaporation and cloud water accretion in a double-moment six-class cloud microphysics scheme were revised to enhance the simulation of low clouds using the Global-Regional Integrated Forecast System(GRIST)model. The validation of the revised scheme using a single-column version of the GRIST demonstrated a reasonable reduction in liquid water biases. The revised parameterization simulated medium-and low-level cloud fractions that were in better agreement with the observations than the original scheme. Long-term global simulations indicate the mitigation of the originally overestimated low-level cloud fraction and cloud-water mixing ratio in mid-to high-latitude regions,primarily owing to enhanced accretion processes and weakened raindrop evaporation. The reduced low clouds with the revised scheme showed better consistency with satellite observations, particularly at mid-and high-latitudes. Further improvements can be observed in the simulated cloud shortwave radiative forcing and vertical distribution of total cloud cover. Annual precipitation in mid-latitude regions has also improved, particularly over the oceans, with significantly increased large-scale and decreased convective precipitation.
基金supported by Russian Science Foundation Grant No.24-62-00032.
文摘Experiments with interacting high-velocity flows in a laser plasma can help answer fundamental questions in plasma physics and improve understanding of the mechanisms behind some astrophysical phenomena,such as the formation of collisionless shock waves,deceleration of accretion flows,and evolution of solar and stellar flares.This work presents the first direct experimental observations of stagnation and redirection of counterstreaming flows(jets)of laser plasma induced by intense laser pulses with intensity I~2×10^(18) W/cm^(2).Hybrid particlein-cell-fluid modeling,which takes into account the kinetic effects of ion motion and the evolution of the pressure tensor for electrons,demonstrates the compression of counterdirected toroidal self-generated magnetic fields embedded in counterstreaming plasma flows.The enhancement of the toroidal magnetic field in the interaction region results in plasma flow stagnation and redirection of the jets across the line of their initial propagation.
基金supported by the National Science Foundation(NSF)of the USA(Grant Nos.TIP-2140489,CBET-2313310,and CBET-2415347).
文摘An unsteady numerical simulation is conducted to examine the dynamic runback characteristics of a water film flow driven by a boundary layer airflow over a solid surface pertinent to the dynamic glaze ice accretion process over aircraft wing surfaces.The multiphase flow simulation results of the wind-driven water runback(WDWR)flow are compared quantitatively with the experimental results in terms of the time-dependent variations of the water film thickness profiles and evolution of the front contact point of the runback water film flow.The underlying mechanism of the intermittent water runback behavior is elucidated by analyzing the time evolution of the airflow velocity and vorticity fields above the runback water film flow over the solid surface.To the best knowledge of the authors,the work presented here is the first successful attempt to numerically examine the transient runback characteristics of WDWR flows.It serves as an excellent benchmark case for the development of best practices to model the important micro-physical processes responsible for the transient water transport over aircraft wing surfaces.
基金supported by the National Natural Science Foundation of China (grant Nos.12173048,12333004,11988101,and 12273057)the National Key R&D Program of China (No.2023YFA1607903)the support from the K.C.Wong Education Foundation。
文摘LB-1 is a binary system that has drawn great attention since its discovery in 2019.The two components of LB-1 are suggested to be a B-type star plus a black hole (BH).In this paper,we first calculate the wind mass-loss rate of the B-type star.We then calculate the mass capture rate by the BH,with which as the initial mass accretion rate,we calculate the truncation radius of the accretion disk and the corresponding emergent spectra of the accretion flow(comprising an inner advection-dominated accretion flow (ADAF)+an outer truncated accretion disk) within the framework of the disk evaporation model.It is found that the predicted truncation radius of the accretion disk with appropriate model parameters is consistent with observations inferred from the observed broad H_(α) emission line.The predicted X-ray luminosity is definitely below the estimated upper limits with the sensitivity of the Chandra X-ray Observatory of the X-ray luminosity≈2×10^(31)ergs^(-1),which is also consistent with observations.Finally,we argue that the disk evaporation model can indeed reflect the intrinsic physics of the accretion flow in LB-1,and has potential to be applied to other quiescent BHs in the future.
文摘Stars can form and evolve within gaseous disks around active galactic nuclei(AGNs).In the sub-parsec region of disks around~10~8M_(☉)black holes,stars accrete rapidly,reaching■200 M_(☉)and settling into a quasi-steady state in which accretion balances wind-driven mass loss.Within this environment,their ultimate fate depends critically on the radiative-zone diffusion coefficient(Dmix),which encapsulates various mixing processes and governs chemical transport between surface and core.Using the MESA stellar evolution code,we simulate AGN stars across a range of mixing efficiencies.We find a critical threshold floor value D_(mix,min)≈1010 cm~2 s^(-1)that separates two distinct fates:1."Immortal stars"—when mixing is over-efficient(D_(mix,min)■10^(10)cm~2 s^(-1)),rapid hydrogen replenishment sustains core hydrogen burning,maintains main-sequence equilibrium,rendering the star effectively“immortal.”2."Metamorphic stars"—when mixing is merely efficient(D_(mix,min)<1010 cm~2 s^(-1)),stars exhaust core hydrogen,evolve off-main-sequence,shed mass to≈15 M_(☉),and produce super-solar a-abundances consistent with AGN observations.We conclude that maintaining a mixing floor below this threshold is sufficient to avoid immortality,as flux-induced extra mixing can be effectively modeled via constant floor values.Our estimates provide a foundation for future work on disk enrichment and stellar evolution.
文摘Changes in the coastline are characterized by accretion and erosion. The aim of this study is to contribute to a better understanding of the dynamics of the coastline and the study areas with a view to mitigating and preventing the risk of coastal erosion in order to propose a coastal occupation model with planned development policies in the future. These phenomena lead to changes in the position of the coastline. After extraction, the satellite images are compiled, then superimposed and processed using Geographic Information Systems (GIS) for statistical calculation of coastline change rates. A morphosedimentary study is carried out using topography and sedimentology. The topographic method is used to calculate sediment volumes using monthly profiles. The sedimentological method is used to determine the granulometric variations in the morphological units by calculating sedimentological indices. With erosion rates of −2.13 m/yr and −2.17 m/yr respectively at Djiffère (Palmarin and Sangomar breccia) and Joal (Joal Fadhiouth and Ngazobil), the EPR index revealed a sediment deficit. Palmarin Ngallou and the island of Fadhiouth are undergoing accretion at rates of +1.43 m/yr and +1.14 m/yr respectively. From a topographical point of view, the respective accumulations of −13.74 m3/m of beach and −8.65 m3/m of beach at Djiffère and Joal respectively point to significant erosion on all the aerial beach units, while for the underwater beaches, accretion was noted with accumulations of +4.00 m3/m of beach and +5.94 m3/m at Djiffère and Joal respectively. As for the sedimentological results, the Mz index shows a decrease in grain size from the high beach to the surf zone. Some points show bimodal deposits, showing the impact of the dune on beach activity, confirmed by the dispersion on the Mz-sigma diagram. The three methods used in this work show that the Djiffère sector in Joal is dominated by erosion, even though accretion points can be noted.
文摘This paper investigates the star formation efficiency(SFE)of blue compact galaxies(BCGs)by analyzing various indicators,including(O II),(H_(α)),infrared emissions,and the 1.4 GHz radio continuum.The assessment of SFE was conducted using publicly accessible data that concentrated on star formation rates(SFRs)and depletion timescales.By employing the fundamental equation,I determined the SFE in relation to its freefall time,which highlighted variations in efficiencies associated with SFR masses derived from different spectra.The results demonstrated that the highest efficiency value was observed in the analysis of the 1.4 GHz radio continuum,emphasizing the importance of studying star-forming clouds across a range of electromagnetic wavelengths to achieve a comprehensive understanding of star formation mechanisms.The study's findings indicated a peak efficiency of star formation value of approximately 0.9%,while the relative minimum obtained from(O II)was around 0.42%,aligning with the range of SFEs documented in the existing literature.Additionally,investigating the factors that influence the variability in efficiency when examining clouds in BCGs across different wave bands is crucial for deepening our comprehension of the underlying processes of star formation.
基金the support of the DST-ANRF(Anusandhan National Research Foundation)Core Research Grant project,Government of India。
文摘Based on the RXTE observations of Sco X-1,we identified a clear dichotomy in the behavior of its X-ray crosscorrelation functions(CCFs)which is possibly associated with two different radio jet ejection events.Twenty six observations display asymmetric CCFs of low correlation coefficients with soft or hard delays of a few hundred seconds along with no oscillation feature in the power density spectrum associated with horizontal branch/hard apex.All the observations belonging to NB/FB have symmetric CCFs with NBO/NBO+HBO/FBO in the PDS.Based on radio observations,two different types of jets were associated with Sco X-1 i.e.,ballistic radio jets and an ultra-relativistic flow(URF).We noted that asymmetric CCFs with delays were connected with ballistic or lobe radio jets and symmetric CCFs with URFs in Sco X-1.We interpret these findings as evidence of two different states of the inner accretion disk.We propose that the launch of a ballistic jet triggers significant instability within the inner accretion region,likely the boundary layer and corona.This disruption explains the observed delays in the CCF and simultaneously quenches the conditions required for coherent oscillations,leaving only flat-topped noise in the PDS.We constrain the size of this unstable region in the inner region of accretion to be~10-40 km.In contrast,the symmetric CCFs and persistent oscillations(NBO/NBO+HBO)during URF events suggest a more stable,steady accretion flow.Although connecting the URF to a specific oscillation is difficult,its strong association with the NB phase of variation suggests a common underlying physical mechanism.Therefore,the CCF's asymmetry acts as a powerful diagnostic,linking ballistic jets to disk instability and URFs to a stable accretion state.Various physical scenarios are discussed to understand the connection among X-ray CCFs,PDS,and radio ejections in Sco X-1.
基金supported by the National Key R&D Program of China(2023YFE0101200)supported by the National Key R&D Program of China(2021YFA0718500)+3 种基金Yunnan Revitalization Talent Support Program(Yun Ling Scholar Award)the National Natural Science Foundation of China(NSFC)grant 12393813the science research grants from the China Manned Space Projectsupported by Hunan Education Department Foundation(grant No.21A0096)。
文摘We conducted an analysis of the continuum during the onset and initial decline phases of the 2023 outburst in transient neutron star low-mass X-ray binary Aql X–1 using broadband observations from the Hard X-ray Modulation Telescope(Insight-HXMT)instrument.To determine the most appropriate model for the continuum of this outburst,we employed three models to explore the evolution of the spectral component.These observations revealed that the source transitions from the hard state to the soft state.The disk-corona and sphere-corona models both adequately described the spectra of the hard state,while the double blackbody model became preferable after the hard X-ray emission(>25 keV)disappeared during the state transition.In the soft state,the total emission is dominated by the accretion disk and two blackbody components.The combination of the sphere-corona model and the double blackbody model is the most suitable model for this outburst.The results suggest that as the source transitioned into the soft state,the emission from the boundary layer was enhanced,and a hot spot occurred.Notably,we identified two type-ⅠX-ray bursts,one of which exhibited a significant hard X-ray deficit(significance~4.82σ),which indicates that Insight-HXMT has the capability to capture the evolution of the corona in a single burst.
文摘Coastal zones are dynamic interfaces responding to complex natural processes and anthropogenic pressures.Monitoring shoreline evolution is essential for sustainable coastal management,particularly given climate change,urban expansion,and sediment flux disruption.This study investigates shoreline changes along Morocco’s northern Atlantic coast from 1990 to 2023,an area of strategic economic importance and environmental vulnerability.Landsat satellite imagery and geospatial techniques,including the Digital Shoreline Analysis System(DSAS v5.1)and the Normalized Difference Water Index(NDWI),provided a high-resolution,diachronic assessment.Shoreline extraction and image enhancement were conducted with ENVI software,and change detection utilized Linear Regression Rate(LRR)and End Point Rate(EPR)indicators.Results revealed significant spatial variability:sectors like northern Moulay Bousselham and Chlihat showed pronounced accretion(+3.2 to+4.7 m/year),while areas such as Tahaddart and southern Mehdia experienced severe erosion(up to−3.4 m/year).The total net eroded area exceeds 58,000 m².Trends correlate strongly with hydrodynamic forces,upstream damming,sediment extraction,and extreme weather events,notably storms in 2014 and 2017.Findings align with studies highlighting compounded effects of sediment starvation and sea-level rise.By integrating remote sensing,time-series analysis,and uncertainty quantification,this research provides insights into the primary drivers of shoreline dynamics,emphasizing the urgent need for adaptive,evidence-based coastal management strategies,including regulation of sand mining,sediment buffer restoration,and soft-engineering solutions.
基金supported by the National Key R&D Program of China with grant No.2023YFA1608100supported by the National Natural Science Foundation of China(NSFC)under grant No.12273013+4 种基金support from the National Key R&D Program of China(grant Nos.2023YFA1607804,2022YFA1602902,and 2023YFA1607800)other NSFC projects(grant Nos.12120101003,12373010,12173051,and 12233008)the China Manned Space Project(No.CMS-CSST-2025-A06)the Strategic Priority Research Program of the Chinese Academy of Sciences with grant Nos.XDB0550100 and XDB0550000supported by the National Science Foundation under grant No.AST-2034437。
文摘Changing-look(CL)active galactic nuclei(AGNs)are an extremely rare type of AGNs,characterized by the appearance(turn-on)or disappearance(turn-off)of broad emission lines within months to years.We construct a sample of 211 CL AGN candidates by systematically searching the Sloan Digital Sky Survey spectra classified as GALAXY,but with the Zwicky Transient Facility and Near-Earth Object Wide-Field Infrared Explorer midinfrared light curves showing significant variability,at the redshift range from 0.1 to 0.75.To examine the success rate of this method,we cross-match the candidate sample with the Large Sky Area Multi-Object Fiber Spectroscopic Telescope and the Dark Energy Spectroscopic Instrument for repeat spectra,finding that the success rate of this method is 50%.By incorporating previously reported CL AGNs,a total of 48 CL AGNs have been identified within our sample,including two recurrent CL AGNs newly confirmed in this work.Our results suggest that the typical rest-frame timescale of CL behavior is from 1.7 to 18.1 yr,which is inconsistent with the obscuration model,and the Eddington ratios of CL AGNs are about 0.01 in the bright state.Additionally,our CL AGN sample follows the M_(BH)–σ_(*)relation of quiescent galaxies.In the future,we expect to detect more galaxies turning into Type 1 AGNs in this CL AGN candidate sample.
基金supported by the National Natural Science Foundation of China(NSFC,Grant No.12073021)。
文摘We present simulation results examining the presence and behavior of standing shocks in zero-energy low angular momentum advective accretion flows and explore their(in)stability properties,taking into account various values of specific angular momentum,λ_(0).Within the range 10-50R_(g)(where R_(g)denotes the Schwarzschild radius),shocks are discernible forλ_(0)≥1.75.In the special relativistic hydrodynamic simulation whenλ_(0)=1.80,we find the merger of two shocks resulted in a dramatic increase in luminosity.We present the impact of external and internal flow collisions from the funnel region on luminosity.Notably,oscillatory behavior characterizes shocks within 1.70≤λ_(0)≤1.80.Using free–free emission as a proxy for analysis,we show that the luminosity oscillations between frequencies of 0.1-10 Hz forλ_(0)range as 1.7≤λ_(0)≤1.80.These findings offer insights into quasiperiodic oscillation emissions from certain black hole X-ray binaries,exemplified by GX 339-4.Furthermore,for the supermassive black hole at the Milky Way's center,Sgr A*,oscillation frequencies between 10^(-6)and 10^(-5)Hz were observed.This frequency range,translating to one cycle every few days,aligns with observational data from X-ray telescopes such as Chandra,Swift,and XMM-Newton.
文摘We present the results of a multi-epoch,detailed spectral and temporal analysis of the ultraluminous X-ray source(ULX),ULX-1,in the galaxy NGC 4088 using XMM-Newton,Chandra,and Swift observations.The presence of a hard powerlaw spectral slope supports the interpretation of ULX-1 as a hard ULX.The observed inner disk temperature of kT_(in)> 1.5 keV is inconsistent with the presence of an intermediate mass black hole,but favors the super-Eddington accretion state.Moreover,the physically acceptable value of the parameter controlling the radial temperature profile of the disk(p) derived from fitting the first XMM-Newton observation with the slim disk model further points towards the possible presence of a broadened disk,indicating the super-Eddington accretion nature.Slight overall long-term flux variability is evident for this ULX,and a hint towards a positive correlation between flux and the powerlaw photon index is also observed when the relatively better data of XMM-Newton and Chandra are considered.The L-T relationship is observed to follow a positive trend,with the L-T profile consistent with either relation(L ∝ T^(4) or L ∝ T^(2)) in both cases.The source exhibits no significant short-term variability at different time binnings of the light curve as indicated by the chi-square probability of constancy and fractional RMS variability values.The power density spectrum created shows no evidence of intrinsic variability of the source above the white noise.Further,no sign of pulsation was detected for this source.Assuming this ULX to be powered by an accreting black hole and using the slim disk geometry,the upper limit of the black hole mass was estimated and found to be less than 100M_(⊙).
