arXiv subjects

Guobao Zhang

Publications and source records attributed to Guobao Zhang.

At least 19 recordsLinked to original sources

arXiv2026

Neutron Star Mass-Radius Constraints for EXO 0748$-$676 from 2008-2025 Quiescent X-ray Spectra

We present new constraints on the mass and radius of the neutron star in the neutron star low-mass X-ray binary EXO 0748$-$676 obtained from a joint analysis of 20 quiescent X-ray observations obtained between 2008 and 2025, including 14 Chandra and 6 XMM-Newton exposures. These data sample two quiescent episodes separated by the 2024$-$2025 outburst. We model the 0.5$-$10 keV spectra with a hydrogen-atmosphere model, assuming a source distance of 7.1 kpc. In a global Markov Chain Monte Carlo analysis in which the hydrogen column density, neutron star mass, and radius are tied across all observations, we obtain a neutron-star mass of $1.77^{+0.17}_{-0.22}\,M_\odot$ and a radius of $12.62^{+0.56}_{-0.74}$ km ($1σ$ credible intervals). We further perform independent fits to the first and second quiescent epochs and find that the combined data set significantly reduces the low-mass tail in the posterior distribution, leading to tighter lower bounds on the neutron-star mass. Incorporating the distance uncertainty of $7.1\pm1.2$ kpc, we conservatively constrain the neutron-star mass and radius to $M\simeq 1.41-2.11~M_{\odot}$ and $R\simeq 10.15-15.13$ km, favoring relatively stiff dense-matter equations of state. We also trace the thermal evolution across two quiescent epochs and find evidence for renewed crust cooling following the 2024$-$2025 outburst, providing a unique opportunity to compare the thermal relaxation behavior after two distinct accretion episodes.

astro-ph.HE↗

arXiv2026

Multiwavelength Analysis of the Einstein Probe X-ray Transient EP240305a

We report multiwavelength observations of EP240305a, an uncatalogued X-ray transient detected by the Einstein Probe on March 5, 2024. The source exhibits distinct characteristics across the X-ray, optical, near-infrared, and radio bands. The soft X-ray observations show two significant flares lasting ~100-250 s, accompanied by rapid flux decay in a few days, and the optical and near-infrared data reveal a faint, candidate counterpart. In contrast, the radio observations expose a long-term spectral evolution from a self-absorbed to an optically thin state within two months, implying discrete jet ejection. We compare EP240305a with known classes of X-ray transients and find that it is unlikely to be associated with long-timescale transients such as jetted tidal disruption events or X-ray binaries. Its properties also disfavor a short-timescale stellar flare origin. Although the absence of optical spectroscopy prevents a redshift determination, the source exhibits properties similar to those of gamma-ray-dark gamma-ray burst-like transients, which may be associated with relativistic jets viewed off-axis or with choked jets. The discovery of EP240305a, along with other uncataloged transients detected by the Einstein Probe, underscores the scientific potential of highly sensitive X-ray survey telescopes and rapid-response multiwavelength follow-up observations in exploring the nature of atypical astronomical transients.

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arXiv2025

Observatory Science with eXTP

Scheduled for launch in 2030, the enhanced X-ray Timing and Polarization (eXTP) telescope is a Chinese space-based mission aimed at studying extreme conditions and phenomena in astrophysics. eXTP will feature three main payloads: Spectroscopy Focusing Arrays (SFAs), Polarimetry Focusing Arrays (PFAs), and a Wide-field Camera (W2C). This white paper outlines observatory science, incorporating key scientific advances and instrumental changes since the publication of the previous white paper [1]. We will discuss perspectives of eXTP on the research domains of flare stars, supernova remnants, pulsar wind nebulae, cataclysmic variables, X-ray binaries, ultraluminous X-ray sources, AGN, and pulsar-based positioning and timekeeping.

astro-ph.IM↗

arXiv2025

Dense Matter in Neutron Stars with eXTP

In this White Paper, we present the potential of the enhanced X-ray Timing and Polarimetry (eXTP) mission to constrain the equation of state of dense matter in neutron stars, exploring regimes not directly accessible to terrestrial experiments. By observing a diverse population of neutron stars - including isolated objects, X-ray bursters, and accreting systems - eXTP's unique combination of timing, spectroscopy, and polarimetry enables high-precision measurements of compactness, spin, surface temperature, polarimetric signals, and timing irregularity. These multifaceted observations, combined with advances in theoretical modeling, pave the way toward a comprehensive description of the properties and phases of dense matter from the crust to the core of neutron stars. Under development by an international Consortium led by the Institute of High Energy Physics of the Chinese Academy of Sciences, the eXTP mission is planned to be launched in early 2030.

astro-ph.HE↗

arXiv2025

Evidence for enhanced mass transfer in the disc preceding the transition to the soft state in MAXI J1820+070

We investigate the 2018-2019 main outburst and the subsequent mini-outbursts of the black hole low-mass X-ray binary MAXI J1820+070 using optical/ultraviolet data from the Las Cumbres Observatory (LCO), the American Association of Variable Star Observers (AAVSO), and $\textit{Swift}$/UVOT, as well as X-ray data from $\textit{Insight}$-HXMT and $\textit{Swift}$/XRT. Given the high-cadence observations, we identify a broad dip-like feature in both the optical and X-ray light curves preceding the transition to the soft state, with the X-ray dip lagging the optical dip by approximately 10 days. We propose that the dip is caused by a brief decrease followed by an increase in the mass accretion rate as it propagates through the disc, ultimately triggering the transition to the soft state. This might be a potential tool to predict impending hard-to-soft state transitions, although such a dip has not yet been observed in many sources. Additionally, we find that optical colour ($g^{\prime}-i^{\prime}$) becomes bluer and less variable before the transition to the intermediate state, preceding a dramatic change in the hardness ratio. This appears to be an unusual case, differing from the typical scenario where the optical colour changes usually along with the transition to the soft state. Finally, we explore the implications of the complex evolution of optical/X-ray correlation during both main outbursts and mini-outbursts. In particular, we find a loop-like evolutionary track before the transition to the soft state, which is linked to the optical/X-ray dips in the light curves.

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arXiv2025

Timing analysis of the black-hole candidate Swift J1727.8-1613: detection of a dip-like feature in the high-energy cross spectrum

We present a timing analysis of observations with the Hard X-ray Modulation Telescope of the black hole X-ray transient Swift J1727.8-1613 during its 2023 outburst. We detect, for the first time in a black hole X-ray binary, a prominent dip at ~ 3-15 Hz in the real part of the cross spectrum between high-energy (>25 keV) and low-energy (<10 keV) photons in the Low Hard and Hard Intermediate States, during which the QPO frequency rapidly increases and then stabilizes at ~ 1.0-1.5 Hz. Remarkably, the real part of the cross spectrum reaches negative values at the frequencies around the minimum of the dip, indicative of a phase lag ranging between $π/2$ and $π$ in this frequency range. We fit the power spectra and the real and imaginary parts of the cross spectra simultaneously using a multi-Lorentzian model. Among the lag models, the Gaussian phase-lag model provides a slightly better reduced $χ^2$ than the constant phase-lag and constant time-lag models, while it also alleviates the degeneracy associated with those models. From the parameters of the Lorentzian that fits the dip, we estimate the size of the accretion flow, which consistently exceeds 10,000 km as the QPO frequency increases from 0.13 Hz to 2.0 Hz. Furthermore, both the energy-dependent phase-lag and fractional-rms spectra of the dip exhibit a change in trend around 15 keV, with the phase lag dropping and rms reaching a local minimum. These spectra closely resemble the shapes predicted by the time-dependent Comptonization model, vKompth, for a low feedback factor, offering a pathway to explain the radiative properties of the corona. Additionally, the coherence function suggests a diversity of variability components, potentially arising from different parts of the corona.

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arXiv2025

Insight-HXMT observations of the 2023 outburst in Aql X-1

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$-$1 using broadband observations from the \textit{Insight-Hard X-ray Modulation Telescope (Insight-HXMT)} instrument. To determine the most appropriate model for the continuum of this outburst, we employed three models to explore the evolution of the spectral component. These observations revealed that the source transitions from the hard state to the soft state. The disk-corona and sphere-corona models both adequately described the spectra of the hard state, while the double blackbody model became preferable after the hard X-ray emission ($>$25 keV) disappeared during the state transition. In the soft state, the total emission is dominated by changes in the disk and other 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-I X-ray bursts, one of which exhibited a significant hard X-ray deficit (significance $\sim$ 4.82 $σ$), which indicates that \textit{Insight-HXMT} has the capability to capture the evolution of the corona in a single burst.

astro-ph.HE↗

arXiv2025

Spectral properties of the neutron star low-mass X-ray binary 4U 1636-53, XTE J1739-285 and MAXI J1816-195

We investigated simultaneous NICER plus NuSTAR observations of three neutron star low-mass X-ray binary 4U 1636-53, XTE J1739-285 and MAXI J1816-195 using the latest reflection models, with the seed photons feeding into the corona originating from either the neutron star (NS) or the accretion disk. We found that, for the sources in the hard spectral state, more than $\sim$ 50% of the NS photons enter into the corona if NS provides seed photons, while only $\sim$ 3%-5% disk photons go to the corona if seed photons come from the disk. This finding, together with the derived small height of the corona, favors the lamp-post geometry or boundary layer scenario where the corona is close to the central neutron star. Additionally, we found that the source of the seed photons has big influence in the significance of the NS radiation, especially for the soft spectral state. This result may help explain why the NS radiation in MAXI J1816-195 is weak in the previous work. More importantly, for the first time, we explored the properties of the corona in the NS systems with the compactness ($l-θ$) diagram. We found that the corona in the NS systems all lie in the left side of the pair-production forbidden region, away from the predicted pair-production lines. This finding indicates that either the corona in these NS systems is not pair-dominated, possibly due to the additional cooling from NS photons, or the corona is composed of both thermal and non-thermal electrons.

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arXiv2025

Science objectives of the Einstein Probe mission

The Einstein Probe (EP) is an interdisciplinary mission of time-domain and X-ray astronomy. Equipped with a wide-field lobster-eye X-ray focusing imager, EP will discover cosmic X-ray transients and monitor the X-ray variability of known sources in 0.5-4 keV, at a combination of detecting sensitivity and cadence that is not accessible to the previous and current wide-field monitoring missions. EP can perform quick characterisation of transients or outbursts with a Wolter-I X-ray telescope onboard. In this paper, the science objectives of the Einstein Probe mission are presented. EP is expected to enlarge the sample of previously known or predicted but rare types of transients with a wide range of timescales. Among them, fast extragalactic transients will be surveyed systematically in soft X-rays, which include γ-ray bursts and their variants, supernova shock breakouts, and the predicted X-ray transients associated with binary neutron star mergers. EP will detect X-ray tidal disruption events and outbursts from active galactic nuclei, possibly at an early phase of the flares for some. EP will monitor the variability and outbursts of X-rays from white dwarfs, neutron stars and black holes in our and neighbouring galaxies at flux levels fainter than those detectable by the current instruments, and is expected to discover new objects. A large sample of stellar X-ray flares will also be detected and characterised. In the era of multi-messenger astronomy, EP has the potential of detecting the possible X-ray counterparts of gravitational wave events, neutrino sources, and ultra-high energy γ-ray and cosmic ray sources. EP is expected to help advance the studies of extreme objects/phenomena and their underlying physical processes revealed in the dynamic X-ray universe, as well as studies in other areas of X-ray astronomy.

astro-ph.HE↗

arXiv2025

Determining the nature of IC 10 X-2: A comprehensive study of the Optical/Infrared emission from an extragalactic BeHMXB

We present a comprehensive analysis of the optical and infrared (IR) properties of high-mass X-ray binary (HMXB) IC 10 X-2, classified as a super-giant HMXB and super-fast X-ray transient (SFXT) by previous work. Our analysis of regular (daily and weekly) observations by both the Zwicky Transient Factory and Las Cumbres Observatory over a ~5 year period indicates both periodic flares and variations in the apparent magnitude and color with a $\sim26.5$~d period -- likely the orbital period of this binary system. The periodic flaring suggests the stellar companion is a Be star, with flares resulting from increased accretion onto the neutron star when it enters the stellar decretion disk. The periodic variations in the optical/IR brightness and color likely result from orbital variations in the Hydrogen column density along the line of sight or a transient accretion disk around the neutron star. Lastly, the numerous, short duration, episodes where IC 10 X-2 is significantly ``redder'' or ``bluer'' than normal likely result from from clumps within this system -- which can accrete onto the neutron star (causing IC 10 X-2 to appear bluer), or pass through the line of sight (causing IC 10 X-2 to appear redder). These results substantially increase our understanding of the evolution of this source, a significant source of ionizing photons in its host galaxy IC 10, a low mass, metal-poor starburst galaxy similar in many respect to those thought to be common in the early Universe.

astro-ph.HE↗

arXiv2024

Investigation into the origin of the soft excess in Ark 564 using principal component analysis

We combined a principal component analysis (PCA) and spectroscopy to investigate the origin of the soft excess in narrow-line Seyfert 1 galaxy Ark 564 with XMM-Newton observations over a period of ten years. We find that the principal components in different epochs are very similar, suggesting stable variability patterns in this source. More importantly, although its spectra could be equally well fitted by the two soft excess models, simulations show that the principal components from the relativistically smeared reflection model match the data well. At the same time, the principal components from the warm corona model show significant inconsistency. This finding indicates that the soft excess in Ark 564 originates from the relativistically smeared reflection, rather than the Comptonization in the warm corona, thereby favoring the reflection origin or the "hybrid" origin of the soft excess. Furthermore, the presence of the narrow absorption features in the spectra suggests that the soft excess is unlikely to originate from absorptions due to possible outflowing winds. Our results indicate that the PCA coupled with spectral analysis is a promising approach to exploring the origin of the soft excess in active galactic nuclei (AGNs).

astro-ph.HE↗

arXiv2024

The bright black hole X-ray binary 4U 1543-47 during 2021 outburst. A clear state transition from super-Eddington to sub-Eddington accretion revealed by Insight-HXMT

We present a detailed analysis of the observations with the Hard X-ray Modulation Telescope of the black hole X-ray transient 4U~1543-47 during its outburst in 2021. We find a clear state transition during the outburst decay of the source. Using previous measurements of the black-hole mass and distance to the source, the source luminosity during this transition is close to the Eddington limit. The light curves before and after the transition can be fitted by two exponential functions with short ($\sim 16$ days) and long ($\sim 130$ days) decay time scales, respectively. We detect strong reflection features in all observations that can be described with either the RelxillNS or Reflionx_bb reflection models, both of which have a black-body incident spectrum. In the super-Eddington state, we observe a Comptonized component characterized by a low electron temperature of approximately 2.0 keV. We suggest that this component appears exclusively within the inner radiation-pressure dominated region of the supercritical disk as a part of the intrinsic spectrum of the accretion disk itself. This feature vanishes as the source transitions into the sub-Eddington state. The emissivity index of the accretion disk in the reflection component is significantly different before and after the transition, $\sim3.0$-$5.0$ and $\sim7.0$-$9.0$ in the super- and sub-Eddington states, respectively. Based on the reflection geometry of returning disk radiation, the geometrically thicker the accretion disk, the smaller the emissivity index. Therefore, we propose that the transition is primarily driven by the change of the accretion flow from a supercritical to a thin disk configuration.

astro-ph.HE↗

arXiv2024

Insight-HXMT observations of thermonuclear X-ray bursts in 4U 1636-53

We conducted an analysis of 45 bursts observed from 4U 1636$-$53. To investigate the mechanism behind the light curve profiles and the impact of thermonuclear X-ray bursts on the accretion environment in accreting neutron star low-mass X-ray binaries. This analysis employed both light curve and time-resolved spectroscopy methodologies, with data collected by the \textit{Insight}-HXMT instrument. We found that 30 bursts exhibited similar light curve profiles and were predominantly in the hard state, and two photospheric radius expansion (PRE) bursts were in the soft state. The light curves of most bursts did not follow a single exponential decay but displayed a dual-exponential behavior. The initial exponent had a duration of approximately 6 s. We utilized both the standard method and the `$f_{\rm a}$' method to fit the burst spectra. The majority of the `$f_{\rm a}$' values exceeded 1, indicating an enhancement of the persistent emission during the burst. Under the two comptonization components assumption, we suggest that the scattering of burst photons by the inner corona may mainly contribute to the persistent emission enhancement. We also observed an inverse correlation between the maximum $f_{\rm a}$ and the persistent emission flux in the non-PRE burst. This anti-correlation suggests that when the accretion rate is lower, there is a greater enhancement of persistent emission during the burst peak. The prediction based on Poynting-Robertson drag (P-R drag) aligns with this observed anti-correlation.

astro-ph.HE↗

arXiv2023

A systematic study of the high-frequency bump in the black-hole low-mass X-ray binary GX 339-4

The high-frequency bump, characterized by a frequency exceeding ~30 Hz, represents a seldom-explored time-variability feature in the power density spectrum (PDS) of black-hole X-ray binaries. In the 2002, 2004, 2007 and 2010 outbursts of GX 339-4, the bump has been occasionally observed in conjunction with type-C quasi-periodic oscillations (QPOs). We systematically study the properties of the bump during these four outbursts observed by Rossi X-ray Timing Explorer (RXTE) in the 2-60 keV bands and detect the bump in 39 observations. While the frequencies of the type-C QPOs are in the range of ~0.1-9 Hz, the root-mean-square (rms) amplitude of the bump shows an evolution in the hardness ratio versus the type-C QPO frequency plot. By comparing the rms amplitude of the bump with the corona temperature and simultaneous radio jet flux of the source, as previously studied in GRS 1915+105, we establish that in the hard state of GX 339-4, the bump is always strong, with the measurements of the rms amplitude in the range of 4-10%. At the same time, the corona temperature is high and the radio flux is low. These findings indicate that, using the bump as a proxy, the majority of the accretion energy is directed towards the hot corona rather than being channeled into the radio jet. We discuss this phenomenon in terms of an inefficient energy transfer mechanism between the corona and jet in GX 339-4.

astro-ph.HE↗

arXiv2023

Investigation into the reflection properties of the neutron star low-mass X-ray binary 4U 1636-53

We present the spectroscopy of the neutron star low-mass X-ray binary 4U 1636-53 using six simultaneous XMM-Newton and Rossi X-ray Timing Explorer observations. We applied different self-consistent reflection models to explore the features when the disk is illuminated by either the corona or the neutron star surface. We found that the spectra could be well fitted by these two types of models, with the derived emissivity index below a typical value of 3. The relative low emissivity can be explained if the neutron star and the corona, working together as an extended illuminator, simultaneously illuminate and ionize the disk. Additionally, the derived ionization parameter in the lamppost geometry is larger than the theoretical prediction. This inconsistency likely suggests that the corona does not emit isotropically in a realistic context. Furthermore, we also found that there is a possible trend between the height of the corona and the normalization of the disk emission. This could be understood either as a variation in the reflected radiation pressure or in the context of a jet base. Finally, we found that the disk is less ionized if it is illuminated by the neutron star, indicating that the illuminating source has significant influence on the physical properties of the disk.

astro-ph.HE↗

arXiv2022

The origin of UV/optical emission in the black hole low-mass X-ray binary Swift J1753.5-0127

The emission from the accreting black holes (BHs) in low-mass X-ray binaries (LMXBs) covers a broad energy band from radio to X-rays. Studying the correlations between emission in different energy bands during outbursts can provide valuable information about the accretion process. We analyse the simultaneous optical, ultraviolet (UV) and X-ray data of the BH-LMXB Swift J1753.5-0127 during its $\sim$ 12-year long outburst with the {\it Neil Gehrels Swift Observatory}. We find that the UV/optical and X-ray emission are strongly correlated during the hard states of the outburst. We fit the relation with a power-law function $F_{UV/optical} \propto F_{X}^β$ and find that the power-law index $β$ increases from $\sim$ 0.24 to $\sim$ 0.33 as the UV/optical wavelength decreases from $\sim$ 5400 Å (V) to $\sim$ 2030 Å (UVW2). We explore the possible reasons for this and suggest that in Swift J1753.5-0127 the UV/optical emission is dominated by a viscously heated accretion disc at large radii. We find that the data that deviate from the correlation correspond to the low-intensity peaks appeared in the X-ray band during the outburst, and suggest that these deviations are driven by the emission from the inner part of the accretion disc.

astro-ph.HE↗

arXiv2021

The harmonic component of the millihertz quasi-periodic oscillations in 4U 1636-53

We studied the harmonics of the millihertz quasi-periodic oscillations (mHz QPOs) in the neutron-star low-mass X-ray binary 4U 1636-53 using the Rossi X-ray Timing Explorer observations. We detected the harmonics of the mHz QPOs in 73 data intervals, with most of them in the transitional spectra state. We found that the ratio between the rms amplitude of the harmonic and that of the fundamental remains constant in a wide range of the fundamental frequency. More importantly, we studied, for the first time, the rms amplitude of the harmonics vs. energy in 4U 1636-53 in the 2-5 keV range. We found that the rms amplitude of both the harmonic and the fundamental shows a decreasing trend as the energy increases, which is different from the behaviors reported in QPOs in certain black hole systems. Furthermore, our results suggest that not all observations with mHz QPOs have the harmonic component, although the reason behind this is still unclear.

astro-ph.HE↗

arXiv2020

The study of multi-peaked type-I X-ray bursts in the neutron-star low mass X-ray binary 4U 1636$-$536 with RXTE

We have found and analysed 16 multi-peaked type-I bursts from the neutron-star low mass X-ray binary 4U 1636$-$53 with the Rossi X-ray Timing Explorer (RXTE). One of the bursts is a rare quadruple-peaked burst which was not previously reported. All 16 bursts show a multi-peaked structure not only in the X-ray light curves but also in the bolometric light curves. Most of the multi-peaked bursts appear in observations during the transition from the hard to the soft state in the colour-colour diagram. We find an anti-correlation between the second peak flux and the separation time between two peaks. We also find that in the double-peaked bursts the peak-flux ratio and the temperature of the thermal component in the pre-burst spectra are correlated. This indicates that the double-peaked structure in the light curve of the bursts may be affected by enhanced accretion rate in the disc, or increased temperature of the neutron star.

astro-ph.HE↗