Low-mass X-ray Research Articles

Investigating the Nature of X-Ray Sources in the Andromeda Galaxy Using Chandra and Hubble Data

Abstract Using data from the Panchromatic Hubble Andromeda Treasury (PHAT) program and the Chandra X-ray Observatory, we analyzed X-ray sources in the Andromeda Galaxy (M31). From this we identified 799 X-ray sources within the PHAT region. Our analysis suggests that the X-ray sources consist mainly of high-mass X-ray binaries (HMXBs) with a few low-mass X-ray binaries. These sources show a cumulative X-ray luminosity function consistent with other star-forming galaxies dominated by HMXBs. The population of evolved stars within M31, as identified by color–magnitude diagrams, further supports the hypothesis that these X-ray sources are predominantly X-ray binaries.

Long-term Evolution of Sco X-1: Implications for the Current Spin Frequency and Ellipticity of the Neutron Star

Abstract Sco X-1 is the brightest observed extrasolar X-ray source, which is a neutron star (NS) low-mass X-ray binary (LMXB) and is thought to have a strong potential for continuous gravitational waves (CW) detection due to its high accretion rate and relative proximity. Here, we compute the long-term evolution of its parameters, particularly the NS spin frequency (ν) and the surface magnetic field (B), to probe its nature and its potential for CW detection. We find that Sco X-1 is an unusually young (∼7 × 106 yr) LMXB and constrain the current NS mass to ∼1.4–1.6 M ⊙. Our computations reveal a rapid B decay, with the maximum current value of ∼1.8 × 108 G, which can be useful to constrain the decay models. Note that the maximum current ν value is ∼550 Hz, implying that, unlike what is generally believed, a CW emission is not required to explain the current source properties. However, ν will exceed an observed cutoff frequency of ∼730 Hz, and perhaps even the NS breakup frequency, in the future without a CW emission. The minimum NS mass quadrupole moment (Q) to avoid this is ∼(2–3) × 1037 g cm2, corresponding to a CW strain of ∼10−26. Our estimation of current ν values can improve the CW search sensitivity.

Singularity-free dark energy star in Rastall gravity

The notion of dark energy presents a potential avenue to avoid the gravitational collapse of compact objects, leading to singularities. The present article is devoted primarily to study a novel singularity-free relativistic solution of the Einstein field equations for dark energy stars in the framework of the Rastall theory of gravity. To analyse our model, we consider the Low-Mass X-ray Binary (LMXB) 4U 1608-52 having a mass of 1.74M⊙ and a radius of 9.3 Km (Güver et al., 2010) as a possible dark energy star candidate. We begin with the dark energy equation of state, where the density of dark energy is proportional to the isotropic perfect fluid distribution through a coupling parameter α. We compute the induced metric and extrinsic curvature tensors at the stellar hyper-surface to evaluate the necessary unknown constants appearing in the model. We conduct a thorough and comprehensive analysis to demonstrate the dependence of the physical behaviour of the model on the Rastall parameter (ξ), and interestingly, we obtain a possible phase transition from the dark to baryonic profile, which is sensitive to both α and ξ. We also compute the percentage of dark energy present in the model by varying ξ. However, for a fixed value of ξ, the percentage variation of dark energy with the mass of a star shows that the percentage of dark energy depends on the mass and radius of a star. The causality and energy conditions are well met in the present model, which describes its physical acceptability. The stability of the stellar configuration is established through the study of stability analysis. The graphical nature of the physical parameters and the present theoretical study show that our proposed model is non-singular and a viable representation of a stable realistic stellar structure in the presence of dark and baryonic matter simultaneously.

X-Ray Dips and Polarization Angle Swings in GX 13+1

Abstract We present the result from the 2024 April observation of the low-mass X-ray binary GX 13+1 with the Imaging X-ray Polarimetry Explorer (IXPE), together with Neutron Star Interior Composition Explorer and Swift- X-Ray Telescope coordinated observations. Two light-curve dips were observed; during them, the harder Comptonized spectral component was dominant and the polarization degree (PD) higher than in the softer, off-dip intervals. Through a joint analysis of the three IXPE observations, which also included the dip from the first observation, we demonstrate that the polarization properties varied in response to the intensity and spectral hardness changes associated with the dips. The PD attained values up to ∼4%. The polarization angle (PA) showed a swing of ∼70∘ across the dip and off-dip states, comparable to the continuous rotation seen during the first IXPE observation. We discuss these results in the context of models for polarized emission from the accretion disk and the boundary/spreading layer on the neutron star surface. We also draw attention to the role that an extended accretion disk corona or disk wind can play in generating high PDs and, possibly, swings of the PA.

Quadrupole Moment of a Magnetically Confined Mountain on an Accreting Neutron Star in General Relativity

Abstract General relativistic corrections are calculated for the quadrupole moment of a magnetically confined mountain on an accreting neutron star. The hydromagnetic structure of the mountain satisfies the general relativistic Grad–Shafranov equation supplemented by the flux-freezing condition of ideal magnetohydrodynamics, as in previous calculations of the magnetic dipole moment. It is found that the ellipticity, and hence the gravitational wave strain, are up to 12% greater than in the analogous Newtonian system. The direct contribution of the magnetic field to the nonaxisymmetric component of the stress-energy tensor is shown to be negligible in accreting systems such as low-mass X-ray binaries.

The evolutionary status of the Galaxy’s X-ray binary stars

The article is devoted to the analysis of the evolutionary status of the X-ray binary stars of the Galaxy. It is shown that the assumption of the conservative evolution of these binary systems leads to an overestimation of the X-ray luminosity of the Galaxy by 3–4 orders. The total observed rate of accretion of matter by relativistic components of X-ray binaries is close to ~10-6⊙/yr, while the theoretically possible rate reaches ~10-2⊙/yr. The contradiction between these estimates is eliminated if two factors are taken into account. The first of them is the formation of a common envelope in massive X-ray binary systems after filling the Roche lobe by the donor and the brief phase of a bright X-ray source. The common envelope eliminates the output of X-ray radiation generated during accretion, and also leads to the loss of part of the donor’s matter from the system. The second factor is the presence of intense stellar wind of donors in massive X-ray binary, as well as the occurrence of induced stellar wind in low-mass donors due to exposure to hard radiation from an accreting relativistic star. At the same time, the generally accepted assumption that donors of X-ray binaries fill their Roche lobes may not be fulfilled. A significant part of the donor’s wind matter may be lost from the system. In addition, radiation can enhance the stellar wind of the accretion disk, and part of this wind will also leave the system. There are other factors that reduce the total number of accreted matter: supernova explosions in X-ray binaries, destroying part of these systems, the impossibility of accretion onto rapidly rotating young neutron stars with a strong magnetic field, as well as a rapid drop in the rate of loss of matter by the donor as its mass decreases, characteristic for low-mass X-ray systems.

A Spectro-temporal View of Normal Branch Oscillations in Cygnus X-2 as Seen by NICER and NuSTAR

We report the spectro-temporal study of the neutron star low mass X-ray binary Cygnus X-2 using NICER and NuSTAR data while the source was in the normal branch (NB). We detect a normal branch oscillation (NBO) feature at ∼5.41 Hz that appears in the middle portion of the NB branch. We note that the NBO appeared only in the 0.5–3 keV energy range, with maximum strength in the 1–2 keV energy band, but was absent in the 3–10 keV energy band of NuSTAR and NICER data. The energy spectrum of the source exhibits an emission feature at ∼1 keV, previously identified as the Fe L transition in the outer region of the accretion disk. Upon considering both the Fe L and NBO features, we suggest that the originating location of the Fe L line and the NBOs may coincide and perhaps be due to the same underlying mechanism. Therefore, lags seen in the frequency-/energy-dependent lag spectra of Cygnus X-2 could be considered to be arising from a region of photoionized material far from the central source. We study the frequency and energy-dependent lag spectra of the source, which exhibited a few millisecond hard lag at the NBO frequency (12–15 ms) and a switch from hard to soft lags at 1 keV. The rms spectrum peaks at 1 keV, and the covariance spectrum clearly resembles a thermal spectrum. We discuss the spectro-temporal behavior of the NBO and attempt to constrain its location of origin.

Representation learning for time-domain high-energy astrophysics: Discovery of extragalactic fast X-ray transient XRT 200515

ABSTRACT We present a novel representation learning method for downstream tasks like anomaly detection, unsupervised classification, and similarity searches in high-energy data sets. This enabled the discovery of a new extragalactic fast X-ray transient (FXT) in Chandra archival data, XRT 200515, a needle-in-the-haystack event and the first Chandra FXT of its kind. Recent serendipitous discoveries in X-ray astronomy, including FXTs from binary neutron star mergers and an extragalactic planetary transit candidate, highlight the need for systematic transient searches in X-ray archives. We introduce new event file representations, $E-t$ maps and $E-t-\mathrm{d}t$ cubes, that effectively encode both temporal and spectral information, enabling the seamless application of machine learning to variable-length event file time series. Our unsupervised learning approach employs PCA or sparse autoencoders to extract low-dimensional, informative features from these data representations, followed by clustering in the embedding space with DBSCAN. New transients are identified within transient-dominant clusters or through nearest-neighbour searches around known transients, producing a catalogue of 3559 candidates (3447 flares and 112 dips). XRT 200515 exhibits unique temporal and spectral variability, including an intense, hard <10 s initial burst, followed by spectral softening in an $\sim$800 s oscillating tail. We interpret XRT 200515 as either the first giant magnetar flare observed at low X-ray energies or the first extragalactic Type I X-ray burst from a faint, previously unknown low-mass X-ray binary in the LMC. Our method extends to data sets from other observatories such as XMM–Newton, Swift-XRT, eROSITA, Einstein Probe, and upcoming missions like AXIS.

Evolution of the Accretion Disk and Corona during the Outburst of the Neutron Star Transient MAXI J1807+132

Low-mass X-ray binaries with a neutron star as the primary object show a complex array of phenomenology during outbursts. The observed variability in X-ray emission primarily arises from changes in the innermost regions of the accretion disk, neutron star surface, and corona. In this work, we present the results of a comprehensive X-ray spectral and timing analysis of the neutron star transient MAXI J1807+132 during its 2023 outburst using data from the NICER observatory. The outburst is marked by a very rapid rise in the count rate by about a factor of 20 in a day. The source undergoes full state transitions and displays the hysteresis effect in the hardness and rms intensity diagrams. Spectral analysis with a three-component model is consistent with disk truncation during the hard states and reaching the last stable orbit during the intermediate and soft states. We discuss the different values of the last stable radius in the context of the possible distance of the source and magnetic field strength. The characteristic frequencies throughout the hard and intermediate states are found to be strongly correlated with the inner radius of the disk. Together with the spectral and fast variability properties, we attempt to trace the evolution of the size of the corona along the outburst. Following the main outburst, the source undergoes a high-amplitude reflare, wherein it shows a complex behavior with relatively high variability (10%), but low hardness.

Can Slow Pulsars in Milky Way Globular Clusters Form via Partial Recycling?

Abstract Alongside the population of several hundred radio millisecond pulsars currently known in Milky Way globular clusters, a subset of six slowly spinning pulsars (spin periods 0.3–4 s) are also observed. With inferred magnetic fields ​​​​​​≳1011 G and characteristic ages ≲​​​​​​108 yr, explaining the formation of these apparently young pulsars in old stellar populations poses a major challenge. One popular explanation is that these objects are not actually young but instead have been partially spun up via accretion from a binary companion. In this scenario, accretion in a typical low-mass X-ray binary (LMXB) is interrupted by a dynamical encounter with a neighboring object in the cluster. Instead of complete spin-up to millisecond spin periods, the accretion is halted prematurely, leaving behind a “partially recycled” neutron star. In this Letter, we use a combination of analytic arguments motivated by LMXB evolution and N-body simulations to show that this partial recycling mechanism is not viable. Realistic globular clusters are not sufficiently dense to interrupt mass transfer on the short timescales required to achieve such slow spin periods. We argue that collapse of massive white dwarfs and/or neutron star collisions are more promising ways to form slow pulsars in old globular clusters.

Long-term optical variations in Swift J1858.6–0814: evidence for ablation and comparisons to radio properties

ABSTRACT We present optical monitoring of the neutron star low-mass X-ray binary Swift J1858.6–0814 during its 2018–2020 outburst and subsequent quiescence. We find that there was strong optical variability present throughout the entire outburst period covered by our monitoring, while the average flux remained steady. The optical spectral energy distribution is blue on most dates, consistent with emission from an accretion disc, interspersed by occasional red flares, likely due to optically thin synchrotron emission. We find that the fractional rms variability has comparable amplitudes in the radio and optical bands. This implies that the long-term variability is likely to be due to accretion changes, seen at optical wavelengths, that propagate into the jet, seen at radio frequencies. We find that the optical flux varies asymmetrically about the orbital period, peaking at phase $\sim$0.7, with a modulation amplitude that is the same across all optical wavebands, suggesting that reprocessing off of the disc, companion star and ablated material is driving the phase dependence. The evidence of ablation found in X-ray binaries is vital in understanding the long-term evolution of neutron star X-ray binaries and how they evolve into (potentially isolated) millisecond pulsars.

New constraints on the central mass contents of Omega Centauri from combined stellar kinematics and pulsar timing

We performed a combined analysis of stellar kinematics with line-of-sight accelerations of millisecond pulsars (MSPs) to probe the mass content of Omega Centauri (OC ). Our mass model includes the stellar mass distribution, a more concentrated mass component linked to the observed MSP population, a generic cluster of stellar remnants (assumed to be more concentrated than the stars and MSPs), and an intermediate-mass black hole (IMBH), allowing us to determine which of these is statistically preferred to account for these observations. We mass-modeled OC using the package GravSphere to solve the Jeans equations, including constraints in the form of proper motions, line-of-sight velocities, the surface density profile of the stars, the spatial distribution of MSPs, and the recently measured line-of-sight accelerations of a subset of these MSPs, self-consistently modeling their intrinsic spin-down. We explore the impact of different assumed centers of OC on our results and we infer the posterior distributions of the model parameters from the combined likelihood using the nested sampling package dynesty Our analysis favors an extended central mass of $ over an IMBH, setting a 3sigma upper limit on the IMBH mass of $6 We find that pulsar timing observations are an important additional constraint, favoring a central mass distribution that is $ 20<!PCT!>$ more massive and extended than implied by models that are constrained by the stellar kinematics alone. Finally, we find a 3sigma confidence level (CL) upper bound of $6 on the total mass traced by the MSPs, with the density profile following $ p (r) star (r)^ with $ 0.3,$ where $ star (r)$ is the stellar mass density and $ is the stellar velocity dispersion profile. This favors models in which MSPs form via stellar encounters, as in the leading paradigm whereby MSPs are the progeny of low-mass X-ray binaries. Our analysis demonstrates how combining stellar kinematics with MSP accelerations produces new constraints on mass models, shedding light on the presence or absence of IMBHs at the centers of globular clusters. Further, we provide the first validation of its kind where MSP positions are linked to their place of formation in globular clusters, which is in excellent agreement with the expectations of stellar encounter models of MSP formation. This sets a promising precedent amid the rapid growth in the number of observations and discoveries currently taking place in this field.

High-Energy Spectra of Black Hole and Neutron Star Low-Mass X-Ray Binaries

In the case of low-mass X-ray binaries, the companion star is often too faint for detection; therefore, there is no chance for dynamical studies to independently determine the mass of the compact object. In the absence of a mass estimate, one cannot make a distinction as to whether the binary hosts a black hole or neutron star. Therefore, the question arises whether this distinction can be made based on the X-ray data alone, even when there are no bursts or pulsations. These would automatically imply a neutron star, but they are not always present. Black hole systems are known to emit radiation with an unbroken power–law shape up to several hundred keV energies in their high/soft states. If the non-thermal Comptonization processes that are responsible for this are somehow related to the lack of a solid surface, and to the fact that more gravitational potential energy can be released for a black hole, then there would be a definite method to reliably distinguish between the two sources. This work intends to review the available observations and studies to compare how these two populations behave during their different spectral states. A conclusion can be made that high/soft-state spectra are really different for black holes and neutron stars, for the low/hard state; however, the same conclusion cannot be safely made.

XMM-Newton High-resolution Spectroscopy of EXO 0748–676 after Its Reemergence from a Long Quiescence

Abstract EXO 0748–676 is a well-studied, high-inclination, dipping and eclipsing neutron star low-mass X-ray binary that has recently emerged from 16 yr of quiescence into a new outburst. We present results from 55.5 ks of XMM-Newton observation, focusing on high-resolution spectroscopy with the same instrument (the Reflection Grating Spectrometer) that produced significant insights during the previous outburst. The XMM-Newton European Photon Imaging Camera light curve reveals a type I X-ray burst that leads to a corresponding optical burst by 3 s. To understand the effects of the burst on the ionization structure, the data are divided into burstless and pre- and postburst spectra, with additional analysis for dip and nondip phases. The primary spectral feature in all phases is a broad O vii recombination line, accompanied by velocity-broadened O viii, N vii, and Ne ix lines. Notably, the Ne ix line shows different ionization states for the preburst (11.65 Å) and postburst (13.56 Å) phases, while the dips also substantially affect the spectral lines. The current outburst mirrors many traits from the earlier one, such as a similar spectral state, plasma components with similar ionization structure, and spectral features from the same elements, implying a stable long-term accretion behavior across outbursts.

Simultaneous radio and X-ray observations of the transitional millisecond pulsar candidate 3FGL J1544.6−1125

ABSTRACT Transitional millisecond pulsars (tMSPs) are neutron star systems that alternate between a rotation-powered radio millisecond pulsar state and an accretion disc-dominated low-mass X-ray binary (LMXB)-like state on multi-year time-scales. During the LMXB-like state, the X-ray emission from tMSPs switches between ‘low’ and ‘high’ X-ray brightness modes on a time-scale of seconds to minutes (or longer), while the radio emission shows variability on time-scales of roughly minutes. Coordinated Very Large Array (VLA) and Chandra observations of the nearby tMSP PSR J1023+0038 uncovered a clear anticorrelation between radio and X-ray luminosities such that the radio emission consistently peaks during the X-ray low modes. In addition, there are sometimes also radio/X-ray flares that show no obvious correlation. In this paper, we present simultaneous radio and X-ray observations of a promising tMSP candidate system, 3FGL J1544.6$-$1125, which shows optical, $\gamma$-ray, and X-ray phenomena similar to PSR J1023+0038, but which is challenging to study because of its greater distance. Using simultaneous VLA and Chandra observations, we find that the radio and X-ray emission are consistent with being anticorrelated in a manner similar to PSR J1023+0038. We discuss how our results help in understanding the origin of bright radio emission from tMSPs. The greater sensitivity of upcoming telescopes such as the Square Kilometre Array will be crucial for studying the correlated radio/X-ray phenomena of tMSP systems.

AstroSat View of the Neutron Star Low-mass X-Ray Binary GX 5-1

We present the spectral and timing study of the bright neutron star low-mass X-ray binary GX 5-1 using AstroSat/Large Area X-ray Proportional Counter (LAXPC) and Soft X-ray Telescope observations conducted in 2018. During the observation, the source traces out the complete horizontal branch (HB) and normal branch (NB) of the Z-track in the hardness–intensity diagram (HID). Understanding the spectral and temporal evolution of the source along the Z-track can probe the accretion process in the vicinity of a neutron star. Spectral analysis was performed in the 0.7–20 keV energy range for different segments in the HID using a multitemperature disk blackbody with an average temperature, kT in ∼ 0.47, and a thermal Comptonization model. It is found that the optical depth of the corona drops from ∼7.07 in HB to ∼2.61 in NB. The timing analysis using the LAXPC instrument indicates the presence of quasiperiodic oscillations in HB and NB of the Z-track. The observed QPO frequencies are similar to the characteristic frequencies of horizontal branch oscillations (HBOs) and normal branch oscillations (NBOs). The HBO frequency increases from ∼12 to 40 Hz toward the hard apex. NBOs are observed at ∼5 Hz. The timing studies conducted in soft and hard bands indicate the association of HBO and NBO origin with the nonthermal component. Further research could explore the implications of this relationship for understanding the dynamics of accretion onto neutron stars.

First spectropolarimetric observation of the neutron star low-mass X-ray binary GX 3+1

We report the first simultaneous X-ray spectropolarimetric observation of the bright atoll neutron star low-mass X-ray binary GX 3+1, performed by the Imaging X-ray Polarimetry Explorer (IXPE) joint with NICER and NuSTAR. The source does not exhibit significant polarization in the 2–8 keV energy band, with an upper limit of 1.3% at a 99% confidence level on the polarization degree. The observed spectra can be well described by a combination of thermal disk emission, the hard Comptonization component, and reflected photons off the accretion disk. In particular, from the broad Fe Kα line profile, we were able to determine the inclination of the system (i ≈ 36°), which is crucial for comparing the observed polarization with theoretical models. Both the spectral and polarization properties of GX 3+1 are consistent with those of other atoll sources observed by IXPE. Therefore, we may expect a similar geometrical configuration for the accreting system and the hot Comptonizing region. The low polarization is also consistent with the low inclination of the system.

Orbital-period Changes of Low-mass X-Ray Binaries Driven by Magnetic Braking

Magnetic braking (MB) plays an important role in driving the evolution of low-mass X-ray binaries (LMXBs). The modified MB prescription, the convection and rotation boosted (CARB) model, is very successful in reproducing the detected mass-transfer rates of persistent neutron star (NS) LMXBs. In this work, we investigate whether the CARB MB prescription could account for the formation and evolution of some NS and black hole (BH) LMXBs with an observed orbital-period derivative. Using the MESA code, we perform a detailed binary evolution model for six NS and three BH LMXBs. Our simulations find that the CARB MB prescription can successfully reproduce the observed donor-star masses, orbital periods, and period derivatives of four NS LMXBs and one BH LMXB. Our calculated effective temperatures are in good agreement with the detected spectral types of two NS LMXBs and one BH LMXB. However, the standard MB model makes it difficult to produce the observed period derivatives of those LMXBs experiencing a rapid orbital shrinkage or expansion.

The enigmatic origin of two dormant BH binaries: Gaia BH1 and Gaia BH2

Abstract The two systems, namely, Gaia BH1 and Gaia BH2, that have been confirmed as dormant (i.e., no X-ray emission detected) black hole (BH) - low-mass star binaries in the latest Gaia mission data release (DR3) are intriguing in the context of their formation and evolution. Both systems consist of $\sim 9\, \mathrm{{\rm M}_{\odot }}$ BH and $\sim 1\, \mathrm{{\rm M}_{\odot }}$ star orbiting each other on a wide, eccentric orbit (e ∼ 0.5). We argue that formation of such Gaia BH-like systems through the isolated binary evolution (IBE) channel, under the standard common envelope assumptions, and from dynamical interactions in young massive and open clusters are equally probable, and that the formation rate of such binaries is of the order of $10^{-7}\, \mathrm{{\rm M}_{\odot }}^{-1}$ for both channels. We estimate that, according to our models, there are at most ∼900 detectable Gaia BH-like binaries in the Milky Way thin disc. What plays an important role in formation of Gaia BH-like systems via the IBE channel is the mutual position of the natal kick velocity vector and the binary angular momentum vector. We find that natal kicks with a median magnitude of ∼40 km/s are preferred for the formation of Gaia BH1-like binaries. Approximately 94% of those binaries are formed with the BH spin misaligned to the orbital axis by less than 40○. Gaia BH2-like binaries form if the low velocity natal kick (of median magnitude ∼20 km/s) is directed within 15○ about the orbital plane. In addition to natal kick, we also discuss the influence of tidal interaction and the adopted common envelope λce parameter prescription on the evolution of Gaia BH-like binaries. We follow the subsequent evolution of the binaries, once formed as Gaia BH1 and Gaia BH2 systems, to investigate their connection with the low-mass X-ray binary population.

Probing the origin of the extended flaring branch of Z-type X-ray binaries GX 340+0 and GX 5-1 using AstroSat

ABSTRACT ‘Z’ type neutron star low-mass X-ray binaries typically show a ‘Z’-like three-branched track in their hardness intensity diagram. However, a few such ‘Z’ sources show an additional branch known as the extended flaring branch (EFB). EFB has been poorly studied, and its origin is not known. It is thought to be an extension of the flaring branch (FB) or associated with Fe K $\alpha$ complex or an additional continuum due to the radiative recombination continuum (RRC) process. Using AstroSat observations, we have detected the EFB from two ‘Z’ sources, GX 340+0 and GX 5–1, and performed a broad-band spectral analysis in the 0.5–22 keV energy range. During EFB, both sources show the presence of a significant RRC component with absorption edges at $7.91^{+0.16}_{-0.15}$ and $8.10^{+0.16}_{-0.17}$ keV, respectively along with blackbody radiation and thermal Comptonization. No signature of RRC was detected during the FB, which is adjoint to the EFB. No Fe K $\alpha$ complex is detected. Interestingly, inside EFB dips of GX 5-1, for the first time, we have detected flaring events of 30–60 s, which can be modelled with a single blackbody radiation. During the FB to EFB transition, an increase in the blackbody radius by a factor of 1.5–2 is observed in both sources. Our analysis strongly suggests that EFB is not an extension of FB or caused by the Fe K $\alpha$ complex. Rather, it is caused by a sudden expansion of the hot, thermalized boundary layer and subsequent rapid cooling.