X-ray Binary GX Research Articles

Comprehensive investigations on spectral and temporal features of GX 5−1 using AstroSat observations

ABSTRACT Comprehensive spectrotemporal analyses of the Z-type neutron star low-mass X-ray binary GX 5−1 were performed using 10 broad-band observations from AstroSat/Soft X-ray Telescope and Large Area X-ray Proportional Counter (LAXPC) instruments. The LAXPC-20 hardness–intensity diagram showed horizontal and normal branches (HBs and NBs) of the Z track which exhibited secular motion. The time-averaged spectra in the energy range 0.7–25.0 keV could be fitted with the model combination – $\tt {constant}\, \times \, \tt {tbabs}\, \times \, \tt {edge}\, \times \, \tt {edge}\, \times \, \tt {thcomp}\, \times \, \tt {diskbb}$. This yielded $\Gamma \, \sim$ 2, $kT_{\mathrm{ e}}\, \sim$ 3.3 keV, and $F_{\mathrm{ disc}}$/$F_{\mathrm{ total}}\, \sim$ 0.8 indicating the soft/intermediate spectral state of the source during the observations. Flux-resolved spectral analysis revealed a positive correlation between $kT_{\mathrm{ in}}$ and $F_{\mathrm{ bol}}$. However, a negative correlation was observed between them in one of the NBs. Time-averaged temporal analysis revealed multiple HB oscillations (HBOs) and NB oscillations (NBOs), and peaked noise components in the $\sim$5–50 Hz range. Furthermore, flux-resolved temporal analysis showed that the frequency of the HBOs correlates positively whereas the strength of HBOs correlates negatively with $F_{\mathrm{ bol}}$, indicating their probable origin from the accretion disc. In contrast, the frequency and strength of NBOs remain fairly constant with $F_{\mathrm{ bol}}$, suggesting that they originate from a different region in the system. Using the relativistic precession model along with highest frequency of the HBO, the upper limits of the magnetic dipole moment ($\mu$) and field strength (B) at the poles of the neutron star in the system were found to be 25.60$\times \, 10^{25}$ G cm3 and 3.64$\times \, 10^{8}$ G, respectively, for $k_{\mathrm{ A}}$ = 1.

NICER Observes the Full Z-track in GX 13+1

We present the temporal analysis of the persistent neutron star low-mass X-ray binary GX 13+1 using NICER data. Classification of this source has been ambiguous so far. We investigate the evolution of the source in its hardness–intensity diagram (HID) and power density spectra (PDS) of the 0.5–10 keV NICER archival data. For the first time, we detect the source tracing out the entire Z-track, distinctly identifying the horizontal branch (HB), normal branch (NB) and flaring branch (FB). We also detect a peaked noise component in the PDS at ∼5.4 Hz, which appears to be present when the source is either in the NB or FB. We note a positive slope of the HB in the HID which could be due to either the high intrinsic absorption of the source or the stronger contribution of the soft spectral components in the soft energy domain.

Accretion Geometry of GX 339–4 in the Hard State: AstroSat View

We perform broadband (0.7–100 keV) spectral analysis of five hard state observations of the low-mass black hole X-ray binary GX 339–4 taken by AstroSat during the rising phase of three outbursts from 2019 to 2022. We find that the outburst in 2021 was the only successful/full outburst, while the source was unable to make the transition to the soft state during the other two outbursts in 2019 and 2022. Our spectral analysis employs two different model combinations, requiring two separate Comptonizing regions and their associated reflection components and soft X-ray excess emission. The harder Comptonizing component dominates the overall bolometric luminosity, while the softer one remains relatively weak. Our spectral fits indicate that the disk evolves with the source luminosity, where the inner disk radius decreases with increasing luminosity. However, the disk remains substantially truncated throughout all the observations at the source luminosity of ∼2%–8%× of the Eddington luminosity. We note that our assumption of the soft X-ray excess emission as disk blackbody may not be realistic, and this kind of soft excess may arise due the nonhomogeneity in the disk/corona geometry. Our temporal analysis deriving the power density spectra suggests that the break frequency increases with the source luminosity. Furthermore, our analysis demonstrates a consistency between the inner disk radii estimated from the break frequency of the power density spectra and those obtained from the reflection modeling, supporting the truncated disk geometry in the hard state.

An in-depth analysis of the variable cyclotron lines in GX 301−2

Context. The high-mass X-ray binary system GX 301−2 is a persistent source with a well-known variable cyclotron line centered at 35 keV. Recently, a second cyclotron line at 50 keV has been reported with a presumably different behavior than the 35 keV line. Aims. We investigate the presence of the newly discovered cyclotron line in the phase-averaged and phase-resolved spectra at higher luminosities than before. We further aim to determine the pulse-phase variability of both lines. Methods. We analyze a NuSTAR observation of GX 301−2 covering the pre-periastron flare, where the source luminosity reached its peak of ∼4 × 1037 erg s−1 in the 5–50 keV range. We analyze the phase-averaged spectra in the NuSTAR energy range from 3.5–79 keV for both the complete observation and three time segments of it. We further analyze the phase-resolved spectra and the pulse-phase variability of continuum and cyclotron line parameters. Results. We confirm that the description of the phase-averaged spectrum requires a second absorption feature at 51.5−1.0+1.1 keV besides the established line at 35 keV. The statistical significance of this feature in the phase-averaged spectrum is > 99.999%. We further find that the 50 keV cyclotron line is present in three out of the eight phase bins. Conclusions. Based on the results of our analysis, we confirm that the detected absorption feature is very likely to be a cyclotron line. We discuss a variety of physical scenarios that could explain the proposed anharmonicity, but also outline circumstances under which the lines are harmonically related. We further present the cyclotron line history of GX 301−2 and evaluate concordance among each other. We also discuss an alternative spectral model including cyclotron line emission wings.

Evidence for a nearly orthogonal rotator in GX 301–2 with phase-resolved cyclotron resonant scattering features

Abstract Cyclotron resonant scattering features (CRSFs) are the absorption features in the X-ray spectra of strongly magnetized accretion neutron stars (NSs), which are probably the most reliable probe to the surface magnetic fields of NSs. The high mass X-ray binary GX 301–2 exhibits a very wide, variable and complicated CRSF in the average spectra, which should be two absorption lines based on NuStar and Insight-HXMT observations. With the Insight-HXMT frequent observations, we performed the phase-resolved spectroscopy and confirmed two cyclotron absorption lines in the phase-resolved spectra, with their centroid energy ratio ∼1.6 − 1.7 in the super-critical luminosity case. A major hindrance in understanding those CRSFs is the very poorly constrained magnetic inclination angle, which is also a fundamental property of a NS and key to understanding the emission characteristics of a pulsar. Comparing the phase-resolved CRSF with simulated X-ray spectra, the magnetic inclination angle is found to be ≳ 70○, i.e., nearly orthogonal between the NS’s spin and magnetic axies. The implications of an orthogonal rotator and magnetic structure evolution in the accreting X-ray binary are also discussed.

Discovery of a variable energy-dependent X-ray polarization in the accreting neutron star GX 5−1

We report on the coordinated observations of the neutron star low-mass X-ray binary (NS-LMXB) GX 5−1 in X-rays (IXPE, NICER, NuSTAR, and INTEGRAL), optical (REM and LCO), near-infrared (REM), mid-infrared (VLT VISIR), and radio (ATCA). This Z-source was observed by IXPE twice in March–April 2023 (Obs. 1 and 2). In the radio band the source was detected, but only upper limits to the linear polarization were obtained at a 3σ level of 6.1% at 5.5 GHz and 5.9% at 9 GHz in Obs. 1 and 12.5% at 5.5 GHz and 20% at 9 GHz in Obs. 2. The mid-IR, near-IR, and optical observations suggest the presence of a compact jet that peaks in the mid- or far-IR. The X-ray polarization degree was found to be 3.7%±0.4% (at 90% confidence level) during Obs. 1 when the source was in the horizontal branch of the Z-track and 1.8%±0.4% during Obs. 2 when the source was in the normal-flaring branch. These results confirm the variation in polarization degree as a function of the position of the source in the color-color diagram, as for previously observed Z-track sources (Cyg X-2 and XTE 1701−462). Evidence of a variation in the polarization angle of ∼20° with energy is found in both observations, likely related to the different, nonorthogonal polarization angles of the disk and Comptonization components, which peak at different energies.

The effects of instrumental deadtime on NICER timing products

Abstract The X-ray timing instrument as part of the neutron star interior composition explorer has the potential to examine the time-domain properties of compact objects in regimes not explored by previous timing instruments, due to its combination of high effective area and timing resolution. We consider the effects of instrumental deadtime at a range of effective countrates in a series of observations of the X-ray binary GX 339-4 to determine what effect deadtime has on photometric and Fourier frequency-domain products. We find that there are no significant inconsistencies across the functional detectors in the instrument, and that in the regimes where instrumental deadtime is a limiting factor on observations that previous approaches to dealing with deadtime, as applied to RXTE and other detectors, are still appropriate, and that performing deadtime corrections to light curves before creating Fourier products are not necessary at the count rates considered in our analysis.

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

ABSTRACT 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 per cent. 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.

Studying the variability of fluorescence emission and the presence of clumpy wind in HMXB GX 301−2 using XMM–Newton

ABSTRACT We present the results from an analysis of data from an XMM–Newton observation of the accreting high-mass X-ray binary pulsar GX 301−2. Spectral analysis in the non-flaring segment of the observation revealed that the equivalent width of the iron fluorescence emission is correlated with the observed absorption column density and the ratio of the iron Kβ and Kα line strength varied with the flux of the source. Coherent pulsations were detected with the spin period of the pulsar of 687.9 ± 0.1 s, and a secondary pulsation was also detected with a period of 671.8 ± 0.2 s, most prominent in the energy band of the iron line. At the spin period of the neutron star, the pulsation of the iron line has a low amplitude and the profile is different from the continuum. Pulse phase-resolved spectroscopy also revealed pulsations of the iron emission line during the non-flaring segment of the light curve. At the secondary period, both the iron line and the continuum have nearly identical pulse fraction and pulse profile. The additional periodicity can be attributed to the beat frequency between the spin of the neutron star and the Keplerian frequency of a stellar wind clump in retrograde motion around the neutron star. Reprocessed X-ray emissions originating from the clump can produce the observed secondary pulsations both in the continuum and the iron fluorescence line. The clump rotating around the neutron star is estimated to be approximately five lt-s away from the neutron star.

Spectral properties of GX 339−4 in the intermediate state using AstroSat observation

ABSTRACT We present the results obtained from the spectral studies of black hole X-ray binary GX 339−4 using AstroSat observations during its 2021 outburst. AstroSat observed the source in the intermediate state for ∼600 ks. The combined spectra of SXT and LAXPC in the 0.7−25 keV energy range are studied with phenomenological and physical models. The spectral study reveals a receding disc and a contracting corona during the observation period. The outflow rate is found to be increased though the accretion rates did not vary during the observation period. The X-ray flux decreases as the disc recedes and the spectrum becomes hard. At the same time, the Comptonized flux decreases with increasing fraction of thermal emission. This could be plausible that episodic jet ejection modified the corona and reduced Comptonized flux. An iron emission line at 6.4 keV is observed in the spectra of all the orbits of observation. We find that the equivalent width of the iron emission line correlates with the photon index, indicating a decrease in the reflection strength as the spectrum becomes hard. We observe that the disc flux does not follow FDBB − T4 relation.

Evolution of low-frequency quasi-periodic oscillations in GX 339-4 during its 2021 outburst using AstroSat data

ABSTRACT The black hole X-ray binary GX 339-4 showed an X-ray outburst during 2021. The AstroSat captured this outburst when the source entered into the intermediate flux state while the count rate was declining. The source showed an alternating flux profile in a time-scale of ≲100 ks, where the hard energy band was more variable than the soft band. The energy-dependent timing study showed that the observed quasi-periodic oscillation (QPO) was prominent in the low-energy bands, with its nearly sub-harmonic and harmonic components. These components appear and disappear with time, as observed in the orbit-wise QPO study. The Q-value, fractional rms, and 4.8–5.6 Hz frequency infer the QPOs as type-B and the spectral state as soft intermediate. The rms spectra of all orbits exhibiting QPOs show an increase in amplitude till ∼10 keV, beyond which it starts decreasing. This may indicate that ∼10 keV photons contributed relatively more in QPOs than other energy band photons. The Lorentzian normalization of the type-B QPO in different energy bands is consistent with the 10 keV peak. The energy-dependent time lag is complex and could be associated with the Comptonizing corona or jet. Finally, we discuss possible reasons behind the origin of different timing properties observed.

Investigating the orbital evolution of the eccentric HMXB GX 301–2 using long-term X-ray light curves

ABSTRACT We report the orbital decay rate of the high-mass X-ray binary GX 301–2 from an analysis of its long-term X-ray light curves and pulsed flux histories from CGRO/BATSE, RXTE/ASM, Swift/BAT, Fermi/GBM, and MAXI by timing the pre-periastron flares over a span of almost 30 yr. The time of arrival of the pre-periastron flares exhibits an energy dependence (hard lag) and the orbital period decay was estimated after correcting for it. This method of orbital decay estimation is unaffected by the fluctuations in the spin rate of the X-ray pulsar associated with variations in the mass accretion rate. The resulting $\dot{P}_\textrm {orb}$ = −(1.98 ± 0.28) × 10−6 s s−1 indicates a rapid evolution time-scale of $|P_\textrm {orb}/\dot{P}_\textrm {orb}|\sim 0.6\times 10^{5}$ yr, making it the high mass X-ray binary with the fastest orbital decay. Our estimate of $\dot{P}$orb is off by a factor of ∼2 from the previously reported value of −(3.7 ± 0.5) × 10−6 s s−1 estimated from pulsar TOA analysis. We discuss various possible mechanisms that could drive this rapid orbital decay and also suggest that GX 301–2 is a prospective Thorne–Żytkow candidate.

NuSTAR and AstroSat observations of GX 9+1: spectral and temporal studies

ABSTRACT We have studied the spectro-temporal properties of the neutron star low-mass X-ray binary GX 9+1 using data from NuSTAR/Focal Plane Module and AstroSat/Soft X-ray Telescope and Large Area X-ray Proportional Counter (LAXPC). The hardness–intensity diagram of the source showed it to be in the soft spectral state (banana branch) during both observations. NuSTAR spectral analysis yielded an inclination angle (θ) = 29${^{+3}_{-4}}^{\circ }$ and inner disc radius (Rin) ≤ 19 km. Assuming that the accretion disc was truncated at the Alfvén radius during the observation, the upper limits of the magnetic dipole moment (μ) and the magnetic field strength (B) at the poles of the neutron star in GX 9+1 were calculated to be 1.5 × 10 26 G cm3 and 2.1 × 10 8 G, respectively (for kA = 1). Furthermore, the thickness of the boundary layer was found to be ≃7.5 km, which yielded the radius of the neutron star to be ≤11.5 km. Flux-resolved spectral analysis with AstroSat data showed the source to be disc dominated (Fdisc/Ftotal ∼ 0.9) with a monotonic increase in mass accretion rate ($\dot{m}$) along the banana branch. The analysis also showed the presence of absorption edges at ∼1.9 and ∼2.4 keV, likely due to Si xiii and S xv, respectively. Temporal analysis with LAXPC-20 data in the 0.02–100 Hz range revealed the presence of noise components, which could be characterized by broad Lorentzian components.

X-ray polarimetry and spectroscopy of the neutron star low-mass X-ray binary GX 9+9: An in-depth study with IXPE and NuSTAR

We report on a comprehensive analysis of simultaneous X-ray polarimetric and spectral data of the bright atoll source GX 9+9 with the Imaging X-ray Polarimetry Explorer (IXPE) and NuSTAR. The source is significantly polarized in the 4–8 keV band, with a degree of 2.2% ± 0.5% (uncertainty at the 68% confidence level). The NuSTAR broad-band spectrum clearly shows an iron line, and is well described by a model including thermal disc emission, a Comptonized component, and reflection. From a spectro-polarimetric fit, we obtain an upper limit to the polarization degree of the disc of 4% (at the 99% confidence level), while the contribution of Comptonized and reflected radiation cannot be conclusively separated. However, the polarization is consistent with resulting from a combination of Comptonization in a boundary or spreading layer, plus reflection off the disc, which significantly contributes in any realistic scenario.

K2 and TESS observations of symbiotic X-ray binaries: GX 1+4 and IGR J16194−2810

I analyze the K2 and TESS data taken in 2016, 2019, and 2021 of the symbiotic X-ray binaries GX 1+4 and IGR J16194−2810. GX 1+4 consists of a pulsar accreting from a red giant companion in a 1160-day orbit. Since 1984, the pulsar has shown a continuous spin-down rate of Ṗ = −0.1177(3) mHZ yr−1. I report the detection of the spin period at an average value of 180.426(1) seconds as observed with the K2 mission and confirm that the spin period continues to increase at a rate of ∼1.61×10−7 s s−1. The K2 and hard X-rays, as observed with Swift/BAT, varied in tandem, in agreement with proposals from other authors that the optical light arises from reprocessed X-ray emission. In the case of IGR J16194−2810, the X-ray and optical spectroscopy have been interpreted as arising from a neutron star accreting from an M2 III red giant companion. Its orbital period is unknown, though I report here the detection of a modulation with a period of 242.837 min and interpret it as the neutron star spin period. IGR J16194−2810 is thus the second symbiotic X-ray binary whose spin period has been detected in optical wavelengths. This period, however, was only detected during the TESS observations of Sector 12 in 2019. The non-detection of this modulation during the observations of Sector 39 in 2021 is perhaps related to a low inclination of the orbit.

Evolution of the truncation radius in GX 339−4 from XMM–Newton spectral timing

ABSTRACT We investigate the changing geometry of the accretion disc in the X-ray binary GX 339−4 using a combination of spectral and timing models to fit six XMM–Newton observations taken during the end of the 2015 X-ray outburst. We use progressively more complex models of the X-ray reflection spectra, from simple disc lines to full relativistic reflection models. For the timing analysis, the Power Spectral Densities (PSDs) are generated from the light curves in the 0.3–0.7 and 0.7–1.5 keV energy bands. We fit PSD models that assume the standard accretion disc truncates at a specific radius, inside of which are two hot-flow zones, one spectrally soft and one spectrally hard, separated by a transition radius. Finally, we combine and jointly fit the full reflection spectral models and the truncated disc PSD models. Our final model is consistent with the spectroscopic and timing data. It suggests that the truncation radius of the disc increases towards the end of the outburst, which is self-consistent with the obtained reflection fraction that is smaller. During this, the source spectrum becomes harder and the soft excess becomes more prominent. The disc truncation radius that increases as the source flux decreases is intermediate between the results of previous studies. However, our analysis of the model performance leads us to believe that higher quality data and better models are required to fully understand the GX 339-4 system and reduce the effect of systematic uncertainties.

Broad-band spectro-temporal investigation of neutron star low-mass X-ray binary GX 349+2

ABSTRACT We report a broad-band investigation of the Z-type neutron star (NS) low mass X-ray binary (LMXB) GX 349+2 using AstroSat and Neutron star Interior Composition Explorer (NICER). AstroSat observed the source exhibiting large scale variability in its normal branch (NB)/flaring branch (FB) vertex and FB and a moderate evolution during NICER observations. The power spectra exhibit very low-frequency noise (VLFN) and low-frequency noise (LFN)/FB noise, described by a power law (PL) and an evolving Lorentzian. We investigate the energy dependence of variability components and their correlation with the spectral state to probe their origin. The joint spectra of GX 349+2 are modeled by two thermal and one non-thermal component. The source moves along the Z track, with the increasing accretion rate, further heating of the NS boundary layer (BL), and increasing temperature/radius of the brightened hotspot at the disc-BL interface/NS surface. A PL well represents the hard non-thermal coronal emission. As predicted by the gravitational redshift, we find a correlation between the line energy detected in NICER spectra and the inner disc radius with the Spearman rank correlation coefficient of 1. Using this correlation, we demonstrate the potential of a method to constrain the accreting compact object properties, including evolving continuum and line spectroscopy. We report the first detection of hard lag providing evidence of the VLFN originating from the accretion disc in NS LMXBs, representing fluctuation of propagation through the disc.

Relativistic X-ray reflection and photoionized absorption in the neutron star low-mass X-ray binary GX 13+1

ABSTRACT We analysed a dedicated NuSTAR observation of the neutron star low-mass X-ray binary Z-source GX 13+1 to study the timing and spectral properties of the source. From the colour–colour diagram, we conclude that during that observation the source transitioned from the normal branch to the flaring branch. We fitted the spectra of the source in each branch with a model consisting of an accretion disc, a Comptonized blackbody, relativistic reflection (relxillNS), and photoionized absorption (warmabs). Thanks to the combination of the large effective area and good energy resolution of NuSTAR at high energies, we found evidence of relativistic reflection in both the Fe K line profile and the Compton hump present in the 10–25 keV energy range. The inner disc radius is Rin ≲ 9.6 rg , which allowed us to further constrain the magnetic field strength to B ≲ 1.8 × 108 G. We also found evidence for the presence of a hot wind leading to photoionized absorption of Fe and Ni, with a Ni overabundance of ∼6 times solar. From the spectral fits, we find that the distance between the ionizing source and the slab of ionized absorbing material is ∼4–40 × 105 km. We also found that the width of the boundary layer extends ∼3 km above the surface of a neutron star, which yielded a neutron star radius RNS ≲ 16 km. The scenario inferred from the spectral modelling becomes self-consistent only for high electron densities in the accretion disc, ne ∼ 1022 − 1023 cm−3, as expected for a Shakura–Sunyaev disc, and significantly above the densities provided by relxillNS models. These results have implications for our understanding of the physical conditions in GX 13+1.

AstroSat observation of rapid type-I thermonuclear burst from low-mass X-ray binary GX 3$$+$$1

We report the results of an observation of low-mass X-ray binary GX 3 $$+$$ 1 with AstroSat’s large area X-ray proportional counter (LAXPC) and soft X-ray telescope (SXT) instruments on-board for the first time. We have detected type-I thermonuclear burst ( $$\sim $$ 15 s) present in the LAXPC 20 light curve with a double peak feature at higher energies and our study of the hardness–intensity diagram reveals that the source was in a soft banana state. The pre-burst emission could be described well by a thermally Comptonized model component. The burst spectra is modeled adopting a time-resolved spectroscopic method using a single color blackbody model added to the pre-burst model, to monitor the parametric changes as the burst decays. Based on our time-resolved spectroscopy, we claim that the detected burst is a photospheric radius expansion (PRE) burst. During the PRE phase, the blackbody flux is found to be approximately constant at an averaged value of $$\sim $$ 2.56 in $$10^{-8}$$ ergs s $$^{-1}$$ cm $$^{-2}$$ units. On the basis of literature survey, we infer that AstroSat/LAXPC 20 has detected a burst from GX 3 $$+$$ 1 after more than a decade, which is also a PRE one. Utilizing the obtained burst parameters, we provide a new estimation to the source distance, which is $$\sim $$ $$9.3 \pm 0.4$$ kpc, calculated for an isotropic burst emission. Finally, we discuss and compare our findings with the published literature reports.

Rapidly alternating flux states of GX 339–4 during its 2021 outburst captured by Insight–HXMT

ABSTRACT The low-mass X-ray binary GX 339–4 entered a new outburst in 2021. At the end of the hard to soft transition of this outburst, Insight–HXMT found that the source rapidly alternated between low flux and high flux states on a time-scale of hours. Two high flux states lasted only for a period comparable to the orbital period of the observatory. Time-resolved spectral analysis shows that the sudden changes of flux are confined in the hard X-ray band (>4 keV). The variable non-thermal emission, including the power-law continuum from the corona and the reflected emission from the inner accretion disc, is responsible for the observed variability. The strength of the disc thermal emission and the inner radius of the accretion disc are consistent between the two flux states. Assuming the lamppost geometry, our best-fitting disc reflection models suggest a very low corona height (within 3 Rg) and there is no evidence of significant variation in the corona geometry either. The observed rapidly alternating flux states suggest that the intrinsic power of the corona must change during the state transition. We discuss possible mechanisms for the observed sudden changes in the coronal power of GX 339–4.