Low Flux State Research Articles

Probing broadband spectral energy distribution and variability of Mrk 501 in the low flux state

We conducted a multi-wavelength analysis of the blazar Mrk501, utilizing observations from AstroSat (SXT, LAXPC), Swift-UVOT, and Fermi-LAT during the period August 15, 2016 to March 27, 2022. The resulting multi-wavelength light curve revealed relatively low activity of the source across the electromagnetic spectrum. Notably, logparabola and broken power-law models provided a better fit to the joint X-ray spectra from AstroSat-SXT/LAXPC instruments compared to the power-law model. During the low activity state, the source showed the characteristic “harder when brighter” trend at the X-ray energies. To gain insights into underlying physical processes responsible for the broadband emission, we performed a detailed broadband spectral analysis using the convolved one-zone leptonic model with different forms of particle distributions such as logparabola (LP), broken power-law (BPL), power-law model with maximum energy (ξmax), and energy-dependent acceleration (EDA) models. Our analysis revealed similar reduced-χ2 values for the four particle distributions. The LP and EDA models exhibited the lowest jet powers. The correlation analyses conducted for the LP and BPL models revealed that there is a positive correlation between jet power and bulk Lorentz factor. Specifically, in the LP model, jet power proved independent of γmin, whereas in the broken power-law model, jet power decreased with an increase in γmin. The jet power in the LP/EDA particle distribution is nearly 10 percent of the Eddington luminosity of a 107 M⊙ black hole. This result suggests that the jet could potentially be fueled by accretion processes.

A second view on the X-ray polarization of NGC 4151 with IXPE

We report on the second observing program of the active galactic nucleus NGC 4151 with simultaneous Imaging X-ray Polarimetry Explorer (IXPE; 750 ks) (sim 60 ks) XMM-Newton (sim 75 ks), and NICER (sim 65 ks) pointings. NGC 4151 is the first Type-1 radio-quiet Seyfert galaxy with constrained polarization properties for the X-ray corona. Despite the lower flux state in which the source was re-observed and the resulting higher contribution of the constant reflection component in the energy band, our results are in agreement with the first detection. From polarimetric analysis, a polarization degree Pi = 4.7 pm 1.3<!PCT!> and polarization angle Psi = 77 pm 8 east of north (68<!PCT!> c.l.) were derived in the 2.0 -- 8.0 keV energy range. Combining the two observations leads to polarization properties that are more constrained than those of the individual detections, showing Pi = 4.5 pm 0.9<!PCT!> and Psi = 81 pm 6 (with a detection significance of sim 4.6sigma ). The observed polarization angle aligns very well with the radio emission in this source, supporting, together with the significant polarization degree, a slab or wedge geometry for the X-ray corona. However, a switch in the polarization angle at low energies ($37 in the 2-3.5 keV bin) suggests the presence of another component. When it is included in the spectro-polarimetric fit, a high polarization degree disfavors an interpretation in terms of a leakage through the absorbers, instead pointing to scattering from some kind of mirror.

Imprint of “Local Opacity” Effect in Gamma-Ray Spectrum of Blazar Jet

Relativistic jets from accreting supermassive black holes at cosmological distances can be powerful emitters of γ-rays. However, the precise mechanisms and locations responsible for the dissipation of energy within these jets, leading to observable γ-ray radiation, remain elusive. We detect evidence for an intrinsic absorption feature in the γ-ray spectrum at energies exceeding 10 GeV, presumably due to the photon–photon pair production of γ-rays with low-ionization lines at the outer edge of broad-line region (BLR), during the high-flux state of the flat-spectrum radio quasar PKS 1424−418. The feature can be discriminated from the turnover at higher energies resulting from γ-ray absorption in the extragalactic background light. It is absent in the low-flux states, supporting the interpretation that powerful dissipation events within or at the edge of the BLR evolve into fainter γ-ray emitting zones outside the BLR, possibly associated with the moving very long baseline interferometry radio knots. The inferred location of the γ-ray emission zone is consistent with the observed variability timescale of the brightest flare, provided that the flare is attributed to external Compton scattering with BLR photons.

Time resolved spectroscopy of a GRS 1915 + 105 flare during its unusual low state using AstroSat

ABSTRACT Since its discovery in 1992, GRS 1915 + 105 has been among the brightest sources in the X-ray sky. However, in early 2018, it dimmed significantly and has stayed in this faint state ever since. We report on AstroSat and NuSTAR observation of GRS 1915 + 105 in its unusual low/hard state during 2019 May. We performed time-resolved spectroscopy of the X-ray flares observed in this state and found that the spectra can be fitted well using highly ionized absorption models. We further show that the spectra can also be fitted using a highly relativistic reflection dominated model, where for the lamp post geometry, the X-ray emitting source is always very close to the central black hole. For both interpretations, the flare can be attributed to a change in the intrinsic flux, rather than dramatic variation in the absorption or geometry. These reflection dominated spectra are very similar to the reflection dominated spectra reported for active galactic nuclei in their low flux states.

Fierce Feedback in an Obscured, Sub-Eddington State of the Seyfert 1.2 Markarian 817

Mrk 817 is a bright and variable Seyfert 1.2 active galactic nucleus (AGN). X-ray monitoring of Mrk 817 with the Neil Gehrels Swift Observatory in 2022 revealed that the source flux had declined to a lower level than recorded at any prior point in the then 19 yr mission. We present an analysis of deep XMM-Newton and NuSTAR observations obtained in this low flux state. The spectra reveal a complex X-ray wind consisting of neutral and ionized absorption zones. Three separate velocity components are detected as part of a structured ultrafast outflow (UFO), with v/c=0.043−0.003+0.007 , v/c=0.079−0.0008+0.003 , and v/c=0.074−0.005+0.004 . These projected velocities suggest that the wind likely arises at radii that are much smaller than the optical broad-line region. In order for each component of the outflow to contribute significant feedback, the volume filling factors must be greater than f ≃ 0.009, f ≃ 0.003, and f ≃ 0.3, respectively. For plausible, data-driven volume filling factors, these limits are passed, and the total outflow likely delivers the fierce feedback required to reshape its host environment, despite a modest radiative Eddington fraction of λ ≃ 0.008–0.016 (this range reflects plausible masses). UFOs are often detected at or above the Eddington limit; this result signals that black hole accretion has the potential to shape host galaxies even at modest Eddington fractions and over a larger fraction of a typical AGN lifetime. We discuss our findings in terms of models for disk winds and black hole feedback in this and other AGN.

Analysis of Bright Source Hardness Ratios in the 4 yr Insight-HXMT Galactic Plane Scanning Survey Catalog

We conduct a statistical analysis of the hardness ratio (HR) for bright sources in the 4 yr Galactic Plane Scanning Survey catalog of Insight-HXMT. Depending on the stable (variable) flux F s (F v) or spectrum S s(S v) of each source, the bright sources are classified into three groups: F v&S v, F v&S s, and F s&S s. Our study of the HR characteristics in different types of sources reveals that accretion-powered neutron star (NS) low-mass X-ray binaries (LMXBs) exhibit softer energy spectra than NS high-mass X-ray binaries (HMXBs), but harder energy spectra than black hole binaries in most cases. This difference is probably due to their different magnetic field strengths. Additionally, F v&S v LMXBs tend to be harder than F v&S s LMXBs below 7 keV, while the opposite is true for HMXBs. Our results suggest that LMXBs may dominate unclassified sources, and NS binaries are likely to be the primary type of X-ray binaries with ambiguous compact stars. By comparing the HR of transient sources in their outburst and low-flux states, it is found that the averaged HR of four sources in the two states are roughly comparable within uncertainties. We also investigate the spatial properties of the three groups and find that F v&S v sources are mainly located in the longitude of −20° < l < 9°, F v&S s sources cross the Galactic Plane, and F s&S s sources are predominantly concentrated in 19° < l < 42°. In addition, analyzing the HR spatial distributions shows the absorption of soft X-rays (primarily below 2 keV) in the Galactic Plane.

Can FSRQ 3C 345 be a very high energy blazar candidate?

ABSTRACT The recent detection of very high energy (VHE) emissions from flat spectrum radio quasars (FSRQs) at high redshifts has revealed that the universe is more transparent to VHE γ-rays than it was expected. It has also questioned the plausible VHE emission mechanism responsible for these objects. Particularly for FSRQs, the γ-ray emission is attributed to the external Compton (EC) process. We perform a detailed spectral study of Fermi-detected FSRQ 3C 345 using synchrotron, synchrotron self-Compton, and EC emission mechanisms. The simultaneous data available in optical, ultraviolet, X-ray, and γ-ray energy bands is statistically fitted under these emission mechanisms using the χ2-minimization technique. Three high flux states and one low flux state are chosen for spectral fitting. The broad-band spectral energy distribution during these flux states is fitted under different target photon temperatures, and the model VHE flux is compared with the 50 h Cherenkov Telescope Array sensitivity. Our results indicate a significant VHE emission could be attained during the high flux state from MJD 59635−59715 when the target photon temperature is within 900–1200 K. Furthermore, our study shows a clear trend of variation in the bulk Lorentz factor of the emission region as the source transits through different flux states. We also note that during high γ-ray flux states, an increase in external photon temperature demands high bulk Lorentz factors, while this behaviour reverses in case of low γ-ray flux state.

Accretion Disk Wind of Hercules X-1 during the Short High State

Hercules X-1 is a nearly edge-on X-ray binary with a warped, precessing accretion disk, which manifests through a 35-day cycle of alternating High and Low flux states. This disk precession introduces a changing line of sight toward the X-ray source, through an ionized accretion disk wind. The sightline variation allows us to uniquely determine how the wind properties vary with height above the disk. All the previous wind measurements were made in the brighter Main High state of Her X-1. Here, we analyze the only Chandra observation during the fainter “Short” High state and significantly detect blueshifted ionized absorption. We find a column density of cm−2, an ionization parameter /erg cm s−1) = , and an outflow velocity of 380 ± 40 km s−1. The properties of the outflow measured during the Short High state are in good agreement with those measured at equivalent precession phases during the Main High state. We conclude that we are sampling the same wind structure, seen during both the Main and Short High states, which is precessing alongside the warped accretion disk every 35 days. Finally, the high spectral resolution of Chandra gratings above 1 keV in this observation enabled us to measure the abundances of certain elements in the outflow. We find Mg/O , Si/O =1.5 ± 0.4, and S/O , whereas in our previous study of Her X-1 with XMM-Newton, we found an overabundance of N, Ne, and Fe compared with O. These peculiar abundance ratios were likely introduced by the pollution of the donor by the supernova that created Her X-1.

Multiwavelength Temporal Variability of the Blazar PKS 1510–089

We perform correlation and periodicity search analyses on long-term multiband light curves of the flat-spectrum radio quasar PKS 1510−089 observed by the space-based Fermi-Large Area Telescope in γ-rays, the SMARTS and Steward Observatory telescopes in optical and near-infrared (NIR), and the 13.7 m radio telescope in Metsähovi Radio Observatory between 2008 and 2018. The z-transform discrete correlation function method is applied to study the correlation and possible time lags among these multiband light curves. Among all pairs of wavelengths, the γ-ray versus optical/NIR and optical versus NIR correlations show zero time lags; however, both the γ-ray and optical/NIR emissions precede the radio radiation. The generalized Lomb–Scargle periodogram, weighted wavelet z-transform, and REDFIT techniques are employed to investigate the unresolved core emission–dominated 37 GHz light curve and yield evidence for a quasi period around 1540 days, although given the length of the whole data set it cannot be claimed to be significant. We also investigate the optical/NIR color variability and find that this source shows a simple redder-when-brighter behavior over time, even in the low-flux state.

The Peculiar Variable X-Ray Spectrum of the Active Galactic Nucleus PKS 2005–489

PKS 2005−489 is a well-known, bright southern BL Lac object that has been detected up to TeV energies. In a low-flux state it exhibits the expected multiwavelength double-peaked spectrum in the radio−γ-ray band. The high-flux state shows extreme flux variations in the X-ray band with a hardening as well as a peculiar curved feature in the spectrum. Thus far, PKS 2005−489 is the only source to exhibit such a feature. To study the X-ray variability further, we obtained the first hard X-ray spectrum of the source with the Nuclear Spectroscopic Telescope Array. We compare quasi-simultaneous radio, optical, UV, soft and hard X-ray, and γ-ray data of PKS 2005−489 to archival data in order to study its broadband behavior. We find a very consistent quiet state in the spectral energy distribution, with little variation in spectral shape or flux between the 2012 and 2020 data. A possible explanation for the peculiar X-ray spectrum in the flaring state is an additional component in the jet, possibly accelerated via magnetic reconnection, that is not co-spatial to the low-flux state emission region.

Probing the nature of the low state in the extreme ultraluminous X-ray pulsar NGC 5907 ULX1

NGC 5907 ULX1 is the most luminous ultra-luminous X-ray pulsar (ULXP) known to date, reaching luminosities in excess of 1041 erg s−1. The pulsar is known for its fast spin-up during the on-state. Here, we present a long-term monitoring of the X-ray flux and the pulse period between 2003 and 2022. We find that the source was in an off- or low-state between mid-2017 to mid-2020. During this state, our pulse period monitoring shows that the source had spun down considerably. We interpret this spin-down as likely being due to the propeller effect, whereby accretion onto the neutron star surface is inhibited. Using state-of-the-art accretion and torque models, we use the spin-up and spin-down episodes to constrain the magnetic field. For the spin-up episode, we find solutions for magnetic field strengths of either around 1012 G or 1013 G, however, the strong spin-down during the off-state seems only to be consistent with a very high magnetic field, namely, &gt; 1013 G. This is the first time a strong spin-down is seen during a low flux state in a ULXP. Based on the assumption that the source entered the propeller regime, this gives us the best estimate so far for the magnetic field of NGC 5907 ULX1.

UV–Optical Disk Reverberation Lags despite a Faint X-Ray Corona in the Active Galactic Nucleus Mrk 335

We present the first results from a 100-day Swift, NICER, and ground-based X-ray–UV–optical reverberation mapping campaign of the Narrow-line Seyfert 1 Mrk 335, when it was in an unprecedented low X-ray flux state. Despite dramatic suppression of the X-ray variability, we still observe UV–optical lags as expected from disk reverberation. Moreover, the UV–optical lags are consistent with archival observations when the X-ray luminosity was >10 times higher. Interestingly, both low- and high-flux states reveal UV–optical lags that are 6–11 times longer than expected from a thin disk. These long lags are often interpreted as due to contamination from the broad line region; however the u-band excess lag (containing the Balmer jump from the diffuse continuum) is less prevalent than in other active galactic nuclei. The Swift campaign showed a low X-ray-to-optical correlation (similar to previous campaigns), but NICER and ground-based monitoring continued for another 2 weeks, during which the optical rose to the highest level of the campaign, followed ∼10 days later by a sharp rise in X-rays. While the low X-ray countrate and relatively large systematic uncertainties in the NICER background make this measurement challenging, if the optical does lead X-rays in this flare, this indicates a departure from the zeroth-order reprocessing picture. If the optical flare is due to an increase in mass accretion rate, this occurs on much shorter than the viscous timescale. Alternatively, the optical could be responding to an intrinsic rise in X-rays that is initially hidden from our line of sight.

Tracking the spectral properties of ESO 511-G030 across different epochs

The Type 1 active galactic nucleus (AGN) ESO 511-G030, a formerly bright and soft excess dominated source, was observed in 2019 in the context of a multi-wavelength monitoring campaign. In the new exposures, the source was found to be in a flux state approximately ten times lower than archival exposures and without any trace of the soft excess. Interestingly, the X-ray weakening observed in the 2019 data corresponds to a comparable fading of the UV flux, suggesting a strong link between these two components. The UV-X-ray spectral energy distribution (SED) of ESO 511-G030 shows remarkable variability. We tested both phenomenological and physically motivated models on the data, finding that the overall emission spectrum of ESO 511-G030 in this extremely low flux state is due to the superposition of a power-law-like continuum (Γ ∼ 1.7) and two reflection components emerging from hot and cold matter. Both the primary X-ray continuum and relativistic reflection are produced in the inner regions close to the supermassive black hole. The prominent variability of ESO 511-G030 and the lack of a soft excess can be explained by the dramatic change in the observed accretion rate, which dropped from an L/LEdd of 2% in 2007 to one of 0.2% in 2019. The X-ray photon index also became harder during the low flux observations from 2019, perhaps as a result of a photon starved X-ray corona.

Probing the Variation of Reverberation Lags along with X-Ray Flux in the AGN Mrk 704

Understanding the variation of lags with respect to the X-ray flux is important to explore the geometry of the inner region of the accretion disk in AGNs. We performed frequency-lag, energy–lag and spectral studies for two sets of observations, in order to investigate the variations in lags with respect to X-ray flux in the AGN source Mrk 704 using the XMM-Newton observatory. We divided one of the light curves into two sections which were noticed to exhibit a flux variation. The frequency-lag spectra in different energy domains revealed that reverberation (soft) lags varied along with the flux. For the first time, we show that the blurred reflection model can consistently explain the soft excess observed in the X-ray spectra for this source. The fluxes of soft (i.e., reflection) and hard components were noted to vary by ∼18% and ∼9% respectively, across the sections. The soft lag amplitude was found to be larger at the high flux state than the amplitude at the low flux state. Most importantly, we found that both frequency-lag and energy–lag spectra do not display significant variation between two observational data sets despite a flux variation of 43%. This phenomenon cannot be explained by the reflection model because the soft lag amplitudes must be larger in the high flux state. The probable scenario is that, in the low flux state, the obscuring cloud delays the reflected soft photons which increases the soft lag amplitude.

Spectral Evolution of Ultraluminous X-Ray Pulsar NGC 300 ULX-1

We report on results from a 1 yr soft X-ray observing campaign of the ultraluminous X-ray pulsar NGC 300 ULX-1 by the Neutron star Interior Composition Explorer (NICER) during 2018–2019. Our analysis also made use of data from Swift/XRT and XMM-Newton in order to model and remove contamination from the nearby eclipsing X-ray binary NGC 300 X-1. We constructed and fitted a series of 5 day averaged NICER spectra of NGC 300 ULX-1 in the 0.4–4.0 keV range to evaluate the long-term spectral evolution of the source, and we found that an absorbed power-law model provided the best fit overall. Over the course of our observations, the source flux (0.4–4.0 keV; absorbed) dimmed from 2 × 10−12 to below 10−13 erg s−1 cm−2 and the spectrum softened, with the photon index going from Γ ≈ 1.6 to Γ ≈ 2.6. We interpret the spectral softening as reprocessed emission from the accretion disk edge coming into view while the pulsar was obscured by the possibly precessing disk. Some spectral fits were significantly improved by the inclusion of a disk blackbody component, and we surmise that this could be due to the pulsar emerging in between obscuration episodes by partial covering absorbers. We posit that we observed a low-flux state of the system (due to line-of-sight absorption) punctuated by the occasional appearance of the pulsar, indicating short-term source variability nested in longer-term accretion disk precession timescales.

X-Ray and GeV-γ-Ray Emission Property of TeV Compact Symmetric Object PKS 1413+135 and Implication for Episodic Jet Activity

PKS 1413+135, a compact symmetric object (CSO) with a two-sided parsec-scale structure in its miniature radio morphology, is spatially associated with 4FGL J1416.1+1320 and recently detected with MAGIC telescopes. We comprehensively analyze its X-ray and gigaelectronvolt γ-ray observation data to reveal its high-energy radiation physics. It is found that the source is in a low-flux stage before MJD 58500 and experiences violent outbursts after MJD 58500 in the gigaelectronvolt band. The flux at 10 GeV varies by ∼3 orders of magnitude, and the gigaelectronvolt-flux variation is accompanied by clear spectral variation, which is characterized as a soft log-parabola spectrum in the low-flux state and a hard power-law spectrum in the bright flares. The amplitude of the variability of X-rays is lower than that of γ-rays, and no correlation of variability between γ-rays and X-rays is observed. Fitting the broadband spectral energy distribution during a gigaelectronvolt outburst with a multi-zone leptonic model, we show that the gigaelectronvolt γ-rays are attributed to the external Compton process while the X-rays are a hybrid of several components. The predicted teraelectronvolt γ-ray flux during the gigaelectronvolt outburst is consistent with the detection of MAGIC telescopes. These results, together with its CSO radio morphology, imply that PKS 1413+135 has episodic nuclear jet activities. The weak γ-ray emission before MJD 58500 may be from its sub-parsec-/parsec-scale jet component powered by previous activities, and the violent outbursts with short timescale variability after MJD 58500 could be attributed to the recently restarted jet activity.

Revisiting the Circumnuclear X-Ray Emission of NGC 2992 in a Historically Low State

The innermost region of the Seyfert galaxy NGC 2992 has long been suspected to be the location of intense AGN–host galaxy interaction, but photon pile up in previous high-resolution observations hampered the study of the soft X-ray excess and the interaction near its nucleus. We present an X-ray imaging and spectroscopic analysis of the circumnuclear (1″–3″) region of NGC 2992 using the zeroth-order image of a 135 ks grating observation obtained with Chandra, which captured the nucleus in a historically low flux state. Extended soft X-ray emission is detected in the circumnuclear region with an observed luminosity of L X ∼ 7 × 1039 erg s−1. The majority of the previous, puzzling detection of soft excess could be associated with the outflow, indicated by the morphological correspondences between the soft X-ray emission and figure-eight-shaped radio bubbles. An anomalous narrow emission line with a centroid energy of ∼4.97 keV is found. If attributed to redshifted highly ionized iron emission (e.g., Fe xxv), the required outflow velocity is ∼0.23 c. An alternative explanation is that this line emission could be produced by the nuclear spallation of iron. We also find extended, asymmetric Fe Kα emission along the galactic disk, which could originate from reflection by cold gas on ∼200 pc scale.

Exploring the Link between the X-Ray Power Spectra and Energy Spectra of Active Galactic Nuclei

Active galactic nuclei (AGN) are generally considered the scaled-up counterparts of X-ray binaries (XRBs). It is known that the power spectral density (PSD) of the X-ray emission of XRBs shows significant evolution with spectral state. It is not clear whether AGN follow a similar evolutionary trend, however, though their X-ray emission and the PSD are both variable. In this work, we study a sample of nine AGN with multiple long observations with XMM-Newton, which exhibit significant X-ray spectral variation. We perform Bayesian PSD analysis to measure the PSD shape and variation. We find that a large change in the X-ray energy spectrum (mainly the change in flux state) is often accompanied by a large change in the PSD shape. The emergence of a high-frequency break in the PSD also depends on the spectral state. Among the four sources with significant high-frequency PSD breaks detected, three show the break only in the high-flux state, while the remaining one shows it only in the low-flux state. Moreover, the X-ray rms variability in different spectral states of an AGN is found to vary by as much as 1.0 dex. These results suggest that the different variability properties observed are likely caused by different physical processes dominating different spectral states. Our results also indicate that the intrinsic PSD variation can introduce a significant fraction of the dispersion as reported for the correlations between various X-ray variability properties and the black hole mass.

The Variability Behavior of NGC 925 ULX-3

We report the results of a 2019–2021 monitoring campaign with Swift and associated target-of-opportunity observations with XMM-Newton and NuSTAR, examining the spectral and timing behavior of the highly variable ultraluminous X-ray source (ULX) NGC 925 ULX-3. We find that the source exhibits a 127–128-day periodicity, with fluxes typically ranging from 1 × 10−13 to 8 × 10−13 erg s−1 cm−2. We do not find strong evidence for a change in period over the time that NGC 925 ULX-3 has been observed, although the source may have been in a much lower flux state when first observed with Chandra in 2005. We do not detect pulsations, and we place an upper limit on the pulsed fraction of ∼40% in the XMM-Newton band, consistent with some previous pulsation detections at low energies in other ULXs. The source exhibits a typical ULX spectrum that turns over in the NuSTAR band and can be fitted using two thermal components. These components have a high temperature ratio that may indicate the lack of extreme inner disk truncation by a magnetar-level magnetic field. We examine the implications for a number of different models for superorbital periods in ULXs, finding that a neutron star with a magnetic field of ∼1012 G may be plausible for this source. The future detection of pulsations from this source would allow for the further testing and constraining of such models.

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 (&amp;gt;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.