IGR J00291 Research Articles

Search for thermonuclear burst oscillations in the Swift/BAT data set

This study comprehensively analyzes type I X-ray bursts observed by Swift/BAT from 2005 to April 2024 to search for X-ray burst oscillations (XBOs) in neutron star low-mass X-ray binaries. XBOs, periodic signals detected within type I X-ray bursts, typically range from 11 to 620 Hz and are often observed in the soft X-ray data of these bursts. Using the high-sensitivity and precise timing capabilities of the Swift/BAT, we found 50 type I X-ray bursts from 37 neutron star low-mass X-ray binaries. We conducted a detailed timing analysis of these bursts. For sources with known burst oscillation frequencies, our findings largely corroborate previous studies. However, many sources displayed low confidence levels in the oscillation signals, with Z12 values between 10 and 20. For sources without known oscillation/spin frequencies, we utilized FFT analysis to search for signals across a broad frequency range. This approach revealed potential oscillation signals, with several bursts showing significance levels exceeding 3σ, including those from MAXI J1421–613, XTE J1701–407, XMM J174457–2850.3, Swift J1734.5–3027, IGR J17473–2721, Swift J174805.3–244637, Swift J181723.1–164300, and X 1832–330.

Unraveling the Hybrid Origins of the X-Ray Nonthermal Emission from IGR J17091–3624

We present a comprehensive study based on multiwavelength observations from the NuSTAR, NICER, Swift, Fermi, NEOWISE, and ATCA telescopes during the 2022 outburst of the black-hole X-ray binary IGR J17091–3624. Our investigation concentrates on the heartbeat-like variability in the X-ray emission, with the aim of using it as a tool to unravel the origin of the nonthermal emission during the heartbeat state. Through X-ray timing and spectral analysis, we observe that the heartbeat-like variability correlates with changes in the disk temperature, supporting the disk radiation pressure instability scenario. Moreover, in addition to a Comptonization component, our time-averaged and phase-resolved spectroscopy reveal the presence of a power-law component that varies independently from the disk component. Combined with the radio–X-ray spectral energy distribution fitting, our results suggest that the power-law component could originate from synchrotron self-Compton radiation in the jet, which requires a strong magnetic field of about B = (0.3–3.5) × 106 G. Additionally, assuming that IGR J17091-3624 and GRS 1915 + 105 share the same radio–X-ray correlation coefficient during both the hard and the heartbeat states, we obtain a distance of 13.7 ± 2.3 kpc for IGR J17091–3624.

A Phase-resolved View of “Heartbeat”-like Variability in IGR J17091-3624 during the 2022 Outburst

IGR J17091–3624, in addition to GRS 1915 + 105, is the only black hole X-ray binary that displays “heartbeat”-like variability, characterized by structured flares with high amplitudes. In this study, we conduct a detailed phase-resolved analysis of the recently identified “heartbeat”-like Class X variability in IGR J17091–3624 during its 2022 outburst, utilizing data from NICER and NuSTAR observations. A shortage in the high-energy (>20 keV) X-ray flux is detected at peak phases of the soft-X-ray flare at a ∼15σ confidence level from the phase-folded light curves. Furthermore, our phase-resolved spectral analysis reveals variations in the spectral shape, particularly showing significant synchronous variations in the disk temperature and flux with the count rate. These findings imply that the flare is primarily driven by instabilities within the accretion disk, consistent with previous studies of the well-known Class ρ variability in GRS 1915+105. However, we also observe a positive correlation between the disk temperature and flux over the flare cycle, which differs from the loop relation between the two parameters found in the Class ρ variability. This could suggest differences in the underlying physical processes between the two variability classes. Variations in the Comptonization component during flares are also observed: the electron temperature and covering fraction show anticorrelations with the disk flux, revealing potential interactions between the accretion disk and the corona during these flares.

Highly Coherent Quasiperiodic Oscillations in the “Heartbeat” Black Hole X-Ray Binary IGR J17091–3624

Highly Coherent Quasiperiodic Oscillations in the “Heartbeat” Black Hole X-Ray Binary IGR J17091–3624

The 2022 Outburst of IGR J17091–3624: Connecting the Exotic GRS 1915+105 to Standard Black Hole X-Ray Binaries

While the standard X-ray variability of black hole X-ray binaries (BHXBs) is stochastic and noisy, there are two known BHXBs that exhibit exotic “heartbeat”-like variability in their lightcurves: GRS 1915+105 and IGR J17091–3624. In 2022, IGR J17091–3624 went into outburst for the first time in the NICER/NuSTAR era. These exquisite data allow us to simultaneously track the exotic variability and the corresponding spectral features with unprecedented detail. We find that as in typical BHXBs, the outburst began in the hard state, then continued in the intermediate state, but then transitioned to an exotic soft state, where we identify two types of heartbeat-like variability (Class V and a new Class X). The flux energy spectra show a broad iron emission line due to relativistic reflection when there is no exotic variability, and absorption features from highly ionized iron when the source exhibits exotic variability. Whether absorption lines from highly ionized iron are detected in IGR J17091–3624 is not determined by the spectral state alone, but rather is determined by the presence of exotic variability; in a soft spectral state, absorption lines are only detected along with exotic variability. Our finding indicates that IGR J17091–3624 can be seen as a bridge between the most peculiar BHXB GRS 1915+105 and “normal” BHXBs, because it alternates between the conventional and exotic behaviors of BHXBs. We discuss the physical nature of the absorbing material and exotic variability in light of this new legacy data set.

Swift/XRT observations of superorbital modulations in wind-fed supergiant X-ray binaries

ABSTRACT We present the first Swift/XRT long-term monitoring of 2S 0114+650, a wind-fed supergiant X-ray binary for which both orbital and superorbital periods are known (Porb ∼ 11.6 d and Psup ∼ 30.8 d). Our campaign, summing up to ∼79 ks, is the most intense and complete sampling of the X-ray light curve of this source with a sensitive pointed X-ray instrument, and covers 17 orbital, and 6 superorbital cycles. The combination of flexibility, sensitivity, and soft X-ray coverage of the X-ray telescope (XRT) allowed us to confirm previously reported spectral changes along the orbital cycle of the source and unveil the variability in its spectral parameters as a function of the superorbital phase. For completeness, we also report on a similar analysis carried out by exploiting XRT archival data on three additional wind-fed supergiant X-ray binaries IGR J16418−4532, IGR J16479−4514, and IGR J16493−4348. For these sources, the archival data provided coverage along several superorbital cycles but our analysis could not reveal any significant spectral variability.

Drop in the Hard Pulsed Fraction and a Candidate Cyclotron Line in IGR J16320–4751 Seen by NuSTAR

We report on a timing and spectral analysis of a 50 ks NuSTAR observation of IGR J16320−4751 (= AX J1631.9−4752), a high-mass X-ray binary hosting a slowly rotating neutron star. In this observation from 2015, the spin period was 1308.8 ± 0.4 s giving a period derivative s s−1 when compared with the period measured in 2004. In addition, the pulsed fraction decreased as a function of energy, as opposed to the constant trend that was seen previously. This suggests a change in the accretion geometry of the system during the intervening 11 yr. The phase-averaged spectra were fit with the typical model for accreting pulsars: a power law with an exponential cutoff. This left positive residuals at 6.4 keV attributable to the known iron Kα line, as well as negative residuals around 14 keV from a candidate cyclotron line detected at a significance of 5σ. We found no significant differences in the spectral parameters across the spin period, other than the expected changes in flux and component normalizations. A flare lasting around 5 ks was captured during the first half of the observation, where the X-ray emission hardened and the local column density decreased. Finally, the binary orbital period was refined to 8.9912 ± 0.0078 days thanks to Swift/BAT monitoring data from 2005–2022.

Investigating the Superorbital Modulations in 4U 1909 + 07, IGR J16418-4532, and IGR J16479-4514 with Swift XRT, BAT, and NuSTAR Observations

A puzzling variety of superorbital modulations has been discovered in several supergiant high mass X-ray binaries (sgHMXBs). To investigate the mechanisms driving these superorbital modulations, we have analyzed long-term Neil Gehrels Swift Observatory (Swift) Burst Alert Telescope (BAT) observations of three sgHMXBs: 4U 1909 + 07, IGR J16418–4532, and IGR J16479–4514, and constructed their dynamic power spectra and superorbital intensity profiles. These Swift BAT observations are complemented by pointed Swift X-ray Telescope (XRT) and Nuclear Spectroscopic Telescope Array (NuSTAR) observations performed near the predicted maximum and minimum phase of a single superorbital cycle for each of these sources. The BAT dynamic power spectra show changes in the strength of the superorbital modulation on timescales of years, with either the peak at the fundamental frequency and/or the second harmonic present at different times for all three sources. The pointed Swift XRT and NuSTAR observations show no significant differences between the pulse profiles and spectral parameters at the superorbital maximum and minimum phase. This is likely due to the fact the superorbital modulation had weakened significantly during the times when the NuSTAR observations were carried out for all three sources. The results from the Swift XRT, BAT, and NuSTAR analysis indicate the possible presence of multiple corotating interaction regions (CIRs) in the stellar winds of the supergiant stars, although a structured stellar wind from the supergiant star due to tidal oscillations cannot be ruled out.

XMM-Newton and Swift observations of supergiant high mass X-ray binaries

Wind-fed supergiant X-ray binaries are precious laboratories not only to study accretion under extreme gravity and magnetic field conditions, but also to probe the still highly debated properties of massive star winds. These include clumps, originating from the inherent instability of line driven winds, and larger structures. In this paper we report on the results of the last (and not yet published) monitoring campaigns that our group has been carrying out since 2007 with both XMM-Newton and the Swift Neil Gehrels observatory. Data collected with the EPIC cameras on board XMM-Newton allow us to carry out a detailed hardness-ratio-resolved spectral analysis that can be used as an efficient way to detect spectral variations associated with the presence of clumps. Long-term observations with the XRT on board Swift, evenly sampling the X-ray emission of supergiant X-ray binaries over many different orbital cycles, are exploited to look for the presence of large-scale structures in the medium surrounding the compact objects. These can be associated either with corotating interaction regions or with accretion and/or photoionization wakes, and with tidal streams. The results reported in this paper represent the outcomes of the concluded observational campaigns we carried out on the supergiant X-ray binaries 4U 1907+09, IGR J16393−4643, IGR J19140+0951, and XTE J1855−026, and on the supergiant fast X-ray transients IGR J17503−2636, IGR J18410−0535, and IGR J11215−5952. All results are discussed in the context of wind-fed supergiant X-ray binaries and ideally serve to optimally shape the next observational campaigns aimed at sources in the same classes. We show in one of the Appendices that IGR J17315−3221, preliminarily classified in the literature as a possible supergiant X-ray binary discovered by INTEGRAL, is the product of a data analysis artifact and should thus be disregarded for future studies.

Statistical Properties of X-Ray Flares from the Supergiant Fast X-Ray Transients

Supergiant fast X-ray transients are a sub-class of high mass X-ray binaries, in which the compact object is a neutron star and it is accreting materials from its non-degenerate companion star. The sample of supergiant fast X-ray transients from XMM-Newton public observations have been intensively studied, and hence the corresponding parameters for identifying X-ray flares have been obtained by Sidoli et al., such as the rise time, the decay time, the duration time, the waiting time, the peak luminosity, the isotropic energy and the mean luminosity. In order to investigate the origin of the X-ray flares from the supergiant fast X-ray transients, we apply the cumulative distribution method with the power-law index to the target sample. We then find that the relevant parameters can be well described by the cumulative distribution with indices 1.0–1.9 for IGR J16418-4532, IGR J16328-4726 and IGR J18450-0435, respectively. This finding indicates that the X-ray flares are likely to be produced by the self-organizing critical process, one possible scenario may be due to reconnection in magnetized stellar wind blobs approaching into the magnetosphere of magnetized neutron star. Therefore, we suggest that the high mass X-ray binaries with similar X-ray flares likely belong to the self-organized criticality systems.

Similarities and differences in accretion flow properties between GRS 1915+105 and IGR J17091-3624: A case study

We perform a comparative spectro-temporal analysis on the variability classes of GRS 1915+105 and IGR J17091-3624 to draw inferences regarding the underlying accretion flow mechanism. The ν, as well as C2 class Rossi X-ray Timing Explorer observation, have been considered for analysis. We investigate the intensity variation of the source in different energy domains that correspond to different components of the accretion flow and infer the relative dominance of these flow components during the dip/flare events. We correlate the dependence of the dynamic photon index (Θ) with intensities in different energy bands and comment on the transition of the source to hard/soft phases during soft dips/flares. We also report the presence of sharp QPOs at ∼7.1 Hz corresponding to both softer and harder domain in the case of ν variability class of GRS 1915+105 and discuss the possible accretion flow configuration it suggests. Sharp QPO around ∼20 mHz is observed in ν and C2 classes of IGR J17091-3624 in low and mid energy band (2.0–6.0 keV and 6.0–15.0 keV), but remains undetected in high energy (15.0–60.0 keV). The 2.5–25.0 keV background-subtracted spectra have also been fitted with TCAF along with a Compton reflection component. A plausible accretion flow mechanism in order to explain the observed variability has been proposed.

Detecting the intrinsic X-ray emission from the O-type donor star and the residual accretion in a supergiant fast X-ray transient in its faintest state

We report on the results of an XMM–Newton observation of the supergiant fast X-ray transient (SFXT) IGR J08408-4503 performed in June 2020. The source is composed of a compact object (likely a neutron star) orbiting around an O8.5Ib-II(f)p star, LM Vel. The X-ray light curve shows a very low level of emission, punctuated by a single, faint flare. We analysed spectra measured during the flare and during quiescence. The quiescent state shows a continuum spectrum that is well deconvolved to three spectral models: two components are from a collisionally ionized plasma (with temperatures of kT1 = 0.24 keV and kT2 = 0.76 keV), together with a power-law model (photon index, Γ, of ∼2.55), dominating above ∼2 keV. The X-ray flux emitted at this lowest level is 3.2 × 10−13 erg cm−2 s−1 (0.5–10 keV, corrected for the interstellar absorption), implying an X-ray luminosity of 1.85 × 1032 erg s−1 (at 2.2 kpc). The two-temperature collisionally ionized plasma is intrinsic to the stellar wind of the donor star, while the power-law can be interpreted as emission due to residual, low-level accretion onto the compact object. The X-ray luminosity contributed by the power-law component only, in the lowest state, is (4.8 ± 1.4)×1031 erg s−1, which is the lowest quiescent luminosity detected from the compact object in an SFXT. Thanks to this very faint X-ray state caught by XMM–Newton, X-ray emission from the wind of the donor star LM Vel could be well-established and studied in detail for the first time, along with a very low level of accretion onto the compact object. The residual accretion rate onto the compact object in IGR J08408-4503 can be interpreted as the Bohm diffusion of (possibly magnetized) plasma entering the neutron star magnetosphere at low Bondi capture rates from the supergiant donor wind at the quasi-spherical, radiation-driven settling accretion stage.

Modelling spin-up episodes in accreting millisecond X-ray pulsars

ABSTRACT Accreting millisecond X-ray pulsars are known to provide a wealth of physical information during their successive states of outburst and quiescence. Based on the observed spin-up and spin-down rates of these objects, it is possible, among other things, to infer the stellar magnetic field strength and test models of accretion disc flow. In this paper, we consider the three accreting X-ray pulsars (XTE J1751–305, IGR J00291+5934 and SAX J1808.4–3658) with the best available timing data, and model their observed spin-up rates with the help of a collection of standard torque models that describe a magnetically threaded accretion disc truncated at the magnetospheric radius. Whilst none of these models is able to explain the observational data, we find that the inclusion of the physically motivated phenomenological parameter ξ, which controls the uncertainty in the location of the magnetospheric radius, leads to an enhanced disc-integrated accretion torque. These ‘new’ torque models are compatible with the observed spin-up rates as well as the inferred magnetic fields of these objects provided that ξ ≈ 0.1−0.5. Our results are supplemented with a discussion of the relevance of additional physics effects that include the presence of a multipolar magnetic field and general relativistic gravity.

Disk Tearing: Implications for Black Hole Accretion and AGN Variability

Abstract Accretion disks around black holes power some of the most luminous objects in the universe. Disks that are misaligned to the black hole spin can become warped over time by Lense–Thirring precession. Recent work has shown that strongly warped disks can become unstable, causing the disk to break into discrete rings producing a more dynamic and variable accretion flow. In a companion paper, we present numerical simulations of this instability and the resulting dynamics. In this paper, we discuss the implications of this dynamics for accreting black hole systems, with particular focus on the variability of active galactic nuclei (AGN). We discuss the timescales on which variability might manifest, as well as the impact of the observer orientation with respect to the black hole spin axis. When the disk warp is unstable near the inner edge of the disk, we find quasi-periodic behavior of the inner disk, which may explain the recent quasi-periodic eruptions observed in, for example, the Seyfert 2 galaxy GSN 069 and in the galactic nucleus of RX J1301.9+2747. These eruptions are thought to be similar to the “heartbeat” modes observed in some X-ray binaries (e.g., GRS 1915+105 and IGR J17091-3624). When the instability manifests at larger radii in the disk, we find that the central accretion rate can vary on timescales that may be commensurate with, e.g., changing-look AGN. We therefore suggest that some of the variability properties of accreting black hole systems may be explained by the disk being significantly warped, leading to disk tearing.

Very low-frequency oscillations from the 11 Hz pulsar in Terzan 5: frame dragging back on the table.

ABSTRACT We present a re-analysis of 47 Rossi X-ray Timing Explorer observations of the 11 Hz accreting pulsar IGR J17480-2446 in Terzan 5 during its 2010 outburst. We studied the fast-time variability properties of the source and searched for quasi-periodic oscillations (QPOs) in a large-frequency range. General relativity predicts that frame dragging occurs in the vicinity of a spinning compact object and induces the precession of matter orbiting said object. The relativistic precession model predicts that this frame dragging can be observed as QPOs with a characteristic frequency in the light curves of accreting compact objects. Such QPOs have historically been classified as horizontal branch oscillations in neutron star (NS) systems, and for a NS spinning at 11 Hz, these oscillations are expected at frequencies below 1 Hz. However, previous studies of IGR J17480-2446 have classified QPOs at 35–50 Hz as horizontal branch oscillations, thus casting doubts on the frame-dragging nature of such QPOs. Here, we report the detection of seven very low-frequency QPOs, previously undetected, with centroid frequencies below 0.3 Hz, and which can be ascribed to frame dragging. We also discuss the possible nature of the QPOs detected at 35–50 Hz in this alternative scenario.

A machine learning approach for classification of accretion states of black hole binaries

ABSTRACT In this paper, we employ Machine Learning algorithms on multimission observations for the classification of accretion states of outbursting black hole X-ray binaries for the first time. Archival data from RXTE, Swift, MAXI, and AstroSat observatories are used to generate the hardness intensity diagrams (HIDs) for outbursts of the sources XTE J1859+226 (1999 outburst), GX 339−4 (2002, 2004, 2007, and 2010 outbursts), IGR J17091−3624 (2016 outburst), and MAXI J1535−571 (2017 outburst). Based on variation of X-ray flux, hardness ratios, presence of various types of quasi-periodic oscillations (QPOs), photon indices, and disc temperature, we apply clustering algorithms like K-Means clustering and Hierarchical clustering to classify the accretion states (clusters) of each outburst. As multiple parameters are involved in the classification process, we show that clustering algorithms club together the observations of similar characteristics more efficiently than the ‘standard’ method of classification. We also infer that K-Means clustering provides more reliable results than Hierarchical clustering. We demonstrate the importance of the classification based on machine learning by comparing it with results from ‘standard’ classification.

AstroSat view of IGR J17091−3624 and GRS 1915 + 105: decoding the ‘pulse’ in the ‘Heartbeat State’

ABSTRACT IGR J17091−3624 is a transient galactic black hole which has a distinct quasi-periodic variability known as ‘heartbeat’, similar to the one observed in GRS 1915 + 105. In this paper, we report the results of ∼125 ks AstroSat observations of this source during the 2016 outburst. For the first time, a double-peaked QPO (DPQ) is detected in a few time segments of this source with a difference of δf ∼ 12 mHz between the two peaks. The nature of the DPQ was studied based on hardness ratios and using the static as well as the dynamic power spectrum. Additionally, a low-frequency (25–48 mHz) ‘heartbeat’ single-peak QPO (SPQ) was observed at different intervals of time along with harmonics (50–95 mHz). Broad-band spectra in the range 0.7–23 keV, obtained with Soft X-ray Telescope and Large Area X-ray Proportional Counter, could be fitted well with combination of a thermal Comptonization and a multicolour disc component model. During AstroSat observation, the source was in the soft-intermediate state (SIMS) as observed with Swift/XRT. We present a comparative study of the ‘heartbeat’ state variability in IGR J17091−3624 with GRS 1915 + 105. Significant difference in the timing properties is observed although spectral parameters (Γ ∼ 2.1–2.4 and Tmax ∼ 0.6–0.8 keV) in the broad energy band remain similar. Spectral properties of segments exhibiting SPQ and DPQ are further studied using simple phase-resolved spectroscopy which does not show a significant difference. Based on the model parameters, we obtain the maximum ratio of mass accretion rate in GRS 1915 + 105 to that in IGR J17091−3624 as ∼25: 1. We discuss the implications of our findings and comment on the physical origin of these exotic variabilities.

A semi-analytical treatment to wind accretion in neutron star supergiant high-mass X-ray binaries – I. Eccentric orbits

ABSTRACT We present in this paper a first step toward a semi-analytical treatment of the accretion process in wind-fed neutron star (NS) supergiant X-ray binaries with eccentric orbits. We consider the case of a spherically symmetric wind for the supergiant star and a simplified model for the accretion on to the compact object. A self-consistent calculation of the photoionization of the stellar wind by the X-rays from the accreting NS is included. This effect is convolved with the modulation of the mass accretion rate induced by the eccentric orbit to obtain the expected X-ray luminosity of a system along the orbit. As part of our results, we first show that the bi-modality of low- and high-X-ray luminosity solutions for supergiant X-ray binaries reported in previous papers is likely to result from the effect of the NS approaching first and then moving away from the companion (without coexisting simultaneously). We propose that episodes of strong wind photoionization can give rise to off-states of the sources. Our calculations are applied to the case of a few classical supergiant X-ray binary systems with known eccentricities (Vela X-1, 4U 1907+09, GX 301-2) and to the case of the only supergiant fast X-ray transient with a confirmed eccentric orbit, IGR J08408-4503. The results are compared with observational findings on these sources. We also discuss the next steps needed to expand the calculations toward a more comprehensive treatment in future publications.

Dust-scattering Halo and Giant Hard X-Ray Flare from the Supergiant Fast X-Ray Transient IGR J16479–4514 Investigated with XMM-Newton and INTEGRAL

Abstract We report results from the analysis of XMM-Newton and INTEGRAL data of IGR J16479−4514. The unpublished XMM-Newton observation, performed in 2012, occurred during the source eclipse. No pointlike X-ray emission was detected from the source; conversely, extended X-ray emission was clearly detected up to a size distance compatible with a dust-scattering halo produced by the source X-ray emission before being eclipsed by its companion donor star. The diffuse emission of the dust-scattering halo could be observed without any contamination from the central point X-ray source, compared to a previous XMM-Newton observation published in 2008. Our comprehensive analysis of the 2012 unpublished spectrum of the diffuse emission, as well as the 2008 reanalyzed spectra extracted from three adjacent time intervals and different extraction regions (optimized for pointlike and extended emission), allowed us to clearly disentangle the scattering halo spectrum from the residual pointlike emission during the 2008 eclipse. Moreover, the pointlike emission detected in 2008 could be separated into two components attributed to the direct emission from the source and scattering in the stellar wind, respectively. From archival unpublished INTEGRAL data, we identified a very strong (∼3 × 10−8 erg cm−2 s−1) and fast (∼25 minute duration) flare that was classified as a giant hard X-ray flare, since the measured peak luminosity is ∼7 × 1037 erg s−1. Giant X-ray flares from supergiant fast X-ray transients are very rare; to date, only one has been reported from a different source. We propose a physical scenario to explain the origin in the case of IGR J16479−4514.

Swift/XRT, Chandra, and XMM–Newton observations of IGR J17091–3624 as it returns into quiescence

ABSTRACT IGR J17091–3624 is a low-mass X-ray binary (LMXB), which received wide attention from the community thanks to its similarities with the bright black hole system GRS 1915+105. Both systems exhibit a wide range of highly structured X-ray variability during outburst, with time-scales from few seconds to tens of minutes, which make them unique in the study of mass accretion in LMXBs. In this work, we present a general overview into the long-term evolution of IGR J17091–3624, using Swift/XRT observations from the onset of the 2011–2013 outburst in 2011 February till the end of the last bright outburst in 2016 November. We found four re-flares during the decay of the 2011 outburst, but no similar re-flares appear to be present in the latter one. We studied, in detail, the period with the lowest flux observed in the last 10 yr, just at the tail end of the 2011–2013 outburst, using Chandra and XMM-Newton observations. We observed changes in flux as high as a factor of 10 during this period of relative quiescence, without strong evidence of softening in the spectra. This result suggests that the source has not been observed at its true quiescence so far. By comparing the spectral properties at low luminosities of IGR J17091–3624 and those observed for a well-studied population of LMXBs, we concluded that IGR J17091–3624 is most likely to host a black hole as a compact companion rather than a neutron star.