mHz Quasi-periodic Oscillations Research Articles

The ongoing spin-down episode of 4U 1626-67

Abstract We report the X-ray characteristics of the persistent X-ray pulsar 4U 1626-67 using simultaneous NuSTAR and NICER observations. The X-ray pulsar 4U 1626-67 has just encountered a torque reversal in 2023 and is presently in the spin-down state. We have examined the temporal and spectral characteristics of the source during its ongoing spin-down episode. The pulse profiles of the source are characterized by multiple substructures at lower energies and a wide asymmetric single-peaked structure at higher energies. The pulse fraction follows an overall increasing trend with energy. We confirm the existence of mHz quasi-periodic oscillation (QPO) exclusively during the current spin-down phase in all the observations. The source is spinning down at 0.00045(4) s yr−1. The broadband spectrum during this phase is described by empirical NPEX model and a soft blackbody component with kT ∼ 0.25 keV. In addition to the iron emission line, we also confirm the presence of cyclotron line at ∼ 36 keV. The source flux continues to decrease during the current spin-down phase, and the corresponding luminosity ∼ (3.3–4.9) × 1036 ergs s−1 lies in the intermediate range of accreting X-ray pulsars that may be associated with a hybrid accretion geometry. The magnetic field strengths estimated using the cyclotron line measurements and QPO frequency are consistent. The evolution of the spectral parameters relative to the pulsed phase is examined using phase-resolved spectroscopy.

4U 1626−67 returns to spin-down: timing features toe the line

ABSTRACT We present a comprehensive analysis of X-ray pulsar 4U 1626−67 during its current spin-down (2SD) state, following a recent torque reversal. Since its discovery, this ultra-compact binary has experienced multiple torque states, transitioning from spin-up (1SU) during 1977–1990 to spin-down (1SD) during 1990–2008, and again spin-up (2SU) until 2023. From Nuclear Spectroscopic Telescope ARray observation of May 2023, we have investigated the timing and spectral properties of this pulsar during its 2SD phase, while also comparing them with previous spin-up-down states. For energies up to 8 keV, a distinct bi-horned pulse profile was observed during the spin-up phase, while several substructures emerged during spin-down. Beyond 8 keV, a broad asymmetric peak was consistently observed across all torque states. The pulse fraction during the 2SD phase was higher than that during 2SU phase. A prominent ∼46.8 mHz quasi-periodic oscillation has been exclusively detected during the spin-down phase. The broad-band spectrum during the 2SD phase is described by empirical Negative and Positive power-law with Exponential cutoff model, cyclotron absorption feature and its first harmonic. The spectrum during 2SU phase requires an additional blackbody component and asymmetry in the cyclotron absorption line. A significant flux drop by a factor of ∼3 in the 2SD was observed.

Broad-band mHz QPOs and spectral study of LMC X-4 with AstroSat

ABSTRACT We report the results of broad-band timing and spectral analysis of data from an AstroSat observation of the high-mass X-ray binary LMC X-4. The Large Area X-ray Proportional Counter (LAXPC) and Soft X-ray Telescope (SXT) instruments onboard the AstroSatobserved the source in 2016 August. A complete X-ray eclipse was detected with the LAXPC. The 3–40 keV power density spectrum showed the presence of coherent pulsations along with a ∼26 mHz quasi-periodic oscillation feature. The spectral properties of LMC X-4 were derived from a joint analysis of the SXT and LAXPC spectral data. The 0.5–25 keV persistent spectrum comprised of an absorbed high-energy cut-off power law with photon index of Γ ∼ 0.8 and cut-off at ∼16 keV, a soft thermal component with kTBB ∼ 0.14 keV, and Gaussian components corresponding to Fe Kα, Ne ix, and Ne x emission lines. Assuming a source distance of 50 kpc, we determined 0.5–25 keV luminosity to be ∼2 × 1038 erg s−1.

Detection of a quasi-periodic oscillation at ∼40 mHz in Cen X-3 with Insight-HXMT

ABSTRACT We investigated the quasi-periodic oscillation (QPO) features in the accretion-powered X-ray pulsar Cen X-3 observed by Insight-HXMT. For two observations in 2020 when Cen X-3 was in an extremely soft state, the power density spectrum revealed the presence of obvious QPO features at ∼40 mHz with an averaged fractional rms amplitude of ∼9 per cent. We study the mHz QPO frequency and rms amplitude over orbital phases, and find that the QPO frequency is ∼33–39 mHz at the orbital phase of 0.1–0.4, increasing to ∼37–43 mHz in the orbital phase of 0.4–0.8, but has no strong dependence on X-ray intensity. We also carried out an energy-dependent QPO analysis, the rms amplitude of the mHz QPOs have a decreasing trend as the energy increases from 2 to 20 keV. In addition, the QPO time-lag analysis shows that the time-delay is ∼20 ms (a hard lag) in the range of ∼5–10 keV, and becomes negative [time-lag of −(20–70) ms] above ∼10 keV. The different QPO theoretical models are summarized and discussed. In the end, we suggest that these energy-dependent timing features as well as the origin of mHz QPOs in Cen X-3 may be ascribed to an instability when the accretion disc is truncated near the corotation radius.

Discovery of Transition from Marginally Stable Burning to Unstable Burning after a Superburst in Aql X-1

Abstract Superbursts are long duration, rare, and extremely energetic thermonuclear explosions of neutron star low-mass X-ray binaries (NS LMXBs), which are proposed to be due to unstable carbon ignition. We report the superburst properties and consequences from Aql X–1 observed by the Neutron Star Interior Composition Explorer (NICER), the Monitor of All-sky X-ray Image, Swift, and Insight Hard X-ray Modulation Telescope (Insight-HXMT) on the MJD 59130.7. We found two faint type I X-ray bursts 9.44 days after the superburst with a short recurrence time of 7.6 minutes, which is the most accurate measurement of the quenching time in all NS LMXBs with observed superbursts. We also discovered mHz quasi-periodic oscillations in the frequency range 2.7–11.3 mHz immediately after the superburst, before and after the resumption of the first type I X-ray burst from NICER, Swift, and Insight-HXMT observations. For the first time, we observed the transition from superburst, via marginally stable burning to unstable burning in NS LMXBs. We compared the quenching time and the recurrence time of type I X-ray bursts with simulations.

The Lense–Thirring timing-accretion plane for ULXs

ABSTRACT Identifying the compact object in ultraluminous X-ray sources (ULXs) has to date required detection of pulsations or a cyclotron resonance scattering feature (CRSF), indicating a magnetized neutron star. However, pulsations are observed to be transient and it is plausible that accretion on to the neutron star may have suppressed the surface magnetic field such that pulsations and CRSFs will be entirely absent. We may therefore lack direct means to identify neutron star systems whilst we presently lack an effective means by which to identify black hole ULXs. Here we present a possible method for separating the ULX population by assuming the X-ray, mHz quasi-periodic oscillations (QPOs), and day time-scale periods/QPOs are associated with Lense–Thirring precession of the inflow and outflowing wind, respectively. The precession time-scales combined with the temperature of the soft X-ray component produce planes where the accretor mass enters as a free parameter. Depending on the properties of the wind, use of these planes may be robust to a range in the angular momentum (spin) and, for high accretion rates, essentially independent of the neutron star’s surface dipole field strength. Our model also predicts the mHz QPO frequency and magnitude of the phase lag imprinted due to propagation through the optically thick wind; in the case of NGC 5408 X-1 we subsequently infer a black hole mass and moderate-to-high spin. Finally, we note that observing secular QPO evolution over sufficient baselines may indicate a neutron star, as the precession responds to spin-up which is not readily observable for black hole primaries.

LAXPC/AstroSat Study of ∼1 and ∼2 mHz Quasi-periodic Oscillations in the Be/X-Ray Binary 4U 0115+63 during Its 2015 Outburst

The Be X-ray Binary 4U 0115+63 was observed by Large Area X-ray Proportional Counter (LAXPC) instrument on AstroSat on 2015 October 24 during the peak of a giant Type II outburst. Prominent intensity oscillations at ~ 1 and ~ 2 mHz frequency were detected during the outburst. Nuclear Spectroscopic Telescope Array (NuSTAR) observations made during the same outburst also show mHz quasi periodic oscillations (QPOs). Details of the oscillations and their characteristics deduced from LAXPC/AstroSat and NuSTAR observations are reported in this paper. Analysis of the archival Rossi X-ray Timing Explorer (RXTE)/Proportional Counter Array (PCA) data during 2001-11 also show presence of mHz QPOs during some of the outbursts and details of these QPOs are also reported. Possible models to explain the origin of the mHz oscillations are examined. Similar QPOs, albeit at higher frequencies, have been reported from other neutron star and black hole sources and both may have a common origin. Current models to explain the instability in the inner accretion disk causing the intense oscillations are discussed.

Phase-resolved spectral analysis of the 11 mHz quasi-periodic oscillation in the black hole candidate H1743–322

H1743-322 is one of the few black hole candidates (BHCs) in low-mass X-ray binaries that shows mHz quasi-periodic oscillations (QPOs) that are not associated with the more common type A, B and C oscillations seen in the X-ray light curves of typical BHCs systems. To better understand the physical origin of the mHz oscillations, we carried out a phase-resolved spectroscopic study of two RXTE observations of this source. As previously reported, the averaged energy spectra of H1743-322 shows a strong iron line at $\sim6.5$ keV. Here we found evidence that the line flux appears to be modulated at twice the frequency of the mHz QPO. This line flux modulation is very similar to the one previously found for the type-C QPO in this source. We interpret the possibly periodic line flux modulation with this mHz QPO in terms of Lense-Thirring precession of the inner flow, and discuss the possible connection with the modulation of the line properties with the type-C QPO frequency.

Broad-band spectral evolution and temporal variability of IGR J17091-3624 during its 2016 outburst: SWIFT and NuSTAR results

We report on the 2016 outburst of the transient Galactic Black Hole candidate IGR J17091-3624 based on the observation campaign carried out with SWIFT and NuSTAR. The outburst profile, as observed with SWIFT-XRT, shows a typical `q'-shape in the Hardness Intensity Diagram (HID). Based on the spectral and temporal evolution of the different parameters, we are able to identify all the spectral states in the q-profile of HID and the Hardness-RMS diagram (HRD). Both XRT and NuSTAR observations show an evolution of low frequency Quasi periodic oscillations (QPOs) during the low hard and hard intermediate states of the outburst rising phase. We also find mHz QPOs along- with distinct coherent class variabilities (heartbeat oscillations) with different timescales, similar to the $\rho$-class (observed in GRS 1915+105). Phenomenological modelling of the broad-band XRT and NuSTAR spectra also reveals the evolution of high energy cut-off and presence of reflection from ionized material during the rising phase of the outburst. Further, we conduct the modelling of X-ray spectra of SWIFT and NuSTAR in 0.5 - 79 keV to understand the accretion flow dynamics based on two component flow model. From this modelling, we constrain the mass of the source to be in the range of 10.62 - 12.33 Msun with 90% confidence, which is consistent with earlier findings.

X-Ray Timing Analysis of Cyg X-3 Using AstroSat/LAXPC: Detection of Milli-hertz Quasi-periodic Oscillations during the Flaring Hard X-Ray State

Abstract We present here results from the X-ray timing and spectral analysis of the X-ray binary Cyg X-3 using observations from the Large Area X-ray proportional Counter on board AstroSat. Consecutive light curves observed over a period of one year show the binary orbital period of 17253.56 ± 0.19 s. Another low-amplitude, slow periodicity of the order of 35.8 ± 1.4 days is observed, which may be due to the orbital precession as suggested earlier by Molteni et al. During the rising binary phase, power density spectra from different observations during the flaring hard X-ray state show quasi-periodic oscillations (QPOs) at ∼5–8 mHz, ∼12–14 mHz, and ∼18–24 mHz frequencies at the minimum confidence of 99%. However, during the consecutive binary decay phase, no QPO is detected up to 2σ significance. Energy-dependent time-lag spectra show soft lag (soft photons lag hard photons) at the mHz QPO frequency and the fractional rms of the QPO increases with the photon energy. During the binary motion, the observation of mHz QPOs during the rising phase of the flaring hard state may be linked to the increase in the supply of the accreting material in the disk and corona via stellar wind from the companion star. During the decay phase, the compact source moves in the outer wind region causing the decrease in supply of material for accretion. This may cause weakening of the mHz QPOs below the detection limit. This is also consistent with the preliminary analysis of the orbital phase-resolved energy spectra presented in this paper.

Evidence of Compton cooling during an X-ray flare supports a neutron star nature of the compact object in 4U1700−37

Abstract Based on new Chandra X-ray telescope data, we present empirical evidence of plasma Compton cooling during a flare in the non-pulsating massive X-ray binary 4U1700−37. This behaviour might be explained by quasi-spherical accretion on to a slowly rotating magnetized neutron star (NS). In quiescence, the NS in 4U1700−37 is surrounded by a hot radiatively cooling shell. Its presence is supported by the detection of mHz quasi-periodic oscillations likely produced by its convection cells. The high plasma temperature and the relatively low X-ray luminosity observed during the quiescence, point to a small emitting area ∼1 km, compatible with a hotspot on an NS surface. The sudden transition from a radiative to a significantly more efficient Compton cooling regime triggers an episode of enhanced accretion resulting in a flare. During the flare, the plasma temperature drops quickly. The predicted luminosity for such transitions, ∼3 × 1035 erg s−1, is very close to the luminosity of 4U1700−37 during quiescence. The transition may be caused by the accretion of a clump in the stellar wind of the donor star. Thus, a magnetized NS nature of the compact object is strongly favoured.

Quasi-periodic Pulse Amplitude Modulation in the Accreting Millisecond Pulsar IGR J00291+5934

Abstract We introduce a new method for analyzing the aperiodic variability of coherent pulsations in accreting millisecond X-ray pulsars (AMXPs). Our method involves applying a complex frequency correction to the time-domain light curve, allowing for the aperiodic modulation of the pulse amplitude to be robustly extracted in the frequency domain. We discuss the statistical properties of the resulting modulation spectrum and show how it can be correlated with the non-pulsed emission to determine if the periodic and aperiodic variability are coupled processes. Using this method, we study the 598.88 Hz coherent pulsations of the AMXP IGR J00291+5934 as observed with the Rossi X-ray Timing Explorer and XMM-Newton. We demonstrate that our method easily confirms the known coupling between the pulsations and a strong 8 mHz quasi-periodic oscillation (QPO) in XMM-Newton observations. Applying our method to the RXTE observations, we further show, for the first time, that the much weaker 20 mHz QPO and its harmonic are also coupled with the pulsations. We discuss the implications of this coupling and indicate how it may be used to extract new information on the underlying accretion process.

MILLIHERTZ QUASI-PERIODIC OSCILLATIONS IN 4U 1636–536: PUTTING POSSIBLE CONSTRAINTS ON THE NEUTRON STAR SIZE

ABSTRACT Based on previous studies of quasi-periodic oscillations (QPOs) in neutron star (NS) low-mass X-ray binaries, mHz QPOs are believed to be related to “marginally stable” burning on the NS surface. Our study of phase-resolved energy spectra of these oscillations in 4U 1636–53 shows that the oscillations are not caused by variations in the blackbody temperature of the NS, but reveals a correlation between the change of the count rate during the mHz QPO pulse and the spatial extent of a region emitting blackbody emission. The maximum size of the emission area, km2, provides direct evidence that the oscillations originate from a variable surface area constrained on the NS and are therefore not related to instabilities in the accretion disk. The obtained lower limit on the size of the NS (11.0 km) rules out equations of state that prefer small NS radii. Observations of mHz QPOs in NS LMXBs with NICER and eXTP will reduce the statistical uncertainty in the lower limit on the NS radius, which together with better estimates of the hardening factor and distance, will allow for improved discrimination between different equations of state and compact star models. Furthermore, future missions will allow us to measure the peak blackbody emission area for a single mHz QPO pulse, which will push the lower limit to larger radii.

XMM–Newtondiscovery of mHz quasi-periodic oscillations in the high-mass X-ray binary IGR J19140+0951

We report on the discovery of mHz quasi-periodic oscillations (QPOs) from the high mass X-ray binary (HMXB) IGRJ19140+0951, during a 40 ks XMM-Newton observation performed in 201 5, which caught the source in its faintest state ever observed. At the start of the observation, IGRJ19140+0951 was at a low flux of 2$\times$10$^{-12}$~erg~cm$^{-2}$~s$^{-1}$ ( 2-10 keV; L$_{\rm X}$=3$\times$10$^{33}$~erg~s$^{-1}$ at 3.6 kpc), then its emission rised reaching a flux 10 times higher, in a flare-like activity. The investigation of the pow er spectrum reveals the presence of QPOs, detected only in the second part of the observation, with a strong peak at a frequency of 1.46$\pm{0.07}$~mHz, together with higher harm onics. The X-ray spectrum is highly absorbed (N$_{\rm H}$=$10^{23}$~cm$^{-2}$), well fitted by a power-law with a photon index in the range 1.2-1.8. The re-analysis of a Chandra archival observation shows a modulation at 0.17+/-0.05mHz, very likely the neutron star spin period (although a QPO cannot be excluded). We discuss the origin of the 1.46 mHz QPO in the framework of both disc-fed and wind-fed HMXBs, favouring the quasi-spherical accretion scenario. The low flux observed by XMM-Newton leads to about three orders of magnit ude the source dynamic range, overlapping with the one observed from Supergiant Fast X-ray Transients (SFXTs). However, since its duty cycle is not as low as in SFXTs, IGRJ19140 +0951 is an intermediate system between persistent supergiant HMXBs and SFXTs, suggesting a smooth transition between these two sub-classes.

Discovery of a correlation between the frequency of the mHz quasi-periodic oscillations and the neutron-star temperature in the low-mass X-ray binary 4U 1636–53

We detected millihertz quasi-periodic oscillations (QPOs) in an XMM–Newton observation of the neutron-star low-mass X-ray binary 4U 1636−53. These QPOs have been interpreted as marginally stable burning on the neutron-star surface. At the beginning of the observation the QPO was at around 8 mHz, together with a possible second harmonic. About 12 ks into the observation a type I X-ray burst occurred and the QPO disappeared; the QPO reappeared ∼25 ks after the burst and it was present until the end of the observation. We divided the observation into four segments to study the evolution of the spectral properties of the source during intervals with and without mHz QPO. We find that the temperature of the neutron-star surface increases from the QPO segment to the non-QPO segment, and vice versa. We also find a strong correlation between the frequency of the mHz QPO and the temperature of a blackbody component in the energy spectrum representing the temperature of neutron-star surface. Our results are consistent with previous results that the frequency of the mHz QPO depends on the variation of the heat flux from the neutron-star crust, and therefore supports the suggestion that the observed QPO frequency drifts could be caused by the cooling of deeper layers.

A ~100 mHz QPO in the X-ray emission from IGR J17361-4441

IGR J17361-4441 was discovered by INTEGRAL undergoing its first detectable X-ray outburst in 2011 and initially classified as an accreting X-ray binary in the globular cluster NGC 6388. A reanalysis of the outburst data collected with INTEGRAL and Swift suggested that the enhanced X-ray emission from IGR J17361-4441 could have been due to a rare tidal disruption event of a terrestrial- icy planet by a white dwarf. In this letter we report on the analysis of XMM-Newton data collected in 2011 during the outburst from IGR J17361-4441. Our analysis revealed the presence of a $\sim$100 mHz quasi-periodic oscillation in the X-ray emission from the source and confirmed the presence of a soft thermal component (kT$\sim$0.08 keV) in its spectrum. We discuss these findings in the context of the different possibilities proposed to explain the nature of IGR J17361-4441.

An X-ray view of the very faint black hole X-ray transient Swift J1357.2–0933 during its 2011 outburst

We report on the X-ray spectral (using XMM–Newton data) and timing behaviour [using XMM–Newton and Rossi X-ray Timing Explorer (RXTE) data] of the very faint X-ray transient and black hole system Swift J1357.2–0933 during its 2011 outburst. The XMM–Newton X-ray spectrum of this source can be adequately fitted with a soft thermal component with a temperature of ∼0.22 keV (using a disc model) and a hard, non-thermal component with a photon index of Γ ∼ 1.6 when using a simple power-law model. In addition, an edge at ∼0.73 keV is needed likely due to interstellar absorption. During the first RXTE observation, we find a 6 mHz quasi-periodic oscillation (QPO) which is not present during any of the later RXTE observations or during the XMM–Newton observation which was taken 3 d after the first RXTE observation. The nature of this QPO is not clear, but it could be related to a similar QPO seen in the black hole system H1743–322 and to the so-called 1 Hz QPO seen in the dipping neutron-star X-ray binaries (although this latter identification is quite speculative). The observed QPO has similar frequencies as the optical dips seen previously in this source during its 2011 outburst, but we cannot conclusively determine that they are due to the same underlying physical mechanism. Besides the QPO, we detect strong band-limited noise in the power-density spectra of the source (as calculated from both the RXTE and the XMM–Newton data) with characteristic frequencies and strengths very similar to other black hole X-ray transients when they are at low X-ray luminosities. We discuss the spectral and timing properties of the source in the context of the proposed very high inclination of this source. We conclude that all the phenomena seen from the source cannot, as yet, be straightforwardly explained neither by an edge-on configuration nor by any other inclination configuration of the orbit.

Quasi-periodic oscillations and energy spectra from the two brightest Ultra-Luminous X-ray sources in M82

Ultra-Luminous X-ray sources are thought to be accreting black holes that might host Intermediate Mass Black Holes (IMBH), proposed to exist by theoretical studies, even though a firm detection (as a class) is still missing. The brightest ULX in M82 (M82 X-1) is probably one of the best candidates to host an IMBH. In this work we analyzed the data of the recent release of observations obtained from M82 X-1 taken by XMM-Newton. We performed a study of the timing and spectral properties of the source. We report on the detection of (46+-2) mHz Quasi-Periodic Oscillations (QPOs) in the power density spectra of two observations. A comparison of the frequency of these high-frequency QPOs with previous detections supports the 1:2:3 frequency distribution as suggested in other studies. We discuss the implications if the (46+-2) mHz QPO detected in M82 X-1 is the fundamental harmonic, in analogy with the High-Frequency QPOs observed in black hole binaries. For one of the observations we have detected for the first time a QPO at 8 mHz (albeit at a low significance), that coincides with a hardening of the spectrum. We suggest that the QPO is a milli-hertz QPO originating from the close-by transient ULX M82 X-2, with analogies to the Low-Frequency QPOs observed in black hole binaries.

Detection of a variable QPO at ∼41 mHz in the Be/X-ray transient pulsar 4U 0115+634

We report the detection of quasi-periodic oscillation (QPO) at ~41 mHz in the transient high-mass Be/X-ray binary pulsar 4U~0115+634 using data from the Rossi X-Ray Timing Explorer (RXTE) observatory. The observations used in the present work were carried out during X-ray outbursts in 1999 March-April, 2004 September-October and 2008 March-April. This frequency of the newly detected QPO was found to vary in 27-46 mHz range. This ~41 mHz QPO was detected in four of the 36 pointed RXTE Proportional Counter Array (PCA) observations during 1999 outburst where as during 2004 and 2008 outbursts, it was detected in four and three times out of 33 and 26 observations, respectively. Though QPOs at ~2 mHz, and ~62 mHz were reported earlier, the ~41 mHz QPO and its first harmonic were detected for the first time in this pulsar. There are three RXTE/PCA observations where multiple QPOs were detected in the power density spectrum of 4U~0115+634. Simultaneous presence of multiple QPOs is rarely seen in accretion powered X-ray pulsars. Spectral analysis of all the pointed RXTE/PCA observations revealed that the 3-30 keV energy spectrum was well described by Negative and Positive Power-law with EXponential cutoff (NPEX) continuum model along with interstellar absorption and cyclotron absorption components. During the three X-ray outbursts, however, no systematic variation in any of the spectral parameters other than the earlier reported anti-correlation between cyclotron absorption energy and luminosity was seen. Presence of any systematic variation of QPO frequency and rms of QPO with source flux were also investigated yielding negative results.

DISCOVERY OF A 7 mHz X-RAY QUASI-PERIODIC OSCILLATION FROM THE MOST MASSIVE STELLAR-MASS BLACK HOLE IC 10 X-1

We report the discovery with XMM-Newton of an approximately 7 mHz X-ray (0.3-10.0 keV) quasi-periodic oscillation (QPO) from the eclipsing, high-inclination black hole binary IC 10 X-1. The QPO is significant at > 4.33 sigma confidence level and has a fractional amplitude (% rms) and a quality factor, Q, of approximately 11 and 4, respectively. The overall X-ray (0.3-10.0 keV) power spectrum in the frequency range 0.0001 - 0.1 Hz can be described by a power-law with an index of -2, and a QPO at 7 mHz. At frequencies > 0.02 Hz there is no evidence for significant variability. The fractional amplitude (rms) of the QPO is roughly energy-independent in the energy range of 0.3-1.5 keV. Above 1.5 keV the low signal to noise ratio of the data does not allow us to detect the QPO. By directly comparing these properties with the wide range of QPOs currently known from accreting black hole and neutron stars, we suggest that the 7 mHz QPO of IC 10 X-1 may be linked to one of the following three categories of QPOs: (1) the "heartbeat" mHz QPOs of the black hole sources GRS 1915+105 and IGR J17091-3624, or (2) the 0.6-2.4 Hz "dipper QPOs" of high-inclination neutron star systems, or (3) the mHz QPOs of Cygnus X-3.