Quasi-periodic Oscillations Frequency Research Articles

An X-ray high-frequency quasi-periodic oscillation in NGC 1365

This study presents the detection of a high-frequency quasi-periodic oscillation (QPO) in the Seyfert galaxy NGC 1365 based on observational data obtained by XMM-Newton in January 2004. Utilizing the weighted wavelet Z-transform (WWZ) and Lomb-Scargle periodogram (LSP) methods, a QPO signal is identified at a frequency of 2.19 × 10−4 Hz (4566 s), with a confidence level of 3.6σ. The signal is notably absent in the lower 0.2–1.0 keV energy band, with the primary contribution emerging from the 2.0–10.0 keV band, where the confidence level reaches 3.9σ. Spectral analysis shows that there are multiple absorption and emission lines in the high-energy band (> 6 keV). The correlation between the QPO frequency (fQPO) and the mass of the central black hole (MBH) of NGC 1365 aligns with the established logarithmic trend observed across black holes, indicating the QPO is of high frequency. This discovery provides new clues for studying the generation mechanism of QPOs in Seyfert galaxies, which helps us understand the accretion process around supermassive black holes and the characteristics of strong gravitational fields in active galactic nuclei.

Accretion tori around rotating neutron stars II. Oscillations and precessions

The four characteristic oscillation frequencies of accretion flows (in addition to the Keplerian orbital frequency) are often discussed in the context of the time variability of black hole and neutron star (NS) low-mass X-ray binaries (LMXBs). These four frequencies are the frequencies of the axisymmetric radial and vertical epicyclic oscillations, and the frequencies of non-axisymmetric oscillations corresponding to the periastron (radial) and Lense-Thirring (vertical) precessions. In this context, we investigated the effect of the quadrupole moment of a slowly rotating NS and provide complete formulae for calculating these oscillation and precession frequencies, as well as convenient approximations. Simple formulae corresponding to the geodesic limit of a slender torus (and test-particle motion) and the limit of a marginally overflowing torus (a torus exhibiting a critical cusp) are presented, and more general approximate formulae are included to allow calculations for arbitrarily thick tori. We provide the Wolfram Mathematica code used for our calculations together with the C++ and PYTHON codes for calculating the frequencies. Our formulae can be used for various calculations regarding the astrophysical signatures of the NS super-dense matter equation of state. For instance, we demonstrate that even for a given fixed number of free parameters, a model that accounts for fluid flow precession matches the frequencies of twin-peak quasiperiodic oscillations observed in NS LMXBs better than a model that uses geodesic precession.

The Bicoherence Analysis of Type-C Quasiperiodic Oscillations in Swift J1727.8−1613

We present the results of bicoherence analysis for Swift J1727.8−1613 during its 2023 outburst, using data from Insight-HXMT. Our analysis focused on observations with quasiperiodic oscillations (QPOs) of frequencies greater than 1 Hz, revealing that all of them belong to type-C QPOs. We found a strong correlation between the QPO frequency and the hardness ratio, as well as a linear relationship between the QPO rms and the hardness ratio. The bicoherence analysis revealed a transition from a “web” pattern to a “hypotenuse” pattern in the low-energy (LE) and high-energy (HE) bands. In the bicoherence patterns, there are correlations between horizontal and vertical bicoherence at f 1 = f 2 = f QPO with count rates. The diagonal structure at f 1 + f 2 = f QPO becomes more prominent with increasing energy. Additionally, we discovered a new bicoherence pattern in the medium energy band from 10 to 20 keV; the diagonal structure at f 1 + f 2 = f har is prominent only in this energy band, which we refer to as the “parallel” pattern. The bicoherence analysis indicates that the source is likely a low-inclination source.

Circular motion and QPOs near black holes in Kalb–Ramond gravity

General relativity (GR) theory modifications include different scalar, vector, and tensor fields with non-minimal gravitational coupling. Kalb–Ramond (KR) gravity is a modified theory formulated based on the presence of the bosonic field. One astrophysical way to test gravity is by studying the motion of test particles in the spacetime of black holes (BHs) using observational data. In the present work, we aimed to test KR gravity through theoretical studies of epicyclic frequencies of particle oscillations using quasi-periodic oscillation (QPO) frequency data from microquasars. First, we derive equations of motion and analyze the effective potential for circular orbits. Also, we studied the energy and angular momentum of particles corresponding to circular orbits. In addition, we analyze the stability of circular orbits. It is shown that the radius of the innermost stable circular orbits is inversely proportional to the KR parameter. We are also interested in how the energy and angular momentum of test particles at ISCO behave around the KR BHs. We found that the Keplerian frequency for the test particles in KR gravity is the same as that in GR. Finally, we study the QPOs by applying epicyclic oscillations in the relativistic precession (RP), warped disc (WD), and epicyclic resonance (ER) models. We also analyze QPO orbits in the resonance cases of upper and lower frequencies 3:2, 4:3, and 5:4 in the QPO as mentioned above models. We obtain constraints on the KR gravity parameter and BH mass using a Monte Carlo Markov Chain simulation in the multidimensional parameter space for the microquasars GRO J1655-40 & XTE J1550-564, M82 X-1, and Sgr A*.

Evolution of QPOs in GX 339–4 and EXO 1846–031 with Insight-HXMT and NICER

We conduct a spectral and timing analysis of GX 339−4 and EXO 1846−031 with the aim of studying the evolution of type-C quasiperiodic oscillations (QPOs) with spectral parameters. The high-cadence data from Insight-HXMT and NICER allow us to track the evolution of QPOs and spectra simultaneously. Type-C QPOs appear at the end of the low–hard state and/or the hard–intermediate state. Our results reveal that the QPO frequency is closely related to the inner disk radius and mass accretion rate in the two sources. This correlation aligns well with the dynamic frequency model of a truncated disk.

Accretion flow dynamics and characteristics of MAXI J153–571-spectral analysis using combination of XSPEC and TCAF models

MAXI J153–571 accretion flow consists of Keplerian (optically thick) and sub-Keplerian (optically thin) flow, and their mass accretion rates seem to regulate other accretion flow characteristics. Hard X-rays are produced when the Keplerian seed soft photons are thermally or inverse comptonized in the Compton cloud/post-shock region by hot electrons. The variations/fluctuations of components of the accretion flow during the hard states create propagating Quasi-periodic oscillation (QPO) when their timescales are roughly matched and resonance phenomena occur. The QPO and its frequency are timing properties and the accretion flow spectra-temporal characteristics can be determined via spectral analysis. In this study, we looked into the accretion flow characteristics of MAXI J153–571 during the hard-intermediate state. Spectral analysis of MAXI J153–571 observed by MAXI/GSC, Swift/BAT, and NuSTAR on the same or close-in epochs was carried out. XSPEC and TCAF models were used in fitting/modeling the data. A robust and statistically acceptable fit spectra with a reduced Chi-squared value of ∼ 0.84 – 1.20 and best-fit photon index of 2.0–2.29 was obtained. The track of the accretion flow characteristics was obtained using models’–fitted parameters and MATLAB written codes of physical equations. Some accretion flow characteristics are positively correlated while others are anti-correlated at different phases and their correlation are statistically significant. The correlation of accretion flow characteristics with one another suggests that saturation effects, variation/fluctuations in the accretion flow, and intermittent/flickering behavior of MAXI J153–571 are tied to the variations/fluctuations of the intrinsic properties; mass accretion rates. Moreover, a resonance condition of 0.70 to 0.83 indicates that the cooling and infall timescales are roughly matched and affirms the presence of QPO in the accretion flow. This suggests that the origin of the photon index–QPO frequency (Γ–vQPO) relation is strongly linked to the variation/fluctuations in mass accretion flow rates. Hence, the accretion flow is dynamic, and independent variations/fluctuations of mass accretion rates could regulate the variation/fluctuations of other accretion flow parameters and perhaps, spectral evolution.

Hard X-rays and QPO in Swift J1727.8−1613: the rise and plateau of the 2023 outburst

ABSTRACT We report on the detection of type-C quasi-periodic oscillations during the initial stages of the outburst of Swift J1727.8–1613 in 2023. Using data of the INTEGRAL observatory along with the data of the Mikhail PavlinskyART-XC telescope on board Spektr-RG and X-ray Telescope (XRT) of the Neil Gehrels Swift observatory the fast growth of the quasi-periodic oscillations (QPO) frequency was traced. We present a hard X-ray light curve that covers the initial stages of the 2023 outburst – the fast rise and plateau – and demonstrate that the QPO frequency was stable during the plateau. The switching from type-C to type-B QPO was detected with the beginning of the source flaring activity. We have constructed a broad-band spectrum of Swift J1727.8−1613 and found an additional hard cutoff power-law spectral component extending at least up to 250 keV. Finally, we have obtained an upper limit on the hard X-ray flux at the beginning of the optical outburst and estimated the delay of the hard X-ray outburst with respect to the optical one.

Timing and Spectral Analysis of the Black Hole X-Ray Binary MAXI J1803-298 with Insight-HXMT Data

We present a comprehensive analysis of the 2021 outburst of MAXI J1803–298 utilizing observations of the Insight-Hard X-ray Modulation Telescope (Insight-HXMT) spanning from the low hard state to the high soft state. Within the Insight-HXMT data set, compared to the previous work, we identify a more prolonged presence of type-C quasi-periodic oscillations (QPOs) with centroid frequencies ranging from ∼0.16 to 6.3 Hz, which present correlations with the hardness ratio and the photon index of the Comptonized component. For QPO frequencies less than ∼2 Hz, the QPO phase lags are hard (photons of 10–19 keV arrive later than those of 1–4 keV), while at higher frequencies, the lags become soft at and above ∼4 Hz. Furthermore, the spectra in all Insight-HXMT observations consist of a multi-color blackbody component and a Comptonized component, as commonly observed in classical black hole X-ray binaries. We analyze state transitions and the evolution of accretion geometry in this work. The fitted inner disk radius increases abnormally during the low hard state, hypothesized to result from the corona condensing onto the inner disk. Additionally, two significant drops in flux are observed during the soft intermediate state, maybe implying changes in the corona/jet and the disk, respectively.

Energy Dependence of the Low-frequency Quasiperiodic Oscillations in Swift J1727.8–1613

Based on observations from the Insight Hard X-ray Modulation Telescope (Insight-HXMT), an analysis of type C quasiperiodic oscillations (QPOs) observed during the outburst of the new black hole candidate Swift J1727.8–1613 in 2023 was conducted. This analysis scrutinized the QPO’s evolution throughout the outburst, particularly noting its rapid frequency escalation during two flare events. Utilizing the energy range covered by Insight-HXMT, a dependency of the QPO frequency on energy was observed. Below approximately 3 Hz, minimal variations in frequency with energy were noted, whereas clear variations with photon energy were observed when it exceeded approximately 3 Hz. Additionally, a sharp drop in the rate of change was observed when the frequency exceeded approximately 8 Hz. This behavior, similar to several previously reported sources, suggests the presence of a common underlying physical mechanism. Moreover, the QPO rms–frequency relationship can be explained by the Lense–Thirring precession model. The relationship between rms energy and phase lag with frequency suggests the black hole system as a high-inclination source.

A Possible X-ray Quasi-periodic Oscillation in the Narrow-line Seyfert 1 Galaxy Mrk 142

A possible quasi-periodic oscillation (QPO) at frequency 7.045 × 10−5 Hz is found in the narrow-line Seyfert 1 galaxy Mrk 142 in the data of XMM-Newton collected on 2020 April 11. We find that the QPO signal is statistically significantly larger than the 5σ level and highly coherent with quality factor Q > 5 at the 0.3–10 keV band by using the method of the Lomb–Scargle Periodogram, the Weighted Wavelet Z-transform and the REDFIT. We analyze the data in 0.3–0.6 keV, 0.6–1 keV, 1–3 keV and 3–10 keV energy bands, and find obvious QPO signals at 0.3–0.6 keV and 1–3 keV bands. We then analyze the time-average spectra and time variability at the QPO frequency of 7.045 × 10−5 Hz, and use a model to fit them. We find that the QPO signal mainly comes from the X-ray hot corona.

Discovery of evolving low-frequency QPOs in hard X-rays (∼100 keV) observed in black hole Swift J1727.8−1613 with AstroSat

ABSTRACT We report the first detection of evolving low-frequency quasi-periodic oscillation (LFQPO) frequencies in hard X-rays upto 100 keV with AstroSat/LAXPC during ‘unusual’ outburst phase of Swift J1727.8−1613 in hard intermediate state (HIMS). The observed LFQPO in 20–100 keV has a centroid $\nu _{_{\rm QPO}}=1.43$ Hz, a coherence factor Q = 7.14 and an amplitude ${\rm rms_{_{\rm QPO}}} = 10.95{{\ \rm per\ cent}}$ with significance σ = 5.46. Type-C QPOs (1.09–2.6 Hz) are found to evolve monotonically during HIMS of the outburst with clear detection in hard X-rays (80−100 keV), where ${\rm rms_{_{\rm QPO}}}$ decreases ($\sim 12\!-\!3{{\ \rm per\ cent}}$) with energy. Further, $\nu _{_{\rm QPO}}$ is seen to correlate (anticorrelate) with low- (high-) energy flux in 2–20 keV (15–50 keV). Wide-band (0.7−40 keV) energy spectrum of NICER/XTI and AstroSat/LAXPC is satisfactorily described by the ‘dominant’ thermal Comptonization contribution (∼88 per cent) in presence of a ‘weak’ signature of disc emissions (kTin ∼ 0.36 keV) indicating the harder spectral distribution. Considering source mass $M_{\rm BH}=10\, \mathrm{M}_\odot$ and distance 1.5 < d (kpc) < 5, the unabsorbed bolometric luminosity is estimated as $\sim 0.03\!-\!0.92{{\ \rm per\ cent}}\, L_{\rm Edd}$. Finally, we discuss the implications of our findings in the context of accretion dynamics around black hole X-ray binaries.

The First Polarimetric View on Quasiperiodic Oscillations in a Black Hole X-Ray Binary

We present the first polarimetric analysis of quasiperiodic oscillations (QPOs) in a black hole binary utilizing IXPE data. Our study focuses on Swift J1727.8–1613, which experienced a massive outburst that was observed by various telescopes across different wavelengths. The IXPE observation we studied was conducted during the hard-intermediate state. The polarization degree (PD) and polarization angle (PA) were measured at 4.28% ± 0.20% and 1.°9 ± 1.°4, respectively. Remarkably, significant QPO signals were detected during this observation, with a QPO frequency of approximately 1.34 Hz and a fractional rms amplitude of about 12.3%. Furthermore, we conducted a phase-resolved analysis of the QPO using the Hilbert–Huang transform technique. The photon index showed a strong modulation with respect to the QPO phase. In contrast, the PD and PA exhibit no modulations in relation to the QPO phase, which is inconsistent with the expectation of the Lense–Thirring precession of the inner flow. Further theoretical studies are needed to conform with the observational results.

First Observational Evidence for an Interconnected Evolution between Time Lag and QPO Frequency among AGNs

Quasiperiodic oscillations (QPOs) have been widely observed in black hole X-ray binaries (BHBs), which often exhibit significant X-ray variations. Extensive research has explored the long-term evolution of the properties of QPOs in BHBs. In contrast, such evolution in active galactic nuclei (AGNs) has remained largely unexplored due to limited observational data. By using the 10 new XMM-Newton observations for the narrow-line Seyfert 1 galaxy RE J1034+396 from publicly available data, we analyze the characteristics of its X-ray QPOs and examine their long-term evolution. The hard-band (1–4 keV) QPOs are found in all 10 observations and the frequency of these QPOs evolves ranging at (2.47–2.83) × 10−4 Hz. Furthermore, QPO signals in the soft (0.3–1 keV) and hard bands exhibit strong coherence, although, at times, the variations in the soft band lead those in the hard band (the hard-lag mode), while at other times, it is the reverse (the soft-lag mode). The observations presented here serendipitously captured two ongoing lag reversals between these two modes within about two weeks, which are first seen in RE J1034+396 and also among all AGNs. A transition in QPO frequency also takes place within a two-week timeframe, two weeks prior to its corresponding lag reversal, indicating a possible coherence between the transitions of QPO frequency and lag mode with delay. The diagram of time lag versus QPO frequency clearly evidences this interconnected evolution with hysteresis, which is, for the first time, observed among AGNs.

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.

Disc corona radii and QPO frequencies in black hole binaries: testing Lense– Thirring precession origin

ABSTRACT Stellar-mass black hole binary systems in the luminous X-ray states show a strong quasi-periodic oscillation (QPO) in their Comptonized emission. The frequency of this feature correlates with the ratio of a disc to Comptonized emission rather than with total luminosity. Hence, it changes dramatically during spectral transitions between the hard and soft states. Its amplitude is also strongest in these intermediate states, making them an important test of QPO models. However, these have complex spectra which generally require a disc and two separate Comptonization components, making it difficult to uniquely derive the spectral parameters. We build a new energy-conserving model of the accretion flow, SSsed model, which assumes a fixed radial emissivity but with a changing emission mechanism. This is similar to the agnsed model in xspec but tuned to be more suitable for stellar mass black holes. It uses a combination of the disc luminosity and temperature to constrain the inner radius of the (colour temperature corrected) blackbody disc, separating this from the more complex Comptonization spectra emitted inwards of this radius. We show a pilot study of this model fit to hundreds of RXTE spectra of the black hole binary XTE J1550 − 564. We show that the derived disc radius tightly anticorrelates with the central frequencies of the low-frequency QPO detected in the same observations. The relation is consistent with the quantitative predictions of Lense–Thirring precession of the entire inner Comptonization regions for the assumed system parameters. This supports the scenario that low-frequency QPOs are caused by Lense–Thirring precession.

Evidence for a dynamic corona in the short-term time lags of black hole X-ray binary MAXI J1820+070

ABSTRACT In X-ray observations of hard state black hole X-ray binaries (BHXRBs), rapid variations in accretion disc and coronal power-law emission are correlated and show Fourier-frequency-dependent time lags. On short ($\sim$0.1 s) time-scales, these lags are thought to be due to reverberation and therefore may depend strongly on the geometry of the corona. Low-frequency quasi-periodic oscillations (QPOs) are variations in X-ray flux that have been suggested to arise because of geometric changes in the corona, possibly due to general relativistic Lense–Thirring precession. Therefore, one might expect the short-term time lags to vary on the QPO time-scale. We performed novel spectral-timing analyses on Neutron Star Interior Composition ExploreR observations of the BHXRB MAXI J1820+070 during the hard state of its outburst in 2018 to investigate how the short-term time lags between a disc-dominated and a coronal power-law-dominated energy band vary on different time-scales. Our method can distinguish between variability due to the QPO and broad-band noise, and we find a linear correlation between the power-law flux and lag amplitude that is strongest at the QPO frequency. We also introduce a new method to resolve the QPO signal and determine the QPO phase dependence of the flux and lag variations, finding that both are very similar. Our results are consistent with a geometric origin of QPOs, but also provide evidence for a dynamic corona with a geometry varying in a similar way over a broad range of time-scales, not just the QPO time-scale.

A spectral study of GRS 1915+105 during its March 2017 NuSTAR observations

ABSTRACT The Nuclear Spectroscopic Telescope Array Mission (NuSTAR) observed the enigmatic microquasar GRS 1915+105 twice, on 2017 March 26 and 28. Using these NuSTAR observations, we performed a time-resolved spectral analysis. We detected a type C quasi-periodic oscillation (QPO) along with the harmonic and the sub-harmonic in both epochs. From our spectral modelling, we found a dual reflection component, where one of the components comes from the hot inner flow and the second component originates from the exterior region of the accretion disc. Additionally, we found strong, highly ionized accretion disc winds as indicated by Fe xxv (on 2017 March 26) and Fe xxvi (on 2017 March 28) absorption lines, likely ultrafast outflows. Finally, at the end of the second observation, we detected variability class, which marked the onset of heartbeat oscillations in the X-ray light curve. We perform time-resolved spectral analysis to comprehend the physical conditions responsible for the onset of the heartbeat. We found that the accretion disc became highly ionized (log ξ > 4), and the high-energy cut-off showed some signs of increase, approaching >100 keV before the onset of the heartbeat class. We have not detected any signature of accretion disc instabilities thought to cause the heartbeat oscillations. We also found a strong positive correlation between the QPO frequency and the power-law index.

Unveiling hidden variability components in accreting X-ray binaries using both the Fourier power and cross-spectra

ABSTRACT We present a novel method for measuring the lags of (weak) variability components in neutron-star and black-hole low-mass X-ray binaries (LMXBs). For this we assume that the power and cross-spectra of these sources consists of a number of components that are coherent in different energy bands, but are incoherent with one another. The technique is based on fitting simultaneously the power spectrum (PS) and the Real and Imaginary parts of the cross-spectrum (CS) with a combination of Lorentzian functions. We show that, because the PS of LMXBs is insensitive to signals with a large Imaginary part and a small Real part in the CS, this approach allows us to uncover new variability components that are only detected in the CS. We also demonstrate that, contrary to earlier claims, the frequency of the type-C quasi-periodic oscillation (QPO) in the black-hole binary GRS 1915+105 does not depend on energy. Rather, the apparent energy dependence of the QPO frequency can be explained by the presence of a separate QPO component with a slightly higher frequency than that of the QPO, whose rms amplitude increases faster with energy than the rms amplitude of the QPO. From all the above we conclude that, as in the case of the PS, the CS of black-hole and neutron-star binaries can be fitted by a combination of Lorentzian components. Our findings provide evidence that the frequency-dependent part of the transfer function of these systems can be described by a combination of responses, each of them acting over relatively well-defined time-scales. This conclusion challenges models that assume that the main contribution to the lags comes from a global, broadband, transfer function of the accreting system.

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.

Fast-varying time lags in the quasi-periodic oscillation in GRS 1915 + 105

ABSTRACT The properties of subsecond time variability of the X-ray emission of the black hole binary GRS 1915 + 105 are very complex and strictly connected to its patterns of variability observed on long time-scales. A key aspect for determining the geometry of the accretion flow is the study of time lags between emission at different energies, as they are associated to key time-scales of the system. In particular, it is important to examine the lags associated to the strong low-frequency quasi-periodic oscillations (QPOs), as the QPOs provide unambiguous special frequencies to sample the variability. We have analysed data from an observation with the AstroSat satellite, in which the frequency of the low-frequency QPO varies smoothly between 2.5 and 6.6 Hz on a time-scale of ∼10 h. The derived phase lags show the same properties and evolution of those observed on time-scales of a few hundred days, indicating that changes in the system geometry can take place on times below one day. We fit selected energy spectra of the source and rms and phase-lag spectra of the QPO with a time-variable Comptonization model, as done previously to RossiXTE data of the same source, and find that indeed the derived parameters match those obtained for variations on much longer time-scales.