arXiv subjects

Mariano Méndez

Publications and source records attributed to Mariano Méndez.

At least 19 recordsLinked to original sources

arXiv2026

Compelling evidence of a link between the lags of the quasi-periodic oscillations and the radio jet in the black-hole X-ray binary GRS 1915+105

GRS 1915+105 is one of the most studied black-hole X-ray binaries, known for its extreme variability and rich phenomenology. Previous studies of this source with the Rossi X-ray Timing Explorer reported a transition of the phase-lags of type-C quasi-periodic oscillations (QPOs) from soft, where low-energy photons lag the high-energy ones, when the QPO frequency is higher than ${\sim} 2 ~{\rm Hz}$, to hard when the QPO frequency decreases below ${\sim} 2 ~{\rm Hz}$. The hard-lags of the QPO coincide with episodes of strong radio emission. We analyse NICER observations of GRS 1915+105 obtained between 2018 and 2020, during a period in which the source flux decreased steadily, and perform a detailed spectral-timing study of the detected type-C QPOs. We find a type-C QPO with frequencies in the range of ${\sim} 1.3$-$3.9 ~{\rm Hz}$, which displays soft lags and, contrary to the RXTE observations, shows no evidence of hard phase lags at frequencies below $2~{\rm Hz}$. Quasi-simultaneous AMI-LA radio observations show consistently low radio flux (${\lesssim} 5 ~{\rm mJy}$) during this period. These results appear to show that the hard QPO lags in GRS 1915+105 are linked to the presence of strong radio activity, suggesting that the relativistic jet is responsible for the hard phase lags, supporting a scenario in which QPO phase lags trace changes in coronal geometry and accretion-ejection coupling in GRS 1915+105.

astro-ph.HE↗

arXiv2026

Cross-spectral Analysis of the Type-C Quasi-periodic Oscillation Shoulder Component in GX 339-4

We revisit Rossi X-ray Timing Explorer (RXTE) observations of GX~339$-$4 during the rising phase of its 2006/2007 outburst and apply a joint power-density-spectrum (PDS)--cross-spectrum (CS) decomposition to the type-C quasi-periodic oscillation (QPO) region. Within this framework, the QPO region is described by a narrow QPO fundamental and a neighboring high-frequency shoulder, whose amplitudes and phase lags can be measured separately. The shoulder is first detected at MJD~54142.04, mainly through the imaginary part of the CS and a narrow local structure in the phase-lag spectrum, before becoming a resolved high-frequency shoulder in the PDS. It follows the QPO frequency evolution on the high-frequency side, with $R_ν=ν_{\rm sh}/ν_{\rm QPO}\simeq1.04$--$1.18$. The QPO lag remains small, typically below $\sim0.17$~rad, whereas the shoulder carries a larger hard lag of $\sim0.5$--$0.8$~rad. Energy-resolved fits show the same separation: the QPO lag is close to zero or only weakly positive across most of the energy band, while the shoulder lag is systematically larger and generally increases with photon energy. The two components have broadly similar rms--energy shapes, although their relative strengths evolve during the observed sequence. Although the shoulder remains broad, with $Q\sim2$--$4$, its lag and rms--energy behavior resemble those of the type-B QPO detected shortly after our observations. This similarity raises the interesting possibility that the shoulder is related to an earlier, broader stage of the variability later seen as the type-B QPO.

astro-ph.HE↗

arXiv2026

Timing and spectral analysis of the 2025 outburst of 4U 1630$-$47 with \textit{NICER}

We analyzed \textit{NICER} observations of the 2025 outburst of the black hole X-ray binary 4U~1630$-$47 to investigate the spectral--timing properties of its transient low-frequency quasi-periodic oscillations (QPOs) and millihertz-scale quasi-regular modulation (QRM). During the rising phase of the outburst, the QPO centroid frequency increased from $\sim 0.24$ Hz to $\sim 3.43$ Hz. Wavelet-based state separation shows that the with-QPO intervals are associated with a higher inner disk temperature and a lower \texttt{diskbb} normalization than the without-QPO intervals, while the photon index ($Γ$) shows weaker changes within the uncertainties. Near the outburst peak, the source displayed a weak QRM at $\sim 0.07$ Hz with a fractional rms amplitude of $\sim 4.7\%$, lower than that of the heartbeat state observed in 2023. Phase-resolved Hilbert--Huang analysis shows that the inner disk temperature is positively correlated with the X-ray flux, the \texttt{diskbb} normalization is anticorrelated, and $Γ$ varies only weakly. Overall, the short-timescale spectral--timing variability is expressed most clearly through the disk-related parameters. The transient QPOs are therefore consistent with short-timescale disk-related variability during the rising phase, whereas the millihertz-scale QRM may represent a weaker heartbeat-like variability mode appearing near the outburst peak.

astro-ph.HE↗

arXiv2026

An atypical X-ray variability component in the black hole candidate AT2019wey

Recent studies have revealed a notable timing feature in several black hole X-ray binaries (BHXBs) during the soft-to-hard transition at the outburst decay. Within a narrow frequency range, the phase lags between high- and low-energy X-ray light curves exhibit a sudden increase, accompanied by a drop in the coherence function. These narrow features have been associated with a quasi-periodic oscillation (QPO) appearing only in the imaginary part of the cross spectrum (CS). This QPO remains undetected in the power density spectrum (PDS) and is known as imaginary QPO. Motivated by these results, we analyse five years of NICER observations of the BHXB AT2019wey during its low-hard state (LHS) and hard-intermediate state (HIMS). We find an imaginary QPO in the CS of AT2019wey, with similar characteristics as those found in other BHXBs, making AT2019wey the fifth BHXB in which such QPOs have been found. As the source hardens, the frequency of the imaginary QPO drops from $\sim$ 5 Hz to $\sim$ 1 Hz, while its phase lag rises from $\sim$ 0.3 rad to $\sim$ 0.7 rad during the HIMS and from $\sim$ 0.5 rad to $\sim$ 0.6 rad during the LHS. During the HIMS, the phase-lag energy spectrum of the imaginary QPO shows a typical U-shaped profile, while the shape changes in the LHS. The rms spectrum of the imaginary QPO rises below $\sim$ 2 keV, peaks at around $\sim$ 2 keV and decreases at higher energies, which may be associated with the presence of a relatively cool corona. We compare the properties of the imaginary QPO with those of the type-B and C QPOs in BHXBs and find a tentative connection to type-C QPOs. Combining the imaginary QPOs detected in AT2019wey with those reported in other sources, we find a systematic increase of QPO phase lags with QPO frequency. However, we cannot conclude whether the phase lags of imaginary QPOs exhibit the inclination dependence previously observed in type-C QPOs.

astro-ph.HE↗

arXiv2026

Discovery of a quasi-periodic oscillation non-harmonically related to the Type-C QPO in the hard intermedidate state of MAXI J1820+070

We present a detailed timing analysis of the transition from the hard-intermediate state (HIMS) to the soft-intermediate state (SIMS) in MAXI J1820+070 using NICER observations. This transition is marked by a sharp drop of the broadband noise across both the soft and hard X-ray bands, the disappearance of the Type-C quasi-periodic oscillation (QPO), the quenching of the steady, optically thick, compact jet, the appearance of a Type-B QPO, and the detection of discrete, optically thin, radio ejections. For the first time, we detect a QPO at 3.5-5.9 Hz in the 2-12 keV power density spectrum of MAXI J1820+070 roughly half a day before the transition, which appears to evolve smoothly into the Type-B QPO observed immediately after the transition. The location of this additional QPO component in the broadband rms vs. QPO frequency plot is consistent with that of the Type-B QPOs in GX 339-4 and GRO J1655-40, suggesting a possible connection between this additional QPO in the HIMS and the Type-B QPO in the SIMS. This result, together with recent findings in Swift J1727.8-1613, suggests that QPOs with these characteristics can emerge prior to the HIMS-to-SIMS transition and are not confined exclusively to the SIMS. If this additional QPO feature is the precursor of the Type-B QPO in the SIMS, its presence before the transition, whereas the bright discrete, optically thin, radio ejections appear at the transition, would imply that there may be no direct physical connection between the Type-B QPO and the discrete radio ejections. Our results further suggest a link between the disappearance of the Type-C QPO, the drop of the broadband noise, and the emergence of discrete radio ejections at the HIMS-to-SIMS transition. We speculate that the simultaneous presence of such a QPO, non-harmonically related to the Type-C QPO in the HIMS, could be compatible with a spine-sheath outflow structure.

astro-ph.HE↗

arXiv2026

Unveiling hidden millihertz quasi-periodic oscillations in 1A 0535+262

Be/X-ray binary pulsars show transient outbursts and complex timing behaviour, including millihertz quasi-periodic oscillations (QPOs), whose physical origin and energy dependence remain poorly understood. We aim to characterise the temporal evolution and energy-dependent properties of the mHz QPO during the 2020 giant outburst of 1A 0535+262. We use the multi-Lorentzian fitting framework to jointly model the power spectra and the real and imaginary parts of the cross-spectrum, incorporating simultaneous broadband X-ray observations from NICER and Insight-HXMT (0.2-120 keV). We report the first detection of weak, but significant, mHz QPOs at low X-ray energies (below 27 keV), extending their detection to a new energy regime. The centroid frequency evolves from 41 to 93 mHz, with the peak root-mean-square (rms) amplitude detected in the 50-65 keV. Throughout the outburst, the QPOs generally exhibit a hard lag between 0.12 pi rad and 0.9 pi rad. However, at the outburst peak, the higher-energy bands (above 35 keV) display a soft lag of up to -0.93 pi rad. We propose that interactions between soft seed photons and an extended outflow located outside the magnetosphere can account for the observed hard lags. Furthermore, we detect a double-peaked mHz QPO only at high energies (E above 35 keV) near peak luminosity. The two peaks maintain an approximately constant separation of 2*nu_spin and exhibit anti-correlated phase evolution. Our results indicate that the mHz QPOs in 1A 0535+262 are closely linked to the coupled evolution of a soft-photon source and a Comptonizing outflow or corona. The joint cross-spectral framework provides a complementary probe of mHz QPOs beyond traditional power-spectral analyses.

astro-ph.HE↗

arXiv2026

A NICER view of the corona through time-dependent Comptonization of the quasi-periodic oscillations in nine black-hole X-ray binaries

We present a systematic study of the evolution of the corona geometry in nine black hole X-ray binaries (BHXRBs) using archival data from NICER. We identify 171 observations exhibiting quasi-periodic oscillations (QPOs) across various spectral states and model the time-averaged energy spectra of the source, as well as the energy-dependent rms and phase-lag spectra of the QPO, with the time-dependent Comptonization model vKompthdk. This allows us to simultaneously constrain the corona size and feedback fraction during outbursts. By using the power color hue diagnostics, we identify different spectral states, and observe that the QPO frequency increases from $\sim$0.1 Hz to $\sim$10 Hz in the low-hard and hard-intermediate states (LHS and HIMS), and remains approximately constant at 4--5~Hz in the soft-intermediate state (SIMS). The corona size shows significant evolution: the corona is large ($\sim10^4$--$10^5$ km) in the LHS, contracts rapidly to $\sim10^3$ km in the HIMS, and exhibits a flare-like expansion near the HIMS-to-SIMS transition. In the SIMS and high-soft state (HSS), the corona becomes compact and stable (4000--8000~km). The feedback fraction of the corona photons increases during the periods in which the corona contracts and decreases during the periods in which the corona expands, indicating a change of the disk-corona coupling. Our results are consistent with previous QPO-based studies using vKompthdk on some individual sources. This work, however, provides the first view of the coronal evolution across outbursts for a diverse BHXRB sample, offering critical insights into coronal behavior as a function of the spectral state of the source.

astro-ph.HE↗

arXiv2026

Exotic optical variability in the black hole X-ray binary IGR J17091-3624

IGR J17091-3624 is a distinctive black hole X-ray binary exhibiting exotic variability, including complex "heartbeat" oscillations in its X-ray light curves, similar to those observed in GRS 1915+105, a system renowned for its structured, rapid X-ray variability but heavily obscured at optical wavelengths. In contrast, IGR J17091-3624 is less obscured, making it a more accessible target for optical investigations. Due to its weak radio emission, optical and infrared data are essential to probe the jet and outer disc behavior of IGR J17091-3624. This study presents the first long-term optical monitoring of IGR J17091-3624, using data from the Las Cumbres Observatory (LCO) over its 2011, 2016, and 2022 outbursts. We combine these observations with quasi-simultaneous X-ray data from Swift/XRT, RXTE, and NICER, employing light curve and variability analysis, spectral energy distributions, color-magnitude diagrams, and optical/X-ray correlations to investigate optical emission mechanisms. We find that the optical and X-ray fluxes are significantly correlated, following a power-law relation with the index 0.40\pm0.04, suggesting that the optical emission in IGR J17091-3624 is dominated by an X-ray-irradiated accretion disk. Based on optical spectral slope constraints, we estimate the extinction toward IGR J17091-3624 A_V = 4.3 to 6.6 mag. The global optical/X-ray correlation suggests a distance estimate of 8-17 kpc, in line with previous findings. High-cadence optical observations show tentative evidence of optical oscillations that may arise from reprocessed X-ray modulations, although confirming this will require higher time-resolution optical data.

astro-ph.HE↗

arXiv2025

Black-hole X-ray binary Swift J1727.8$-$1613 shows simultaneous Type-B and Type-C quasi-periodic oscillations across the hard-intermediate and soft-intermediate states

We present a timing analysis of \textit{Insight}-HXMT observations of the black-hole X-ray binary Swift J1727.8$-$1613 across a bright soft X-ray flare on 2023 September 19 (MJD 60206). At the peak of the flare, the source undergoes a brief transition from the hard-intermediate state (HIMS) into the soft-intermediate state (SIMS), marked by the simultaneous appearance of three discrete radio jet ejections, a drop in broadband noise in the 2$-$10 keV band, and the presence of a narrow quasi-periodic oscillation (QPO) with a characteristic ``U''-shaped phase-lag spectrum and a quality factor of $Q \geq 6$, features that robustly identify it as a Type-B QPO. The Type-C QPO, which was clearly detected in the HIMS prior to the flare, is not observed at the flare's peak and only reappears afterward. Most notably, we find that the Type-B QPO is not restricted to the SIMS: it is present throughout all our observations, including those taken in the HIMS, where it appears as a broad shoulder of the Type-C QPO. During the flare, the Type-B and Type-C QPOs exhibit distinct evolutionary trends in frequency, fractional rms amplitude, and phase lag. These results challenge the traditional view that Type-B QPOs are exclusive to the SIMS, a state that is, in fact, defined by their appearance in the power spectrum, and directly linked to discrete jet ejections. Instead, our findings suggest that the physical conditions giving rise to Type-B QPOs occur more broadly within the inner accretion flow.

astro-ph.HE↗

arXiv2025

Detection of a Type-C QPO during the soft-to-hard transition in Swift J1727.8-1613

Timing analysis of accreting systems is key to probe the structure and dynamics around compact objects. In Black-Hole Low-Mass X-ray Binaries (BH LMXBs), the compact object accretes matter from a low-mass companion star via Roche Lobe overflow, forming an accretion disk, and occasionally exhibiting bright eruptions. The BH LMXB Swift J1727.8-1613 (hereafter J1727), recently underwent one of the brightest outbursts ever recorded in X-rays, in August 2023. This analysis aims to study the timing properties of J1727, in the decaying phase of its outburst, using high-time resolution XMM-Newton data. We analyzed J1727's power spectrum (PS) and cross spectrum (CS), which we modeled with Lorentzians. The PS reveals how the source's power is distributed across frequencies, and the Real and Imaginary parts of the CS compare the displacement of the light curves in different energy bands across the observations. Finally, we simultaneously derived the phase lags and the coherence, using a constant phase lag model. While the first (soft-state) observation does not show any strong variability, the two harder observations exhibit quasi-periodic oscillations (QPOs). Because the QPO is more significantly detected in the Imaginary part of the CS than in the PS, we refer to it as the 'Imaginary QPO'. The QPO is more prominent in the soft 0.3-2 keV band than in the hard 2-12 keV band. As the source evolves towards the hard state, the Imaginary QPO shifts to lower frequencies, the broadband fractional rms amplitude in the 0.3-2 keV energy band increases, while the rms covariance of the Imaginary QPO decreases. Simultaneously, the phase lags increase and the coherence function drops at the Imaginary QPO frequency. In the elusive soft-to-hard transition of J1727, the first XMM-Newton observations of the source reveal an Imaginary QPO also detected in the PS, exhibiting the properties of a type-C QPO.

astro-ph.HE↗

arXiv2025

Evidence for enhanced mass transfer in the disc preceding the transition to the soft state in MAXI J1820+070

We investigate the 2018-2019 main outburst and the subsequent mini-outbursts of the black hole low-mass X-ray binary MAXI J1820+070 using optical/ultraviolet data from the Las Cumbres Observatory (LCO), the American Association of Variable Star Observers (AAVSO), and $\textit{Swift}$/UVOT, as well as X-ray data from $\textit{Insight}$-HXMT and $\textit{Swift}$/XRT. Given the high-cadence observations, we identify a broad dip-like feature in both the optical and X-ray light curves preceding the transition to the soft state, with the X-ray dip lagging the optical dip by approximately 10 days. We propose that the dip is caused by a brief decrease followed by an increase in the mass accretion rate as it propagates through the disc, ultimately triggering the transition to the soft state. This might be a potential tool to predict impending hard-to-soft state transitions, although such a dip has not yet been observed in many sources. Additionally, we find that optical colour ($g^{\prime}-i^{\prime}$) becomes bluer and less variable before the transition to the intermediate state, preceding a dramatic change in the hardness ratio. This appears to be an unusual case, differing from the typical scenario where the optical colour changes usually along with the transition to the soft state. Finally, we explore the implications of the complex evolution of optical/X-ray correlation during both main outbursts and mini-outbursts. In particular, we find a loop-like evolutionary track before the transition to the soft state, which is linked to the optical/X-ray dips in the light curves.

astro-ph.HE↗

arXiv2025

Fast Transitions of X-ray Variability in the Neutron Star Low Mass X-ray Binary Cygnus X-2

We present a spectral-timing analysis of two NICER observations of the weakly magnetized neutron star low-mass X-ray binary Cygnus X-2. During these observations, we detect a rapid transition from a narrow 50-Hz horizontal-branch oscillation to a broad 5-Hz normal-branch oscillation, accompanied by an increase in source flux and a decrease in spectral hardness. Thanks to the large effective area of NICER, we are able to conduct a detailed comparison of the spectra associated with different types of quasi-periodic oscillations (QPOs) on short timescales. By fitting the spectra with a model that includes a disc and Comptonization components plus two emission lines, we find that the parameters of the disc component do not change significantly during the transition. However, assuming a fixed electron temperature, the optical depth of the Comptonization component decreases significantly. This drop in optical depth may be attributed to the expansion of the boundary layer or spreading layer.In addition, we find that the rms spectra for both the HBO and NBO are hard, suggesting that the boundary layer or spreading layer is driving the variability. We discuss the potential physical origin of the different types of QPOs.

astro-ph.HE↗

arXiv2025

Timing analysis of the black-hole candidate Swift J1727.8-1613: detection of a dip-like feature in the high-energy cross spectrum

We present a timing analysis of observations with the Hard X-ray Modulation Telescope of the black hole X-ray transient Swift J1727.8-1613 during its 2023 outburst. We detect, for the first time in a black hole X-ray binary, a prominent dip at ~ 3-15 Hz in the real part of the cross spectrum between high-energy (>25 keV) and low-energy (<10 keV) photons in the Low Hard and Hard Intermediate States, during which the QPO frequency rapidly increases and then stabilizes at ~ 1.0-1.5 Hz. Remarkably, the real part of the cross spectrum reaches negative values at the frequencies around the minimum of the dip, indicative of a phase lag ranging between $π/2$ and $π$ in this frequency range. We fit the power spectra and the real and imaginary parts of the cross spectra simultaneously using a multi-Lorentzian model. Among the lag models, the Gaussian phase-lag model provides a slightly better reduced $χ^2$ than the constant phase-lag and constant time-lag models, while it also alleviates the degeneracy associated with those models. From the parameters of the Lorentzian that fits the dip, we estimate the size of the accretion flow, which consistently exceeds 10,000 km as the QPO frequency increases from 0.13 Hz to 2.0 Hz. Furthermore, both the energy-dependent phase-lag and fractional-rms spectra of the dip exhibit a change in trend around 15 keV, with the phase lag dropping and rms reaching a local minimum. These spectra closely resemble the shapes predicted by the time-dependent Comptonization model, vKompth, for a low feedback factor, offering a pathway to explain the radiative properties of the corona. Additionally, the coherence function suggests a diversity of variability components, potentially arising from different parts of the corona.

astro-ph.HE↗

arXiv2025

Evolution of the Comptonizing medium of the black-hole candidate Swift J1727.8$-$1613 along the hard to hard-intermediate state transition using NICER

We analyse the properties of the Comptonizing medium in the black-hole X-ray binary Swift J1727.8$-$1613 using the time-dependent Comptonization model vkompth, using NICER observations of type-C QPOs in the hard and hard-intermediate states. During the 2023 outburst of the source, we measure the rms and phase lags of the QPO across 45 observations as the QPO frequency, $ν_{\rm QPO}$, evolves from $\sim 0.3$ Hz to $\sim 7$ Hz. By simultaneously fitting the time-averaged spectrum of the source and the rms and lag spectra of the QPO, we derive the evolution of the disk and corona parameters. At $ν_{\rm QPO} = 0.34$ Hz, the QPO phase lags are hard, with 10 keV photons lagging 0.5 keV photons by $\sim 0.5$ rad. As $ν_{\rm QPO}$ increases, the lags for the same energy bands decrease, reaching near zero at $ν_{\rm QPO} \sim 1.2$ Hz, and then reverse to soft lags of $\sim -1.1$ rad at $ν_{\rm QPO} \sim 7$ Hz. Initially, the inner radius of the accretion disk is truncated at $\sim 30-40 R_g$ (assuming a 10 solar-mass black hole) and, as the QPO frequency increases, the truncation radius decreases down to $\sim 10 R_g$. Initially, two coronas of sizes of $\sim 6.5 \times 10^3$ km and $\sim 2 \times 10^3$ km, extend over the disk and are illuminated by different regions of the disk. As the QPO frequency increases, both the coronas shrink to $\sim 2 \times 10^3$ km at $ν_{\rm QPO} = 2.5$ Hz. Following a data gap, one corona expands again, peaking at a size of $\sim 2 \times 10^4$ km. We interpret the evolution of the coronal size in the context of accompanying radio observations, discussing its implications for the interplay between the corona and the jet.

astro-ph.HE↗

arXiv2025

Insight-HXMT observations of the 2023 outburst in Aql X-1

We conducted an analysis of the continuum during the onset and initial decline phases of the 2023 outburst in transient neutron star low-mass X-ray binary Aql X$-$1 using broadband observations from the \textit{Insight-Hard X-ray Modulation Telescope (Insight-HXMT)} instrument. To determine the most appropriate model for the continuum of this outburst, we employed three models to explore the evolution of the spectral component. These observations revealed that the source transitions from the hard state to the soft state. The disk-corona and sphere-corona models both adequately described the spectra of the hard state, while the double blackbody model became preferable after the hard X-ray emission ($>$25 keV) disappeared during the state transition. In the soft state, the total emission is dominated by changes in the disk and other blackbody components. The combination of the sphere-corona model and the double blackbody model is the most suitable model for this outburst. The results suggest that as the source transitioned into the soft state, the emission from the boundary layer was enhanced, and a hot spot occurred. Notably, we identified two type-I X-ray bursts, one of which exhibited a significant hard X-ray deficit (significance $\sim$ 4.82 $σ$), which indicates that \textit{Insight-HXMT} has the capability to capture the evolution of the corona in a single burst.

astro-ph.HE↗

arXiv2025

A hidden quasi-periodic oscillation in Cygnus X-1 revealed by NICER

Cygnus X-1 is a high-mass black hole binary extensively studied since its discovery in 1964. Its rapid X-ray variability provides insights into accretion physics. Unlike other black hole X-ray binaries, its power spectra are generally featureless and modeled with two broad Lorentzians, without requiring narrow quasi-periodic oscillations. We investigate the possibility that some undetected variability components in power spectra may appear in the imaginary part of the cross spectra and the coherence function. Using NICER observations up to Cycle 6, we study the power, cross, and lag spectra, along with the coherence function, searching for these "imaginary" components. We simultaneously fit the power spectra in two energy bands, 0.3-2 keV and 2-12 keV, and the real and imaginary parts of the cross-spectrum with a multi-Lorentzian model. Assuming each Lorentzian is coherent between the two bands but incoherent with others, we predict intrinsic coherence and phase lags. he intrinsic coherence shows a narrow dip at a frequency increasing from ~1 Hz to ~6 Hz as the power-law index of the Comptonized component increases from ~1.8 to ~2.4. Simultaneously, the phase lags exhibit a steep increase (the "cliff") at the same frequencies. These features vanish when using energy bands similar to RXTE (e.g., 3-5 keV and 5-12 keV). A narrow Lorentzian component with low fractional rms and large phase lag is required to reproduce the coherence drop. Its rms and phase-lag spectra evolve systematically in the hardness-intensity diagram. This "imaginary" QPO behaves like a type-C QPO despite being undetectable in power spectra alone. Similar features in MAXI J1348-630 and MAXI J1820+070 support this interpretation, suggesting this may be the first detection of a type-C QPO in Cygnus X-1.

astro-ph.HE↗

arXiv2025

The nature of an imaginary quasi-periodic oscillation in the soft-to-hard transition of MAXI J1820+070

A recent study shows that if the power spectra (PS) of accreting compact objects consist of a combination of Lorentzian functions that are coherent in different energy bands but incoherent with each other, the same is true for the Real and Imaginary parts of the cross spectrum (CS). Using this idea, we discovered imaginary quasi-periodic oscillations (QPOs) in NICER observations of the black hole candidate MAXI J1820+070. The imaginary QPOs appear as narrow features with a small Real and large Imaginary part in the CS but are not significantly detected in the PS when they overlap in frequency with other variability components. The coherence function drops and the phase lags increase abruptly at the frequency of the imaginary QPO. We show that the multi-Lorentzian model that fits the PS and CS of the source in two energy bands correctly reproduces the lags and the coherence, and that the narrow drop of the coherence is caused by the interaction of the imaginary QPO with other variability components. The imaginary QPO appears only in the decay of the outburst, during the transition from the high-soft to the low-hard state of MAXI J1820+070, and its frequency decreases from approximately 5 Hz to around 1 Hz as the source spectrum hardens. We also analysed the earlier observations of the transition, where no narrow features were seen, and we identified a QPO in the PS that appears to evolve into the imaginary QPO as the source hardens. As for the type-B and C QPOs in this source, the rms spectrum of the imaginary QPO increases with energy. The lags of the imaginary QPO are similar to those of the type-B and C QPOs above 2 keV but differ from the lags of those other QPOs below that energy. While the properties of this imaginary QPO resemble those of type-C QPOs, we cannot rule out that it is a new type of QPO.

astro-ph.HE↗

arXiv2024

Geometry of the comptonization region of MAXI J1348$-$630 through type-C quasi-periodic oscillations with NICER

We use the rms and lag spectra of the type-C quasi-periodic oscillation (QPO) to study the properties of the Comptonisation region (aka corona) during the low/hard and hard-intermediate states of the main outburst and reflare of MAXI J1348$-$630. We simultaneously fit the time-averaged energy spectrum of the source and the fractional rms and phase-lag spectra of the QPO with the time-dependent Comptonization model vKompth. The data can be explained by two physically connected coronae interacting with the accretion disc via a feedback loop of X-ray photons. The best-fitting model consists of a corona of $\sim$10$^3$ km located at the inner edge of the disc and a second corona of $\sim$10$^4$ km horizontally extended and covering the inner parts of the accretion disc. The properties of both coronae during the reflare are similar to those during the low/hard state of the main outburst, reinforcing the idea that both the outburst and the reflare are driven by the same physical mechanisms. We combine our results for the type-C QPO with those from previous work focused on the study of type-A and type-B QPOs with the same model to study the evolution of the geometry of the corona through the whole outburst, including the reflare of MAXI J1348$-$630. Finally, we show that the sudden increase in the phase-lag frequency spectrum and the sharp drop in the coherence function previously observed in MAXI J1348$-$630 are due to the type-C QPO during the decay of the outburst and can be explained in terms of the geometry of the coronae.

astro-ph.HE↗