Quasi-periodic Oscillations Research Articles

Probable low-frequency quasi-periodic oscillations in blazars from the ZTF survey

ABSTRACT We investigate the possible presence of quasi-periodic oscillation (QPO) signals in 2103 blazars from the Zwicky Transient Facility (ZTF) time-domain survey. We detect a low-frequency QPO signal in five blazars observed over these 3.8-yr-long optical r-band ZTF light curves. These periods range from 144 to 196 d detected at ≳4σ significance levels in both the Lomb–Scargle periodogram and weighted wavelet Z-transform analyses. We find consistent results using the phase dispersion minimization technique. A similar peak is detected in the g-band light curves at a slightly lower significance of 3σ. Such nearly periodic signals on these time-scales in optical wavebands most likely originate from a precessing jet with high Lorentz factor, closely aligned to the observer’s line of sight or the movement of plasma blobs along a helical structure in the jet.

Study of X-ray spectral and timing properties of astronomical sources using low-cost stratospheric balloons

ABSTRACTAstronomical objects, such as the Sun, accreting matter around black holes, neutron stars, white dwarfs, etc. are known to emit X-rays and gamma-rays. Some of these sources, such as highly magnetized neutron stars, show periodic pulsations. Even accreting matter around black holes and neutron stars shows timing properties including quasi-periodic oscillations. In this paper, we use the data of low-cost, lightweight, high-altitude (∼42 km) small, stratospheric balloon-borne missions of the Indian Centre for Space Physics. We measure the intensity of the secondary cosmic rays, the radiation fluxes, and the spectra of persistent X-ray sources or transient events such as solar flares and gamma-ray bursts, apart from routine weather parameters. In this study, we present the source detection method without an onboard pointing system and the temporal and spectral behaviour of the quiet Sun. We also analyse the data containing Crab in the field of view to find the pulsation of the Crab pulsar. During the analysis of the source detection and spectrum of the solar X-rays, we calculate the detector’s background radiation (mainly secondary cosmic rays) using physical assumptions and also take care of the atmospheric absorption effects. Finally, we show the source detection processes for the strong sources such as the Sun, Crab, and Cyg X-1, obtaining the quiet Sun’s spectrum. Among the interesting timing properties, we present the result for the Crab pulsar and find the well-known ∼33 Hz pulsations whenever our instrument pointed to the Crab.

Flow instability of supercritical hydrocarbon fuel in a multi-channel cooling system

Thermally-induced flow instabilities are a critical issue in multi-channel regenerative cooling systems. In particular, the interactions between Density-Wave Oscillations (DWO) and Flow Maldistribution (FMD) can result in complex and disastrous instability phenomena. This study investigates the instability behaviors of hydrocarbon fluid in a four-channel system with a constant heat flux ratio using both frequency- and time-domain methods. As the heat flux increases, the in-tube flow sequentially destabilizes in each channel and converges to new equilibrium states, leading to the emergence of FMD phenomena. This also causes the system eigenvalue to change repeatedly from negative to positive rather than increasing monotonically. Additionally, the system eigenvalues are between those of the two most unstable channels, indicating that the stability behavior of the entire system is dictated by the most unstable channel. After FMD occurs, flow oscillations are activated in channels with weak stability, and the in-tube flow is observed to evolve into various flow patterns, including stable flow, self-sustained oscillation, oscillation divergence, quasi-periodic oscillation, and oscillation excursion. The novel instability mode of oscillation excursion involves a spontaneous transition of operating states. It oscillates from an equilibrium state and then stabilizes at a new operational state after oscillation-induced redistribution. However, the newfound stable state may also be only temporary, with the in-tube flow regressing to the initial state, resulting in quasi-periodic oscillation.

Quasiperiodic Peak Energy Oscillations in X-Ray Bursts from SGR J1935+2154

Magnetars are young neutron stars powered by the strongest magnetic fields in the Universe (1013–15 G). Their transient X-ray emission usually manifests as short (a few hundred milliseconds), bright, energetic (∼1040–41 erg) X-ray bursts. Since its discovery in 2014, SGR J1935+2154 has become one of the most prolific magnetars, exhibiting very active bursting episodes and other fascinating events, such as pulse timing antiglitches and fast radio bursts. Here we present evidence for possible 42 Hz (24 ms) quasiperiodic oscillations in the ν F ν spectrum peak energy (E p ) identified in a unique burst detected with the Fermi Gamma-ray Burst Monitor in 2022 January. While quasiperiodic oscillations have been previously reported in the intensity of magnetar burst light curves, quasiperiodic oscillations in E p have not. We also find an additional event from the same outburst that appears to exhibit a similar character in E p , albeit of lower statistical quality. For these two exceptional transients, such E p oscillations can be explained by magnetospheric density and pressure perturbations. For burst-emitting plasma consisting purely of e + e − pairs, these acoustic modes propagate along a highly magnetized flux tube of length up to around L ∼ 130 neutron star radii, with L being lower if ions are present in the emission zone. Detailed time-resolved analyses of other magnetar bursts are encouraged to evaluate the rarity of these events and their underlying mechanisms.

Quasi-periodic oscillation detected in γ-rays in blazar PKS 0346−27

Aims. We present a variability study of the blazar PKS 0346−27 conducted between December 2018 and January 2022 using archival γ-ray observations from Fermi-LAT. Methods. We used Lomb–Scargle periodogram and weighted wavelet transform methods to detect the presence of periodicity or quasi-periodicity and localize this feature in time and frequency space. We estimated the significance of the periodicity feature using a Monte Carlo simulation approach. We also determined the global significance of the periodicity to test the robustness of our claim. To explore the most probable scenario, we modeled the light curve with both a straight-jet and a curved-jet model. Results. We detect a periodicity feature of ∼100 days in the entire period of observation with a statistical significance of 3σ, which amounts to a 99.7% confidence level. The global significance of this feature is found to be 96.96%. Based on the Akaike information criterion, the most probable explanation is that the observed emission is enhanced due to the helical motion of a blob within a curved jet. Conclusions. The origin of this quasi-periodic oscillation is very likely a region of enhanced emission moving helically inside a curved jet. This work presents strong evidence for jet curvature in the source and an independent (albeit a little serendipitous) procedure to estimate the curvature in blazar jets.

Detection of quasi-periodic oscillation in the optical light curve of the blazar S5 0716+714

Detection of quasi-periodic oscillation in the optical light curve of the blazar S5 0716+714

Photometric analysis of the dwarf nova SU UMa based on TESS

We report the photometric analysis of SU UMa based on the observations of the Transiting Exoplanet Survey Satellite (TESS). TESS has released a large amount of data, which contains the light curves of a complete superoutburst and three normal outbursts of SU UMa. Based on the observations, the evolution of superhumps during the superoutburst was analyzed. By using the O - C method, the three stages were determined, and the superhump period for each stage was calculated. The mass ratio q=0.137(1) was estimated based on the stage A superhump method. A periodic oscillation with a period of 2.17(9) days was found in the superhump minimum O - C diagram, which is related to the precession of the accretion disk. We investigated the frequency information of quasiperiodic oscillations (QPOs) in the light curves. An ultralong period QPO of 0.18 days at the time of normal outburst and two ultralong periods QPOs of 0.19 days and 0.285 days in quiescence were found. No QPO was found in the superoutburst.

Correction to: The X-ray corona in the black-hole binary GRO J1655–40 from the properties of non-harmonically related quasi-periodic oscillations

Correction to: The X-ray corona in the black-hole binary GRO J1655–40 from the properties of non-harmonically related quasi-periodic oscillations

Evolution of Negative Superhumps, Quasiperiodic Oscillations, and Outbursts in the Z Cam-type Dwarf Nova AH Her

AH Her is a Z Cam-type dwarf nova with an orbital period of ∼0.258 days. Dwarf nova oscillations and long-period dwarf nova oscillations have been detected, but no quasiperiodic oscillations (QPOs) and negative superhumps (NSHs) have been found. We investigated the association between NSHs, QPOs, and outbursts of AH Her based on Transiting Exoplanet Survey Satellite photometry. We find for the first time NSHs with a period of 0.24497(1) days in AH Her, and trace the variation in the amplitude and period of NSHs with the outburst. The amplitude of the NSHs is the most significant at quiescence, weakening as the outburst rises, becoming undetectable at the top, rebounding and weakening at the plateau, and strengthening again as the outburst declines. The variation in the accretion disk radius can explain the NSH amplitude variation except for the plateau, so we suggest that the relationship between the NSH amplitude and outburst can be used as a window to study the accretion disk instability and the origin of NSHs. In addition, we find periodic variations in the amplitude, maxima, and shape of the NSHs ranging from 2.33(2) to 2.68(5) days, which may be related to the precession of the tilted disk. Finally, we find QPOs at the top of AH Her’s long outburst with ∼2800 s similar to those of HS 2325+8205, suggesting that the presence of QPOs at the top of Z Cam’s long outburst may be a general phenomenon.

Charged particles motion and quasiperiodic oscillation in Simpson–Visser spacetime in the presence of external magnetic fields

The present work is devoted to the study of the dynamics of charged particles around Simpson–Visser black holes (with the length parameter lle 2) and wormholes (l>2) immersed in an external asymptotically uniform magnetic field. To do this, first, we solve the Maxwell equation for 4-potentials of the electromagnetic field and show that the difference between the numerical solution and Wald’s solution is small enough to neglect it, which may allow us to use the solution obtained by Wald. We also study fundamental frequencies of in the vertical and radial oscillations of charged particles around circular stable orbits around the magnetized black hole. The effects of the magnetic interaction and length parameters on the fundamental frequencies. We investigate the quasiperiodic oscillations (QPOs) around the black hole in relativistic precession and epicyclic resonance models. It is also shown that the combined effects of magnetic interaction for negatively charged particles and length parameters can mimic the spacetime effects of the Schwarzschild black hole compensating for their effects, as well as the spin of rotating Kerr black holes. The distance between an orbit where a QPO is generated with the ratio of upper and lower frequencies 3: 2 and innermost stable circular orbits is also studied. It is found that the QPO orbits are very close to ISCO in the RP model at l<2. This implies that the obtained result helps to determine the ISCO around black holes. We also study the applications of observed QPOs around stellar-mass black holes in microquasars and supermassive black holes.

Quasiperiodic Oscillation in Short Gamma-Ray Bursts from Black Hole–Neutron Star Mergers

Short-duration gamma-ray bursts (sGRBs) are commonly attributed to the mergers of double neutron stars (NSs) or the mergers of a neutron star with a black hole (BH). While the former scenario was confirmed by the event GW170817, the latter remains elusive. Here, we consider the latter scenario in which an NS is tidally disrupted by a fast-spinning low-mass BH and the accretion onto the BH launches a relativistic jet and hence produces an sGRB. The merging binary’s orbit is likely misaligned with the BH’s spin. Hence, the Lense–Thirring precession around the BH may cause a hyperaccreting thick disk to precess in a solid-body manner. We propose that a jet, initially aligned with the BH spin, is deflected and collimated by the wind from the disk, therefore being forced to precess along with the disk. This would result in a quasiperiodic oscillation or modulation in the gamma-ray light curve of the sGRB, with a quasi-period of ∼0.01–0.1 s. The appearance of the modulation may be delayed respective to the triggering of the light curve. This feature, unique to the BH–NS merger, may have already revealed itself in a few observed sGRBs (such as GRB 130310A), and it carries the spin–orbit orientation information of the merging system. Identification of this feature would be a new approach to reveal spin–orbit misaligned merging BH–NS systems, which are likely missed by the current gravitational-wave searching strategy that is principally targeting aligned systems.

AstroSat View of the Neutron Star Low-mass X-Ray Binary GX 340+0

Understanding the spectral evolution along the “Z”-shaped track in the hardness–intensity diagram of Z sources, which are a class of luminous neutron star low-mass X-ray binaries, is crucial to probe accretion processes close to the neutron star. Here, we study the horizontal branch (HB) and the normal branch (NB) of the Z source GX 340+0 using AstroSat data. We find that the HB and the NB appear as two different types of X-ray intensity dips, which can appear in any sequence and with various depths. Our 0.8–25 keV spectra of dips and the hard apex can be modeled by the emissions from an accretion disk, a Comptonizing corona covering the inner disk, and the neutron star surface. We find that as the source moves onto the HB, the corona is replenished and energized by the disk and a reduced amount of disk matter reaches the neutron star surface. We also conclude that quasiperiodic oscillations during HB/NB are strongly associated with the corona and explain the evolution of strength and hard lag of this timing feature using the estimated coronal optical depth evolution.

Unveiling the accretion scenario of BH-ULXs using XMM–Newton observations

ABSTRACT We present a comprehensive spectrotemporal analysis of five ultraluminous X-ray sources (ULXs) with central object likely being a black hole, using archival XMM–Newton observations. These sources, namely NGC1313 X−1, NGC5408 X-1, NGC6946 X−1, M82 X−1, and IC342 X−1, reveal short-term variability with fractional variance of 1.42$-27.28~{{\ \rm per\ cent}}\, $ and exhibit Quasi-periodic Oscillations (QPOs) with frequency νQPO ∼ 8−667 mHz. Long-term evolution of ULXs energy spectra (0.3–10 keV; excluding M82 X−1) are described satisfactorily with a model combination that comprises a thermal Comptonization component (nthComp, yielding Γnth ∼ 1.48–2.65, kTe ∼ 1.62–3.76 keV, τ ∼ 8–20, y-par∼1.16–6.24) along with a standard disc component (diskbb, kTin ∼ 0.16–0.54 keV). We find that these ULXs generally demonstrate anticorrelation between disc luminosity and inner disc temperature as $L_{\rm disc} \propto T_{\rm in}^\alpha$, where α = −3.58 ± 0.04 for NGC1313 X−1 and IC342 X−1, α = −8.93 ± 0.11 for NGC6946 X−1, and α = −10.31 ± 0.10 for NGC5408 X−1. We also obtain a linear correlation between bolometric luminosity Lbol and Γnth that indicates spectral softening of the sources when Lbol increases. We observe that in presence of QPO, Comptonized seed photon fraction varies in between $\sim 5-20~{{\ \rm per\ cent}}\, $, while the Comptonized flux contribution (50$-90~{{\ \rm per\ cent}}\, $) dominates over disc flux. Utilizing νQPO and Lbol, we constrain ULXs mass by varying their spin (ak) and accretion rate ($\dot{m}$). We find that NGC6946 X−1 and NGC5408 X−1 seem to accrete at sub-Eddington accretion rate provided their central sources are rapidly rotating, whereas IC342 X−1 and NGC1313 X−1 can accrete in sub/super-Eddington limit irrespective to their spin values.

QPOs in Compact Sources as a Nonlinear Hydrodynamical Resonance: Determining Spin of Compact Objects

The origin of wide varieties of quasiperiodic oscillations (QPOs) observed in compact sources is still not well established. Its frequencies range from millihertz to kilohertz spanning all compact objects. Are different QPOs, with different frequencies, originating from different physics? We propose that the emergence of QPOs is the result of nonlinear resonance of fundamental modes present in accretion disks forced by external modes including that of the spin of the underlying compact object. Depending on the properties of accreting flow, e.g., its velocity and gradient, resonances (and any mode-locking) take place at different frequencies, exhibiting low- to high-frequency QPOs. We explicitly demonstrate the origin of higher-frequency QPOs for black holes and neutron stars by a unified model and outline how the same physics could be responsible for producing lower-frequency QPOs. The model also predicts the spin of black holes, and constrains the radius of neutron stars and the mass of both.

A sub-pc BBH system in SDSS J1609+1756 through optical QPOs in ZTF light curves

ABSTRACT Optical quasi-periodic oscillations (QPOs) are the most preferred signs of sub-pc binary black hole (BBH) systems in active galactic nucleus (AGN). In this manuscript, robust optical QPOs are reported in quasar SDSS J1609+1756 at z = 0.347. In order to detect reliable optical QPOs, four different methods are applied to analyse the 4.45 yr-long Zwicky Transient Facility (ZTF) g/r/i-band light curves of SDSS J1609+1756, direct fitting results by sine function, generalized Lomb–Scargle periodogram, autocross correlation function, and weighted wavelet Z-transform method. The four different methods can lead to well determined reliable optical QPOs with periodicities ∼340 d with confidence levels higher than 5σ, to guarantee the robustness of the optical QPOs in SDSS J1609+1756. Meanwhile, based on simulated light curves through continuous autoregressive process to trace intrinsic AGN activities, confidence level higher than 3σ can be confirmed that the optical QPOs are not misdetected in intrinsic AGN activities, reconfirming the robust optical QPOs and strongly indicating a central sub-pc BBH system in SDSS J1609+1756. Furthermore, based on apparent redshifted shoulders in broad Balmer emission lines in SDSS J1609+1756, space separation of the expected central BBH system can be estimated to be smaller than 107 ± 60 light-days, accepted upper limit of total BH mass ∼(1.03 ± 0.22) × 108 M⊙. Therefore, to detect and report BBH system expected optical QPOs with periodicities around 1 yr is efficiently practicable through ZTF light curves, and combining with peculiar broad-line emission features, further clues should be given on space separations of BBH systems in broad-line AGN in the near future.

Prediction of evolution behavior of Internet bottleneck delay based on improved Logistic equation

As massive real-time systems and various types of networks connect to the Internet, real-time Internet connections are becoming increasingly prominent. Research on bottleneck delays provides a reference for solving real-time performance issues in data transmission for real-time systems on the Internet. In this study, under the Internet macro-topology, we utilized real probing data to conduct an in-depth study on the evolution behavior of bottleneck delays in end-to-end delays. We demonstrate that more than 90% of valid paths were 9 hops on average, and the ratio of the maximum link delay to the one-way delay was above 1/3. In addition, bottleneck delays on the valid paths were mainly concentrated in the range of [0, 130 ms], and the relative spatial positions of the bottleneck delays appearing in the links of one-way delays tended to be near the middle. The timing evolution sequence of the bottleneck delays exhibited a slow upward trend and quasiperiodic oscillations. On this basis, the Logistic equation was introduced to describe this evolution and establish the prediction model, and the gray Wolf Optimizer (GWO) algorithm was used to fit and optimize the model parameters. Finally, experiments were conducted to verify the validity and accuracy of the models. Their prediction accuracies exceeded 90% according to a quantitative evaluation, and the models fit well according to a χ2 goodness-of-fit test. This validates our modeling approach and demonstrates that the model can predict bottleneck delays accurately in a short period of time.

Dynamic modeling of hard-magnetic soft actuators: Unraveling the role of polymer chain entanglements, crosslinks, and finite extensibility

In recent years, there has been increasing interest in hard-magnetic soft materials (HMSMs) due to their ability to retain high residual magnetization and undergo large deformations under external magnetic loading. The performance of these materials in the dynamic mode of actuation is significantly influenced by internal properties, such as entanglements, crosslinks, and the finite extensibility of polymer chains. This article presents a theoretical framework for modeling the dynamic behavior of a hard-magnetic soft material-based planar actuator. A physics-based nonaffine material model is utilized to consider the inherent properties of polymer chain networks. The governing equation for dynamic motion is derived using Euler–Lagrange’s equation of motion for conservative systems. The devised dynamic model is utilized to examine the dynamic response, stability, periodicity, and resonance properties of a planar hard-magnetic soft actuator for different values of polymer chain entanglements, crosslinks, and finite extensibility parameters. The Poincaré maps and phase-plane plots are presented to analyze the stability and periodicity of the nonlinear vibrations of the actuator. The results reveal that transitions between aperiodic and quasi-periodic oscillations occur when the density of polymer chain entanglements and cross-linking changes. The findings from the present investigation can serve as an initial step towards the design and manufacturing of remotely controlled actuators for various futuristic applications.

Nonlinear dynamic characteristics of self-excited thermoacoustic instabilities in premixed swirling flames

The nonlinear behavior of thermoacoustic oscillation for premixed swirl flames fueled with CH4/air mixtures is experimentally studied by changing the equivalence ratio Φ, combustor inlet bulk velocity Uin and swirl number S. The nonlinear dynamic characteristics of the system are analyzed by the methods of phase space reconstruction, recurrence plot, Poincaré section and 0–1 test for chaos. Five kinds of flame modes are observed in the combustion system with the change of Φ, Uin, and S, which are lean/rich blow-off, stable combustion, quasi-periodic oscillation, and limit cycle oscillation. Subcritical Hopf bifurcation of the oscillation is captured either by changing Φ or Uin, and Φ and Uin are found to have similar effects on the flame mode transition under specific operating conditions. It is observed that the appearance of flame in the outer circulation zone (ORZ) and the enhancement of its burning intensity are matched with the occurrence of quasi-periodic and limit cycle oscillations, respectively. It is examined that the lean/rich blow-off limits and oscillation range are extended with a larger S, and the stable combustion regime is wider with a smaller S. This study contributes to understanding the nonlinear thermoacoustic instability of swirl flames under broad flame conditions, which helps to develop effective instability prediction and control methods in stable, efficient, and low-emission gas turbines.

Intermittent properties of the quasi-periodic oscillations of MAXI J1820+070 revealed by Insight-HXMT

We investigate the dynamical properties of low-frequency quasi-periodic oscillations (QPOs) observed from the black hole X-ray binary MAXI J1820+070 during the early part of its 2018 outburst, when the system was in a bright hard state. To do this, we used a series of observations from the Hard X-ray Modulation Telescope Insight-HXMT and applied a wavelet decomposition (weighted wavelet Z-transforms) to the X-ray light curve. We find that the QPO phenomenon is intermittent within each individual observation, with some subintervals in which the oscillation is strongly detected (high root-mean-square amplitude) and others in which it is weak or absent. The average lifetime of individual QPO segments is about 5 oscillation cycles, with a 3σ tail of up to about 20 cycles. There is no substantial difference between the energy spectra during intervals with strong and weak or absent QPOs. Alternatively, we suggest that the changes in oscillation amplitude can be caused by changes in the jet speed. We discuss a possible reason for the intermittent QPO strength within the precessing-jet model previously proposed for MAXI J1820+070: we suggest that the changes in oscillation amplitude are caused by changes in the jet speed. We argue that a misaligned precessing-jet scenario is also consistent with other recent observational findings that suggest that the oscillation of the Compton reflection component is in phase with the QPOs.

Test Particles and Quasiperiodic Oscillations around Gravitational Aether Black Holes

This paper is devoted to the analysis of the dynamics of test particles in the vicinity of a black hole within the framework of a gravitational aether model. First, we explored the structure of spacetime by analyzing the curvature scalars. Then, we studied particle dynamics around a black hole using the Hamilton–Jacobi equation.The influence of the aether on the effective potential of the radial motion of test particles around the black hole has been investigated. The dependence of the innermost stable circular orbits (ISCO) on the aether parameter has also been investigated. We also considered particle collision near the black hole in the presence of aether, and studied the fundamental frequencies of the orbital motion of the test particles around the black hole in the presence of aether. Further, we applied the obtained results to the analysis of the upper and lower frequencies of twin-peaked quasiperiodic oscillations (QPOs) occurring near black holes. Finally, we use theoretical and numerical results to obtain constraints on model parameters using observation data in QPO.