Quasi-periodic Oscillation Feature Research Articles

Quasi-periodic Oscillations in GX 339−4 during the 2021 Outburst Observed with Insight-HXMT

A new outburst of GX 339−4 in 2021 was monitored by the Hard X-ray Modulation Telescope (Insight-HXMT). By using the data of Insight-HXMT from 2021 February to March, we make an X-ray timing analysis of this new outburst. Based on the results of the count rates, hardness–intensity diagram, and power density spectrum (PDS), we confirm that the source exhibits spectral transitions from the low-hard state (LHS) to the hard-intermediate state (HIMS). During the transition from the LHS to the HIMS, low-frequency quasi-periodic oscillations (QPOs) are detected in the PDS. We found that these QPOs are all type C QPOs with centroid frequencies evolving from 0.1 to 0.6 Hz in the LHS and in the 1–3 Hz frequency range in HIMS. The QPO features above 50 keV are reported for the first time in this black hole by Insight-HXMT. The QPO rms stays stable with time but decreases with energy at a higher energy above ∼10 keV. We also find that the phase lag of the type C QPO is close to zero in the early outburst stage but becomes positive as the outburst evolves, with a hard lag of ∼0.6–1.2 rad in 50–100 keV. The implications of the phase lag in high energy bands and the possible physical mechanisms to explain those observations are also discussed.

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 quasi-periodic oscillations in the persistent X-ray emission of accreting binary X-ray pulsar LMC X-4

ABSTRACTWe report the discovery of quasi-periodic oscillations (QPOs) in the high-mass X-ray binary (HMXB) pulsar LMC X-4 in its non-flaring (persistent) state using observations with XMM–Newton. In addition to the 74 mHz coherent pulsations, the persistent emission light curve shows a QPO feature in the frequency range of 20–30 mHz. Quasi-periodic flares have been previously observed from LMC X-4 in observations made with Rossi X-ray Timing Explorer (RXTE). However, this is the first time QPOs have been observed in the persistent emission observations of LMC X-4. QPOs in X-ray binaries are generally thought to be related to the rotation of the inhomogeneous matter distribution in the inner accretion disc. In HMXBs such as LMC X-4 where the compact object is a neutron star with a high magnetic field, the radius of the inner accretion disc is determined by the mass accretion rate and the magnetic moment of the neutron star. In such systems, the QPO feature, along with the pulse period and X-ray luminosity measurement, helps us to constrain the magnetic field strength of the neutron star. We use considerations of magnetospheric accretion to have an approximate value of the magnetic field strength of the neutron star in LMC X-4.

Detection of a QPO in five XMM–Newton observations of RE J1034+396

Abstract The X-ray quasi-periodic oscillation (QPO) at ∼2.6 × 10−4 Hz in RE J1034+396 has been robustly observed in the 0.2–10 keV band of one ∼90 ks XMM–Newton observation, but was not detected in subsequent observations. Here, we investigate the power spectral density (PSD) of eight archival XMM–Newton observations of RE J1034+396, and search for the presence of QPOs in three energy bands: soft (0.3–0.8 keV); hard (1.0–4.0 keV) and total (0.2–10.0 keV). We find a significant detection of a QPO feature in the hard-band PSD of five low-flux/spectrally harder observations. The QPO frequency has remained persistent at ∼2.6 × 10−4 Hz over ∼4 yr of observations, though it is no longer detected in the soft band, except in the original observation. This result increases the duration where the QPO is present by a factor of 3 (now ∼250 ks), allowing for a better understanding of the QPO phenomenon observed in both active galactic nuclei and black hole X-ray Binaries.

A SPECTRAL STUDY OF THE RAPID TRANSITIONS OF TYPE-B QUASI-PERIODIC OSCILLATIONS IN THE BLACK HOLE TRANSIENT XTE J1859+226

The evolution of different types of quasi-periodic oscillations (QPOs) and the coupled radiative/physical changes in the accretion disk are still poorly understood. In a few black hole binaries it was found that fast evolution of QPOs is associated with spectral variations. Such studies in other black hole binaries are important to understand the QPO phenomenon. For the black hole transient XTE J1817-330, we study fast QPO transitions and accompanying spectral variations to investigate what causes the spectral variation during the QPO transition. Roy et al. (2011) found QPOs in ten RXTE observations of XTE J1817-330. We found that, among the ten observations, only one observation shows erratic dips in its X-ray light curve. The power density spectra and the corresponding energy spectra were extracted and analyzed for the dip and non-dip sections of the light curve. We found that type-B $\sim$6 Hz QPO changes into type-A QPO in a few tens of seconds along with a flux decrease. This transient evolution is accompanied with a significant spectral variation. We report a transient QPO feature and accompanying spectral variation in XTE J1817-330. Based on our findings, we discuss the origin of fast evolution of QPOs and spectral variations.

Fast transition of type-B quasi-periodic oscillation in the black hole transient XTE J1817-330

Context. The evolution of different types of quasi-periodic oscillations (QPOs) and the coupled radiative/physical changes in the accretion disk are still poorly understood. In a few black hole binaries it was found that fast evolution of QPOs is associated with spectral variations. These studies in other black hole binaries are important to understand the QPO phenomenon. Aims. We study fast QPO transitions and accompanying spectral variations of the black hole transient XTE J1817-330 to investigate the causes of the spectral variation during the QPO transition. Methods. Recently, QPOs in ten RXTE observations of XTE J1817-330 were found. We found that among these, only one observation shows erratic dips in its X-ray light curve. The power density spectra and the corresponding energy spectra were extracted and analyzed for the dip and non-dip sections of the light curve. Results. We found that type-B ∼6 Hz QPO changes into type-A QPO in a few tens of seconds along with a flux decrease. This transient evolution is accompanied by a significant spectral variation. Conclusions. We report a transient QPO feature and accompanying spectral variation in XTE J1817-330. Based on our findings, we discuss the origin of fast evolution of QPOs and spectral variations.

NEAR-INFRARED AND X-RAY QUASI-PERIODIC OSCILLATIONS IN NUMERICAL MODELS OF Sgr A*

We report transient quasi-periodic oscillations (QPOs) on minute timescales in relativistic, radiative models of the galactic center source Sgr A*. The QPOs result from nonaxisymmetric $m=1$ structure in the accretion flow excited by MHD turbulence. Near-infrared (NIR) and X-ray power spectra show significant peaks at frequencies comparable to the orbital frequency at the innermost stable circular orbit (ISCO) $f_o$. The excess power is associated with inward propagating magnetic filaments inside the ISCO. The amplitudes of the QPOs are sensitive to the electron distribution function. We argue that transient QPOs appear at a range of frequencies in the neighborhood of $f_o$ and that the power spectra, averaged over long times, likely show a broad bump near $f_o$ rather than distinct, narrow QPO features.

Timing and spectral studies of the transient X-ray pulsar GX 304−1 during an outburst

We present the timing and spectral properties of the transient X-ray pulsar GX 304-1 during its recent outburst in 2010 August, using observations carried out with the Proportional Counter Array (PCA) instrument on-board the Rossi X-ray Timing Explorer (RXTE) satellite. We detected strong intensity and energy dependent variations in the pulse profiles during the outburst. The pulse profile showed significant evolution over the outburst. It showed complex structures consisting of a main peak with steps on both sides during the start of the outburst. On some days, a sharp dip like feature was seen which disappeared at the end of the outburst; when the profile evolved into a sinusoidal shape. At low energies, the pulse profiles appeared complex, consisting of multiple peaks and a narrow minimum. The amplitude of the second brightest peak in low energies decreased with energy, and above 12 keV, the shape of the pulse profile changed to a single broad peak with a dip like feature. The dip had energy dependence, both in phase and in width. We detected Quasi-periodic Oscillations (QPO) at 0.125 Hz with a harmonic. The QPO feature had a low rms value of 2.9% and it showed a positive energy dependence up to 40 keV with the rms value increasing to 9% at 40 keV. The QPO frequency decreased from 0.128 Hz to 0.108 Hz in 12 days. During most of the outburst, the 3-30 keV spectrum of GX 304-1 can be well fitted with a partial covering power-law model with a high energy cut-off and iron fluorescent line emission. For a few of the observations carried out during the decay of the outburst, the partial covering absorption component is found to change to single component absorption. We also found that the partial covering and high energy cut-off parameters vary significantly with the pulse phase.

Flares, broadening of the pulse-frequency peak and quasi-periodic oscillations in the transient X-ray pulsar 4U 1901+03

After a long quiescence of three decades, the transient X-ray pulsar 4U 1901+03 became highly active in 2003 February. From the analysis of a large number of Rossi X-ray Timing Explorer/ Proportional Counter Array (RXTE/PCA) observations of this source, we report here the detection of X-ray flares, a broadening of the pulse frequency peak and Quasi Periodic Oscillations (QPOs). The X-ray flares showed spectral changes, had a duration of 100 s - 300 s, and were more frequent and stronger during the peak of the outburst. In most of the observations during the outburst we also detected a broadening of the pulse frequency peak. We have also found intensity dependent changes in the pulse profile at very short timescales. This reveals a coupling between the periodic and the low frequency aperiodic variabilities. In addition, near the end of the outburst we have detected a strong QPO feature centered at ~ 0.135 Hz. The QPO feature is broad with a quality factor of 3.3 and with an rms value of 18.5+/-3.1%. Using the QPO frequency and the X-ray luminosity during the QPO detection period we estimated the magnetic field strength of the neutron star as 0.31*10^12 G which is consistent with the value inferred earlier under the assumption of spin equilibrium.

Quasi‐Periodic Oscillations in Cen X‐3 and the Long‐Term Intensity Variations

We have investigated the properties of the quasi-periodic oscillation (QPO) features in the accretion-powered X-ray pulsar Cen X-3 over a period of about 4 years using observations carried out with the Proportional Counter Array (PCA) of the Rossi X-ray Timing Explorer (RXTE). The observations cover a wide range of X-ray intensity of the source in excess of the binary intensity modulation. In 11 of 81 pointings with the PCA, we have detected QPOs with rms intensity fluctuation up to 10%. The QPO peak frequency shows clustering around 40 and 90 mHz, with the QPO frequency having no dependence on X-ray intensity. This indicates that either (1) the observed X-ray luminosity of the source is not related to the mass accretion rate or inner radius of the accretion disk, or (2) the QPO generation mechanism in Cen X-3 is different from the beat frequency model or Keplerian frequency model that is believed to operate in most other transient and persistent X-ray pulsars. We have also found that the rms variation in the 40 mHz QPO feature is not dependent on the X-ray energy, indicating that the origin of the QPO is unlikely to be related to disk absorption.

Quasi‐periodic Oscillations in XTE J0111.2−7317: Highest Frequency among the HMXB Pulsars

We report here discovery of Quasi Periodic Oscillations (QPOs) in the High Mass X-ray Binary (HMXB) Pulsar XTE J0111.20-7317 during a transient outburst in this source in December 1998. Using observations made with the proportional counter array of the Rossi X-ray Timing Explorer during the second peak and the declining phase of this outburst we have discovered a QPO feature at a frequency of 1.27 Hz. We have ruled out the possibility that the observed QPOs can instead be from the neighbouring bright X-ray pulsar SMC X-1. This is the highest frequency QPO feature ever detected in any HMXB pulsar. In the absence of a cyclotron absorption feature in the X-ray spectrum, the QPO feature, along with the pulse period and X-ray flux measurement measurement helps us to constrain the magnetic field strength of the neutron star.

Multicolour observations of V404 Cyg with ULTRACAM

We present high time-resolution multicolour observations of the quiescent soft X-ray transient V404 Cyg obtained with ULTRACAM. Superimposed on the secondary star's ellipsoidal modulation are large flares on timescales of a few hours, as well as several distinct rapid flares on timescales of tens of mins. The rapid flares, most of which show further variability and unresolved peaks, cover shorter timescales than those reported in previous observations. The power density spectrum (PDS) of the 5 s time-resolution data shows a quasi-periodic oscillation (QPO) feature at 0.78 mHz (=21.5 min). Assuming this periodicity represents the Keplerian period at the transition between the thin and advective disc regions, we determine the transition radius. We discuss the possible origins for the QPO feature in the context of the advection-dominated accretion flow model. We determine the colour of the large flares and find that the i' band flux per unit frequency interval is larger than that in the g' band. The colour is consistent with optically thin gas with a temperature of ~8000 K arising from a region with an equivalent blackbody radius of at least 2 Ro, which covers 3 percent of the accretion disc's surface. Our timing and spectral analysis results support the idea that the rapid flares (i.e. the QPO feature) most likely arise from regions near the transition radius.

Effects of Resonance in Quasi-periodic Oscillators of Neutron Star Binaries

Using a large quantity of Rossi X-Ray Timing Explorer data presented in the literature, I offer a detailed investigation into the accuracy of the quasi-periodic oscillation (QPO) frequency determination. The QPO phenomenon seen in X-ray binaries is possibly a result of the resonance of the intrinsic (eigen) oscillations and harmonic driving forces of the system. I show that the resonances, in the presence of the damping of oscillations, occur at frequencies that are systematically and randomly shifted with respect to the eigenfrequencies of the system. The shift value strongly depends on the damping rate that is measured by the half-width of the QPO feature. Taking into account this effect, I analyze the QPO data for four Z sources, Scorpius X-I, GX 340+0, GX 5-1, and GX 17+2, and two atoll sources, 4U 1728-34 and 4U 0614+09. The transition-layer model (TLM) predicts the existence of the invariant quantity delta, an inclination angle of the magnetospheric axis with respect to the normal to the disk. I calculate delta and the error bars of delta using the resonance shift, and I find that the inferred delta-values are consistent with constants for these four Z sources, in which horizontal-branch oscillation and kilohertz frequencies have been detected and correctly identified. It is shown that the inferred delta are in the range between 5.5 deg and 6.5 deg. I conclude that the TLM seems to be compatible with the data.

Discovery of Kilohertz Fluctuations in Centaurus X‐3: Evidence for Photon Bubble Oscillations (PBO) and Turbulence in a High‐Mass X‐Ray Binary Pulsar

We report the discovery of kilohertz fluctuations, including quasiperiodic oscillations (QPO) at {approx}330 Hz (260-407 Hz) and {approx}760 Hz (671-849 Hz) and a broadband kilohertz continuum in the power-density spectrum of the high-mass X-ray binary pulsar Centaurus X-3. These observations of Cen X-3 were carried out with the Rossi X-Ray Timing Explorer (RXTE). The fluctuation spectrum is flat from millihertz to a few hertz, then steepens to f{sup -2} behavior between a few hertz and {approx}100 Hz. Above 100 Hz, the spectrum shows the QPO features, plus a flat continuum extending to {approx}1200 Hz and then falling out to {approx}1800 Hz. These results, which required the co-adding three days of observations of Cen X-3, are at least as fast as the fastest known variations in X-ray emission from an accreting compact object (kilohertz QPO in LMXB sources) and probably faster since extension to {approx}1800 Hz is indicated by the most likely parameterization of the data. Multidimensional radiation hydrodynamics simulations of optically thick plasma flow onto the magnetic poles of an accreting neutron star show that the fluctuations at frequencies above 100 Hz are consistent with photon bubble turbulence and oscillations (PBO) previously predicted to be observable in this source. Wemore » show that previous observations of Cen X-3 constrain the models to depend on only one parameter, the size of the polar cap. For a polar cap opening angle of 0.25 radians (polar cap radius {approx}2.5 km and area {approx}20 km2, for a neutron star radius of 10 km), we show that the spectral form above 100 Hz is reproduced by the simulations, including the frequencies of the QPO and the relative power in the QPO and the kilohertz continuum. This has resulted in the first model-dependent measurement of the polar cap size of an X-ray pulsar. The simulations underpredict the overall amplitude of the observed spectrum, which we suggest is the consequence of a two-dimensional axisymmetric simulation of an intrinsically three-dimensional phenomenon. The power-density spectrum of Cen X-3 shows a dramatic decrease above {approx}1000 Hz, which suggests an optical depth {approx}30 across the accretion mound consistent with effects of radiative diffusion in the simulations. We identify this decline at high frequency as the first direct evidence of radiative diffusion near the surface of a neutron star (NS). We suggest the fluctuations observed at frequencies below 100 Hz, whose spectrum has a different form from that of the kilohertz phenomena, reflect intermittency in the mass transfer mechanism that carries plasma from the accretion disk to field aligned flow onto the neutron star's polar caps. Using simple estimates based on Rayleigh-Taylor instabilities, possibly modulated by intrinsic disk turbulence, we show that mass transfer in ''blobs'' forming through Rayleigh-Taylor disruption of the disk's inner edge can explain the large amplitude fluctuations required by the spectrum at frequencies f{approx}1 Hz, but only if magnetic pressure in the disk's innermost regions inflates the disk until its scale height is comparable to the magnetosphere's size {approx}4300 km. The observational results required the development of a procedure for the careful determination of the dead-time effects of the PCA. This procedure is described in Appendix A. As a consequence of the use of observations of Cyg X-1 for the estimation of dead-time corrections for the observations of Cen X-3, we have also demonstrated that the black hole Cyg X-1 shows clear evidence of variability up to a frequency of {approx}280 Hz. Also observations of GX 17+2 were used to validate the procedure for dead-time corrections. This analysis of GX 17+2 clearly indicates the presence of a kilohertz QPO and the absence of any significant simultaneous kilohertz continuum. (c) 2000 The American Astronomical Society.« less

Low frequency quasi-periodic oscillations in the hard X-ray emission from Cygnus X-1

The black-hole candidate Cyg X-1 was observed in a balloon flight experiment with large area Xenon-filled Multi-layer Proportional Counter (XMPC) telescope on April 5, 1992. The XMPC telescope sensitive in 20–100 keV range, observed Cyg X-1 in its hard state with an average counting rate of 51 counts per second for about 140 minutes. The timing analysis of the data show presence of Quasi-Periodic Oscillations (QPOs) in hard x-ray emission from the source. A broad QPO feature is seen clearly at a frequency of 0.06 Hz in the power density spectrum. Details of the results are presented and compared with reports of similar QPOs observed in Cyg X-1. The x-ray light curve also shows short time scale random intensity variations which are described by a shot noise model.

Low frequency quasi-periodic oscillations in the hard x-ray emission from cygnus x-1

The observations of the black hole binary Cygnus X-l were made in the energy band of 20–100keV with a balloon-borne Xenon-filled multiwire proportional counter telescope on 5th April 1992. Timing analysis of the data revealed the presence of Quasi-Periodic Oscillations (QPO) in the hard X-ray emission from the source. The QPO feature in the power density spectrum is broad with a peak at a frequency of 0.06 Hz. This result is compared with similar reports of QPOs in Cyg X-l in soft and hard X-rays. Short time scale random intensity variations in the X-ray light curve are described with a shot noise model.

CLC]k[/CLC]H[CLC]z[/CLC] Quasi-periodic Oscillation in Island State of 4U 1608−52 as Observed with [ITAL]RXTE[/ITAL]/PCA

We report RXTE/PCA observations of 4U 1608-52 on March 15, 18, and 22 immediately after the outburst in early 1996. The persistent count rates ranged from 190 to 450 counts s-1 (1-60 keV). During this period of time, 4U 1608-52 was in the island state. We detected quasi-periodic oscillation (QPO) features in the power density spectra (PDS) at 567-800 Hz on March 15 and 22, with source fractional root mean square (rms) amplitude of 13%-17% and widths of 78-180 Hz. The average rms amplitude of these QPO features is positively correlated with the energy. Our results imply that the neutron star spin frequency is possibly between 300 and 365 Hz.

The Main Characteristics of GRO J1744−28 Observed by the Proportional Counter Array Experiment on the [ITAL]Rossi X-Ray Timing Explorer[/ITAL

In this Letter, we provide an introduction to the main features seen in a series of observations of the bursting pulsar GRO J1744-28. The observations were made from 1996 January through May with the proportional counter array on the Rossi X-Ray Timing Explorer spacecraft. In the 2-10 keV band, GRO J1744-28 emitted large bursts of ~10 s duration at a rate of about 2 per hour. The peak flux during these bursts was ~6-40 times greater than that in the quiescent, or nonbursting, periods. For the earliest bursts, the inferred peak luminosity approaches 100 times the Eddington limit, which is suggestive of some kind of beaming mechanism. A range of smaller bursts and quasi-periodic oscillation features were also seen. All this activity was superposed on an almost perfect sinusoidal modulation at a frequency of 2.14 Hz with an amplitude of ~10% of the nonbursting flux. The source's persistent flux declined in a roughly linear trend from late January until mid-May, by which time its intensity was confused with several other sources.

X-Ray Timing and Spectral Evolution of Circinus X-1 versus Orbital Phase with the [ITAL]Rossi X-Ray Timing Explorer[/ITAL

We have carried out a study of Cir X-1 through (1) detailed sampling over a single 16.5 day intensity cycle with the Rossi X-Ray Timing Explorer (RXTE) proportional counter array and (2) complementary monitoring over six such cycles with the RXTE all-sky monitor (ASM). We report here the temporal and spectral evolution at nonflaring phases, during which Cir X-1 remained unusually bright (~1.0 crab) and relatively steady. The Fourier power density spectrum of the source was observed to vary with strong correlations among low-frequency flat-topped power (1-10 Hz), a quasi-periodic oscillation (QPO) peak centered at 1.3-12 Hz, and a broad QPO peak centered at ~20 Hz up to ~100 Hz. As orbital phase increased within the cycle, the rms amplitude of the flat-topped power generally (but not monotonically) decreased while the QPO features generally evolved toward higher frequency. The 1.5-12 keV spectrum was observed to generally harden during the nonflaring phases of each of the six 16.5 day periods monitored by the ASM.