Significant Quasi-periodic Oscillations Research Articles

Discovery of correlated optical/X-ray quasi-periodic oscillations in black hole binary SWIFT J1753.5–0127

We report the discovery of the correlated optical/X-ray low-frequency quasi-periodic oscillations (QPOs) in black hole binary SWIFT J1753.5-0127. The phase lag between two light-curves at the QPO frequency is close to zero. This result puts strong constraints on the nature of the optical emission in this object and on the origin of the QPOs in general. We demonstrate that the QPO signal and the broadband variability can be explained in terms of the hot accretion flow radiating in both optical and X-ray bands. In this model, the QPO appears due to the Lense-Thirring precession of entire flow, while the broadband variability in the optical is produced by two components: the hot flow and the irradiated disc. Using the phase-lag spectra, we put a lower limit on the orbital inclination i>50 deg, which can be used to constrain the mass of the compact object.

Searching for the trigger of the active galactic nucleus quasi-periodic oscillation: 8 years of RE J1034+396

RE J1034+396 is one of the most extreme Narrow-line Seyfert 1s detected thus far, showing the only quasi-periodic oscillation (QPO) reliably detected in an Active Galactic Nucleus (AGN). Comparison with similar spectral and timing properties observed in the black hole X-ray binary (BHB) GRS 1915+105 suggests that RE J1034+396 is a super-Eddington accretor. A more complete understanding of the behaviour of RE J1034+396 can therefore lead to a unification of the accretion physics between such extreme AGN and super-Eddington BHBs. Here we report on our latest XMM-Newton observations of RE J1034+396, which no longer show the QPO, indicating that this source shows a non-stationary power-spectrum. We use spectral and temporal analysis across all five XMM-Newton observations of the source to probe the evolution of the object. The combination of the shape of the fractional variability with energy and the inferred velocity of absorbing material in the line-of-sight rules out an absorption-only method of creating the QPO. Instead the periodically changing absorption may be produced by the QPO causing a change in ionization state. We extend our analysis by including the covariance spectra which give much better signal to noise than an rms spectrum. These reveal a new aspect of the QPO, which is that there is also a small contribution from a soft component which is hotter than the soft excess seen in the mean spectrum. Folding the lightcurve on the QPO period shows that this component lags behind the hard X-rays. If this is due to re-processing then the lag corresponding to a light travel time across 30 R_g. Some of the remaining observations have similar energy spectra and covariance spectra, but none of them show a significant QPO, so we conclude that none of these features are the trigger for the appearance of the QPO in this object.

Characteristics and causes of the extreme precipitation anomaly in Lake Poyang area

鄱阳湖区域4-7月极端降水总量存在显著的2-4年周期变化,且鄱阳湖4-7月极端降水总量的变率存在比较显著的增加趋势.1998年6月鄱阳湖地区极端降水总量异常偏多,而2001年6月异常偏少,仅为1998年的13.5%左右.通过分析大气环流场、水汽输送等方面的差异,发现1998年6月500hPa高度场上乌拉尔山高脊和鄂霍次克海高脊发展,贝加尔湖大槽加深,为鄱阳湖极端降水的产生提供了有利的环流背景,850hPa高度场上西太平洋副高增强,使得副高西北侧的西南气流增强北上与由贝加尔湖低槽引导南下的偏北风气流交绥于长江流域,并且增强的西南气流把大量的水汽带到鄱阳湖地区,同时对应高层南亚高压增强,从而使得鄱阳湖地区有充足的水汽来源并产生强烈的上升运动.而2001年6月500hPa高度场上中高纬地区环流经向发展,850hPa西太平洋副高位置相对偏东,水汽不易到达鄱阳湖地区,且贝加尔湖低槽减弱,使得北方冷空气不易到达鄱阳湖区域,同时对应高层南亚高压偏弱偏西,从而使得鄱阳湖地区上升运动减弱.;The extreme precipitation (EP) amount from April to July in Lake Poyang area exhibits significant quasi-periodic oscillations of 2-4 years, and its variance has remarkable ascendant trend. June EP amount in Lake Poyang area was abundant in 1998, while it decreased sharply in 2001, which only accounted for 13.5% of annual precipitation in 1998. Tlirough analyzing the difference of atmospheric circulation and moisture transport between the two periods, the results show that the developed high ridge near the Ural mountains and Okhotsk Sea and the stronger trough around Lake Baikal have offered a favorable circulation background for the EP in Lake Poyang area in June, 1998. Because of the strengthened Western Pacific subtropical high(WPSH), the stronger southwesterly in the northwest part of WPSH transported plentifiil moisture to the area, and it converged with the northerly around the Yangtze River valley. Meanwhile, the South Asia high was reinforced at upper layers, which led to strong ascending motion in Lake Poyang area. While the eastward shifted WPSH and ihe meridional developed circulation over the middle and high latitude at 500hPa reduced the moisture transport in Lake Poyang anea significantly in June, 2001. Moreover, the weakened north cold air from the trough around Lake Baikal was difficult to reach Lake Poyang area, and the weakened South Asia high decreased the ascending motion in the area.

Observations of the quiescent X-ray transients GRS 1124-684 (=GU Mus) and Cen X-4 (=V822 Cen) taken with ULTRACAM on the VLT

We present high time-resolution multicolour optical observations of the quiescent X-ray transients GRS1124-684 (=GU Mus) and Cen X-4 (=V822 Cen) obtained with ULTRACAM. Superimposed on the secondary stars' ellipsoidal modulation in both objects are large flares on time-scales of 30-60 min, as well as several distinct rapid flares on time-scales of a few minutes, most of which show further variability and unresolved structure. Not significant quasi-periodic oscillations are observed and the power density spectra of GRS1124-684 and Cen X-4 can be described by a power-law. From the colour-colour diagrams of the flare events, for GRS1124-684 we find that the flares can be described by hydrogen gas with a density of N_H~10^24 nucleons cm^-2, a temperature of ~8000 K and arising from a radius of ~0.3 Rsun. Finally we compile the values for the transition radius (the radius of the hot advection-dominated accretion flow) estimated from quasi-periodic oscillations and/or breaks in the power density spectrum for a variety of X-ray transients in different X-ray states. As expected, we find a strong correlation between the bolometric luminosity and the transition radius.

QUASI-PERIODIC OSCILLATIONS FROM RANDOM X-RAY BURSTS AROUND ROTATING BLACK HOLES

We continue our earlier studies of quasi-periodic oscillations (QPOs) in the power spectra of accreting, rapidly rotating black holes that originate from the geometric echoes of X-ray flares occurring within the black hole ergosphere. Our present work extends our previous treatment to three-dimensional photon emission and orbits to allow for arbitrary latitudes in the positions of the distant observers and the X-ray sources in place of the mainly equatorial positions and photon orbits of the earlier consideration. Following the trajectories of a large number of photons, we calculate the response functions of a given geometry and use them to produce model light curves which we subsequently analyze to compute their power spectra and autocorrelation functions. In the case of an optically thin environment, relevant to advection-dominated accretion flows, we consistently find QPOs at frequencies on the order of ~kHz for stellar-mass black hole candidates, while on the order of ~mHz for typical active galactic nuclei (~107 M ☉) for a wide range of viewing angles (30°-80°) from X-ray sources predominantly concentrated toward the equator within the ergosphere. As in our previous treatment, here too, the QPO signal is produced by the frame-dragging of the photons by the rapidly rotating black hole, which results in photon bunches separated by constant time lags, the result of multiple photon orbits around the hole. Our model predicts for various source/observer configurations the robust presence of a new class of QPOs, which is inevitably generic to a curved spacetime structure in rotating black hole systems.

High-frequency quasi-periodic oscillations from GRS 1915+105 in its C state

We report the results of a systematic timing analysis of RXTE observations of GRS 1915+105 when the source was in its variability class 0, characterized by alternating soft and hard states on a time-scale of a few hundred seconds. The aim was to examine the high-frequency part of the power spectrum in order to confirm the hectohertz quasi-periodic oscillations (QPO) previously reported from observations from mixed variability behaviours. During the hard intervals (corresponding to state C in the classification of Belloni et al.), we find a significant QPO at a frequency of ∼170 Hz, although much broader (Q∼2) than previously reported. No other significant peak is observed at frequencies >30 Hz. A time-resolved spectral analysis of selected observations shows that the hard intervals from class θ show a stronger and steeper (r = 2.8-3.0) power-law component than hard intervals from other classes. We discuss these results in the framework of hectohertz QPOs reported from GRS 1915+105 and other black hole binaries.

On the incidence of kilohertz quasi-periodic oscillations in the neutron star system 4U 1636−53

Nearly 1.32 x 10 6 s of non-zero count rate data were obtained from five years of Rossi X-ray Timing Explorer (RXTE) observations of the atoll source 4U 1636-53. The data were divided into 10 309 segments of 128 s each. The histogram of the number of segments as a function of count rate shows that the system can naturally be classified into four flux states. For each segment, an automated search for kilohertz quasi-periodic oscillations (QPOs) was undertaken and the histograms of the maximum Leahy normalized power P max for the four flux states were created. These were fitted by probability distribution functions under the assumption that the intrinsic amplitude of the QPO has a normal distribution. The best-fitting distribution functions showed that the fraction f of the time the system exhibits a kHz QPO decreases from near unity for the lowest flux state (1400 count s -1 ) to zero for the highest (>2550 count s -1 ), whereas the average amplitude remains nearly constant. Based on the best-fitting probability distribution functions, a threshold P thre was defined, such that 95 per cent of the segments with P max > P thre correspond to a real QPO signal. For segments selected by this criterion, the frequency versus count rate plot reproduced the earlier known 'parallel track' variation with the tracks coinciding roughly with the flux states. It is shown that the gaps between the tracks are not caused by uneven sampling, but rather the QPO phenomenon is absent or weak when the system flux level is intermediate between two flux states.

Simultaneous [ITAL]EXTREME ULTRAVIOLET EXPLORER[/ITAL][ITAL]Extreme Ultraviolet Explorer[/ITAL] and [ITAL]ROSSI X-RAY TIMING EXPLORER[/ITAL][ITAL]Rossi X-Ray Timing Explorer[/ITAL] Observations of AM Herculis

We present coordinated Extreme Ultraviolet Explorer (EUVE) and Rossi X-Ray Timing Explorer (RXTE) target of opportunity observations of the 1998 August high state of AM Herculis. The EUVE light curve sampled phases ~0.0–0.3 and ~0.5–0.8 and showed AM Her to be uncharacteristically bright during these phases but lacking the expected counts at phase ≈0.5, possibly because of absorption. The RXTE proportional counter array (PCA) 2–20 keV and HEXTE 15–60 keV light curves clearly show the 3.09 hr orbital modulation. No correlations between the soft and hard bands predicted in the simple one-pole accretion model were detected in the ≈1600 s of true, simultaneous EUVE and RXTE data. Quasi-periodic oscillations (QPOs) of 1–3 s found in optical data for several magnetic cataclysmic variables were not detected in PCA high time resolution data of AM Her with a 90% confidence upper limit for the rms amplitude of 3%. No significant QPOs were detected in the 10s–100s of seconds range with EUVE or RXTE. The EUVE spectrum was well fitted with a 19 eV blackbody. The phase-averaged PCA spectrum was well fitted with a 17.6 keV thermal bremsstrahlung (TB) continuum plus a broad iron line centered at 6.5 keV and absorption edge at 9.2 keV. HEXTE detections extended to 100 keV and preferred a highly absorbed TB continuum with temperature of 37 keV. The thermal bremsstrahlung plus emission-line (Eline = 6.7 keV) and edge model fit to PCA spectra as a function of phase could be described as a single TB temperature of 18.9 ± 0.2 keV with a variable column density of 7.1–53.5 × 1021 cm-2 and did not require more complex continuum models. The ratio of EUV to X-ray luminosity showed no soft excess and was more in accord with the simple one-pole model than previous observations.

Hard X‐Ray Variability of the Black Hole Candidate GRO J0422+32 during Its 1992 Outburst

We have studied the hard X-ray variability of the soft X-ray transient GRO J0422+32 with BATSE (Burst and Transient Source Experiment) in the 20-100 keV energy band. Our analysis covers 180 days following the first X-ray detection of the source on August 5, 1992, fully covering its primary and secondary X-ray outbursts. We compute power density spectra (PDSs) in the 20-50, 50-100, and 20-100 keV energy bands, in the frequency interval 0.002-0.488 Hz. The PDSs of GRO J0422+32 are approximately flat up to a break frequency, and decay as a power law above it, with index approximately 1. During the first 70 days of the X-ray outburst, the PDSs of GRO J0422+32 show a significant quasi-periodic oscillation (QPO) peak near approximately 0.2 Hz, superposed on the power-law tail. The break frequency of the PDSs obtained during the primary X-ray outburst of GRO J0422+32 occurs at 0.041 +/- 0.006 Hz; during the secondary outburst, the break is at 0.081 +/- 0.015 Hz. The power density at the break ranged between 44% and 89% / square root of Hz (20-100 keV). The canonical anticorrelation between the break frequency and the power density at the break, observed in Cyg X-1 and other BHCs in the low state, is not observed in the PDSs of GRO J0422+32. We compare our results with those of similar variability studies of Cyg X-1. The relation between the spectral slope and the amplitude of the X-ray variations of GRO J0422+32 is similar to that of Cyg X-1; however, the relation between the hard X-ray flux and the amplitude of its variation is opposite to what has been found in Cyg X-1. Phase lags between the X-ray flux variations of GRO J0422+32 at high and low photon energies could only be derived during the first 30 days of its outburst. During this period, the variations in the 50-100 keV band lag those in the 20-50 keV energy band by an approximately constant phase difference of 0.039(3) rad in the frequency interval 0.02-0.20 Hz. The time lags of GRO J0422+32 during the first 30 days of the outburst decrease with frequency as a power law with index 0.9 for v > 0.01 Hz.

Fast X‐Ray Oscillations: Probes of an Accreting Neutron Star

I have discovered fast X-ray oscillations near 1000 Hz in the flux of the low-mass X-ray binary 4U 06141091 in observations with NASA’s Rossi X-Ray Timing Explorer (RXTE). Such fast oscillations in this and other X-ray binaries (see M. van der Klis et al., Proc. NATO/ASI Ser., in press [1998]) are likely produced very close to the neutron star and provide constraints on the mass and radius of the neutron star and a measurement of its spin period. The quasi-periodic oscillation (QPO) signals are observed as peaks in the power density spectra (see Fig. 1). They are usually present in pairs, the higher frequency QPO appearing between 500 and 1145 Hz with the lower frequency QPO between 480 and 800 Hz. The difference in frequency between these two signals is constant at Hz (Ford, E. C., et 323 5 4 al., 1997, ApJ, 475, L123). The existence of two QPOs with a constant frequency separation is predicted by the “beat-frequency” model. The higher frequency QPO is modulated by the spinning neutron star, producing another QPO simultaneously at lower frequency. The higher frequency QPO signal may be produced by a modulation from gas in Keplerian orbital motion in the inner accretion disk. At the highest frequencies, or smallest inner disk radii, the QPOs are produced closest to the neutron star and serve as probes of this region of strong gravitational fields. Measurements of the fastest QPO frequencies provide upper limits to the mass and radius of the neutron star, since the oscillations must originate outside the radius of the marginally stable orbit predicted by general relativity (Kaaret, P., Ford, E. C., & Chen, K., 1997, ApJ, 482, L167). The upper limits for the neutron star mass and radius in 4U 06141091 are 2.1 M, and 19 km, respectively. I have observed a robust correlation between the frequency of the QPO and the flux of a blackbody component of the Xray spectrum (Ford, E. C., et al., 1997, ApJ, 486, L47). This correlation is explained if the higher frequency QPO is associated with the Keplerian motion at an inner disk edge. An increasing mass accretion rate, reflected in an increasing blackbody flux, causes the inner disk radius to shrink. As the inner disk radius shrinks, the QPO frequency increases. From the QPO frequencies, one can measure the spin period of the neutron star in the low-mass X-ray binary, one of the observational goals of the last two decades. The beat-frequency model predicts that the neutron star spin frequency is equal to the frequency difference between the two QPOs. In 4U 06141091, the frequency separation implies a neutron star spin period of 3.1 ms.

Discovery of Kilohertz Quasi-periodic Oscillations from 4U 1820−303 with [ITAL]Rossi X-Ray Timing Explorer[/ITAL

We have detected high-frequency (HF) quasi-periodic oscillations (QPOs) from the low-mass X-ray binary 4U 1820-303 during observations performed in 1996 October using the Rossi X-ray Timing Explorer. The QPOs are visible when the source occupies the low-state luminosity range LX = 2.4-3.1 × 1037 ergs s-1 (2-20 keV, at 6.4 kpc); the centroid frequency of the main QPO peak varies between 546 ± 2 Hz and 796 ± 6 Hz and is tightly correlated with the source count rate. The measured QPO widths are typically ~20 Hz, with mean rms amplitude 4.1% ± 0.3%. At the upper end of this luminosity range a second significant QPO peak appears with frequency 1065 ± 7 Hz, width 40 ± 20 Hz, and rms amplitude 3.2% ± 0.8%. When both QPOs are visible simultaneously, the difference between their frequencies is 275 ± 8 Hz. When the source brightens beyond LX = 3.1 × 1037 ergs s-1 (~10% of the Eddington limit for a helium-rich envelope), neither QPO is detected. Neither the magnetospheric beat frequency model nor the sonic point model of HF QPOs provides a perfect explanation of the phenomenology we observe. These results represent the first detection of kilohertz QPO activity in a globular cluster X-ray binary, and provide a new method of directly comparing the properties of cluster and noncluster neutron star binaries. If the highest QPO frequency we observe is identified with the marginally stable orbit in the accretion disk, the neutron star mass may be ~2 M☉, 35%-50% more massive than usually assumed. This may have consequences for the current evolutionary scenarios for this source and also for the debate about the evolution of millisecond pulsars in globular clusters.

Timescale Invariance of Rapid X-Ray Variability of the Black Hole Candidate GRO J1719−24

We present the results of an analysis of the time variability of the soft X-ray transient GRO J1719-24 (Nova Oph 1993), as observed with BATSE. Our analysis covers the entire ~80 day outburst, beginning with the first detection of this black hole candidate on 1993 September 25. We obtained power density spectra (PDSs) of the data in the 20-100 keV energy band, covering the frequency interval 0.002-0.488 Hz. The PDSs show a significant quasi-periodic oscillation peak, the centroid frequency of which increased from ~0.04 Hz at the onset of the outburst to ~0.3 Hz at the end. Additional noise is present in the PDSs, which we describe in terms of two components. We find that the evolution of the PDSs can be described as a gradual stretching by a factor of ~7.5 in frequency of the power spectrum, accompanied by a decrease of the power level by the same factor, such that the integrated power in a scaled frequency interval remains constant.

The discovery of 0.2 HZ quasi-periodic oscillations in the X-ray flux of the transient 42 second pulsar EXO 2030 + 375

view Abstract Citations (80) References (27) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Discovery of 0.2 HZ Quasi-periodic Oscillations in the X-Ray Flux of the Transient 42 Second Pulsar EXO 2030+374 Angelini, L. ; Stella, L. ; Parmar, A. N. Abstract The discovery of about 0.2 Hz quasi-periodic oscillations (QPOs) in the 1-15 keV X-ray flux of the transient 42 s pulsar EXO 2030 + 375 is reported. The detection of QPOs in the power spectra of this pulsar was made possible by the application of a newly developed technique to decouple the power spectrum peaks. QPOs were detected during an outburst, when the source was close to its maximum observed luminosity. Upper limits to the QPO strength during the rest of the outburst are consistent with the mean value observed of about 3.5 percent rms. During the outburst the QPOs centroid frequency decreased by about 12 percent, as the luminosity decreased by about 25 percent. The observed QPOs strength, frequency, and frequency-luminosity dependence are all in remarkable agreement with the predictions of the beat frequency model for QPOs and provide the first quantitative confirmation of it. Simple models in which the QPOs signal is produced close to the magnetospheric boundary are ruled out as only about 0.2 percent of the total accretion luminosity can be liberated at this distance from the neutron star surface. Publication: The Astrophysical Journal Pub Date: November 1989 DOI: 10.1086/168070 Bibcode: 1989ApJ...346..906A Keywords: Pulsars; Stellar Oscillations; X Ray Binaries; Accretion Disks; Astronomical Spectroscopy; Power Spectra; Stellar Luminosity; Astrophysics; PULSARS; STARS: ACCRETION; STARS: BINARIES; STARS: PULSATION; STARS: X-RAYS full text sources ADS | data products SIMBAD (1)

An overview of observations quasi-periodic oscillations in low mass X-ray binaries

In this review of some of the key observational of fast (≥- 1 H z) quasi-periodic oscillations (QPO) in low mass X-ray binaries are surveyed. These include the strength, colour and coherence of the QPO, and the shape of the QPO signal. Different QPO modes have been identified based on the range of QPO frequency, the QPO frequency - source intensity relation, the properties of the red noise, and the spectral state of the source. It is shown that the two classifications of QPO which have been proposed largely overlap.