Quasi-periodic Research Articles

A Study of the Long‐Term Evolution of Quasi‐Periodic Oscillations in the Accretion‐Powered X‐Ray Pulsar 4U 1626−67

We report here a study of the long term properties of Quasi Periodic Oscillations (QPO) in an unusual accreting X-ray pulsar, 4U 1626--67. This is a unique accretion powered X-ray pulsar in which we have found the QPOs to be present during all sufficiently long X-ray observations with a wide range of X-ray observatories. In the present spin-down era of this source, the QPO central frequency is found to be decreasing. In the earlier spin-up era of this source, there are only two reports of QPO detections, in 1983 with EXOSAT and 1988 with GINGA with an increasing trend. The QPO frequency evolution in 4U 1626--67 during the last 22 years changed from a positive to a negative trend, somewhat coincident with the torque reversal in this source. In the accretion powered X-ray pulsars, the QPO frequency is directly related to the inner radius of the accretion disk, as per Keplerian Frequency Model (KFM) and Beat Frequency Model (BFM). A gradual depletion of accretion disk is reported earlier from the X-ray spectral, flux and pulse profile measurements. The present QPO frequency evolution study shows that X-ray flux and mass accretion rate may not change by the same factor, hence the simple KFM and BFM are not able to explain the QPO evolution in this source. This is the only X-ray pulsar to show persistent QPOs and is also the first accreting X-ray pulsar in which the QPO history is reported for a long time scale relating it with the long term evolution of the accretion disk.

RECURRENT DIMENSIONS AND EXTENDED COMMON MULTIPLES OF QUASI-PERIODIC ORBITS GIVEN BY SOLUTIONS OF SECOND ORDER NONLINEAR EVOLUTION EQUATIONS

In our previous paper we introduced the sequence of Extended Common Multiples (abr. ECM) and the ECM condition for a pair of rationally independent irrational numbers to estimate the recurrent dimensions of quasiperiodic orbits. In this paper first we define the quasi-periodic properties for the functions, which have recurrent properties related with some ECM sequences. Next we study a second order nonlinear evolution equation with a quasi-periodic perturbation term, which gives an ECM sequence, and we show the existence of solutions, which have the quasi-periodic properties induced by this ECM sequence. Furthermore, we estimate the recurrent dimensions of the discrete orbits given by this solution for the case where the irrational frequencies of the q.p. pertubations satisfy the ECM condition.

Quasi periodicities of extreme precipitation events in the Yangtze River basin, China

Daily precipitation data of 148 measuring stations in the Yangtze River basin (P.R. China) for the period from 1961 to 2002 have been analyzed to detect quasi periodicities in the frequency of occurrence of extreme precipitation events. Extreme precipitation events of duration 1-day and 10-day were defined in terms of 90th percentile threshold exceedances for each station. The time series were standardized, averaged and detrended for each subbasin. Distinct temporal agglomerations of Gaussian peaks in the smoothed time series for all subbasins were detected for certain years. Quasi periodicities features in the time series were analyzed by means of spectrum analyses. Characteristic differences were found for the subbasins with typical significant quasi periodicities in the 2–3 and 3–4 year ranges. Wavelet analyses revealed that the stability of the observed quasi periodicities over time also varied between the subbasins. This results in various degrees of confidence that can be placed on the future extrapolation of Fourier emulations. Our extrapolations suggest occurrences of peaks in the frequencies of extreme precipitation events for most subbasins around the years 2012–2013 and 2017–2018 even though major differences between the subbasins are expected.

Observational evidence of coupling between quasi-periodic echoes and medium scale traveling ionospheric disturbances

Abstract. We have found that quasi-periodic (QP) echoes in the E region were well defined when medium scale traveling ionospheric disturbances (MSTIDs) in the F region were present. The appearance and disappearance of the MSTIDs observed with the dense GPS receiver network are well correlated with the development and decay of QP echoes observed with the Middle-and-Upper atmosphere (MU) radar. Interferometric imaging of the QP echoes obtained using the MU radar shows that bands of echoing regions aligned northwest to southeast drift southwestward, and their wavefront and propagation direction are the same as those of MSTIDs. This result confirms the expectation of Hysell et al. (2002) who observed band structures in QP echoes by using the MU radar and suggested their relation to MSTIDs. We found observational evidence that the midlatitude E- and F regions are coupled through the geomagnetic field line, although we could not clearly ascertain which of the two regions is the source.

Spatial distribution of random velocity fluctuations of 3‐m field‐aligned irregularities in ionospheric sporadic E layer

In this article we attempt to estimate the spatial distribution of the random velocity fluctuation of 3‐m field‐aligned irregularities (FAI) in sporadic E (Es) region, which were responsible for the quasi‐periodic (QP) echoes in range‐time‐intensity (RTI) plot. As previously shown by Chu and Wang (2003), beam broadening effect may be significant in spectral width measurements. However, using interferometry technique this effect could be removed by estimating the zonal drift velocity and the interferometry‐resolved zonal extent of the plasma irregularities. By applying this correction to 1 hour of data we found that the ratio of the beam broadening spectral width to the observed spectral width vary between 5 and 40% with a mean value of 14.2% and a standard deviation of 6.3%. After removing the beam‐broadening contribution from observed Doppler spectral width, the mean velocity fluctuations of the 3‐m plasma irregularities at a height of 105 km are in the range of 5 to 12 m/s. In addition, the spatial distribution of the FAI velocity fluctuation is nearly uniform in the echoing region over a span of 8° (corresponding to a horizontal extent of about 20 km) in geomagnetic zonal direction. The plausible mechanism responsible for the uniform distribution of the random velocity fluctuation of the 3‐m FAI in the nighttime Es region is discussed.

Simultaneous observations of nighttime medium‐scale traveling ionospheric disturbances and E region field‐aligned irregularities at midlatitude

We report on simultaneous observations of very high frequency (VHF) radar backscatter from field‐aligned irregularities (FAI) in the E region and medium‐scale traveling ionospheric disturbances (MSTID) in 630‐nm airglow images. On the night of 6 August 2002, the powerful 46.5‐MHz middle and upper atmosphere (MU) radar with five beam directions at Shigaraki, Japan, observed typical quasiperiodic (QP) echoes from FAI in the E region. During this QP echo event, an all‐sky charge‐coupled device imager at the MU radar site detected MSTID in 630‐nm airglow images. The MSTID also propagated southwestward at 80 m/s with fluctuation amplitudes of 50% from the background intensity and a wavelength of 380 km. Period of the MSTID was almost same as that of the Doppler velocity in the QP echoes. The airglow enhancement (depletion) caused by the MSTID coincided with southeastward (northwestward) velocity in the QP echo. These results indicate that the electric fields associated with the F region MSTID could be closely coupled to those associated with QP echoes in the E region.

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.

Noise correlations of fermions and hard core bosons in a quasi-periodic potential

We use Hanbury-Brown-Twiss interferometry (HBTI) to study various quantum phases of hard core bosons (HCBs) and ideal fermions confined in a one-dimensional quasi-periodic (QP) potential. For HCBs, the QP potential induces a cascade of Mott-like band-insulator phases in the extended regime, in addition to the Mott insulator, Bose glass, and superfluid phases. At critical filling factors, the appearance of these insulating phases is heralded by a peak to dip transition in the interferogram, which reflects the fermionic aspect of HCBs. On the other hand, ideal fermions in the extended phase display various complexities of incommensurate structures such as devil’s staircases and Arnold tongues. In the localized phase, the HCB and the fermion correlations are identical except for the sign of the peaks. Finally, we demonstrate that HBTI provides an effective method to distinguish Mott and glassy phases.

QUASI-PERIODIC OSCILLATIONS AND FREQUENCIES IN AN ACCRETION DISK AND COMPARISON WITH THE NUMERICAL RESULTS FROM NON-ROTATING BLACK HOLE COMPUTED BY THE GRH CODE

The shocked wave created on the accretion disk after different physical phenomena (accretion flows with pressure gradients, star-disk interaction etc.) may be responsible observed Quasi Periodic Oscillations (QPOs) in X-ray binaries. We present the set of characteristics frequencies associated with accretion disk around the rotating and non-rotating black holes for one particle case. These persistent frequencies are results of the rotating pattern in an accretion disk. We compare the frequency's from two different numerical results for fluid flow around the non-rotating black hole with one particle case. The numerical results are taken from Refs. 1 and 2 using fully general relativistic hydrodynamical code with non-selfgravitating disk. While the first numerical result has a relativistic tori around the black hole, the second one includes one-armed spiral shock wave produced from star-disk interaction. Some physical modes presented in the QPOs can be excited in numerical simulation of relativistic tori and spiral waves on the accretion disk. The results of these different dynamical structures on the accretion disk responsible for QPOs are discussed in detail.

雑音に乱される準周期生体信号から周期を推定するための観測器

雑音に乱される準周期生体信号から周期を推定するための観測器

Understanding AXPs and SGRs through the December 2004 SGR 1806–20's Hyperflare

On 27th December 2004 SGR 1806?20, one of the most active Soft ?-ray Repeaters (SGRs), displayed an extremely rare event, also known as giant flare, during which up to 1047 erg were released in the ~1?1000 keV range in less than 1s. Follow-up VLA observations after the giant flare largely increased the position accuracy of SGR 1806?20, allowing to perform sensitive searches at shorter wavelengths. In particular we carried out observations by using IR adaptive optics (NAOS-CONICA) mounted on VLT which provided images of unprecedented quality (FWHM better than 0.1''). We discovered the likely IR counterpart to SGR 1806?20 based on positional coincidence with the VLA uncertainty region and flux variability of a factor of about 2 correlated with that at higher energies. Among the several high energy datasets obtained during the 27th December 2004 event, we analyzed those taken from the Rossi?XTE which provided the highest ever statistics for a giant flare. We discovered rapid Quasi Periodic Oscillations (QPOs) in RXTE/PCA data of the pulsating tail of the 27th December 2004 giant flare. QPOs at ~92.5 Hz are detected in a 50 s interval starting 170 s after the onset of the giant flare. This is the first time that QPOs are unambiguously detected in the flux of a Soft Gamma-ray Repeater, or any other magnetar candidate. We interpret the highest QPOs in terms of the coupling of toroidal seismic modes with Alfv?n waves propagating along magnetospheric field lines.

Plasma structures responsible for sporadic E region quasi-periodic echoes

We used the interferometry technique to reconstruct the spatial structures of 3-m field-aligned irregularities (FAI) in the nighttime sporadic E (Es) layer responsible for quasi-periodic (QP) echoes. We found that the scattering regions that give rise to the QP striations with positive range rates occurred in patchy patterns with zonal and vertical extents of approximately 5–20 and 3–8 km, respectively. These echo patterns appeared quasi-periodically with periods of 2–6 min and moved horizontally from southeast to northwest at an approximately constant height. One of the important findings of this study is that the zonal trace velocities of the kilometer-scale plasma structures were approximately the same as the zonal drift velocities of the 3-m FAI. The former were deduced from the temporal displacement of the echo patterns on the horizontal plane, and the latter were estimated from the observed Doppler velocities of the 3-m FAI combined with the angular positions resolved by the interferometry technique. This feature strongly implies that the small-scale FAI were “frozen” within the large-scale plasma structures responsible for the QP echoes. On the basis of this frozen-in property, the meridional component of the drift velocity of the plasma structures can be obtained from that of the 3-m FAI, irrespective of the limitations of the extraordinarily narrow width of the expected echoing region in the north–south direction. Our results show that the maximum drift velocity of the plasma structures is as large as approximately 135 m/s in the direction from southeast to northwest. On the basis of the long dwelling time (greater than 6 min) and small horizontal dimension (average of approximately 10 km) of the QP scattering regions, we speculate that the configuration of the large-scale plasma structures responsible for the QP echoes is likely to be a horizontally elongate shape whose major axes are primarily aligned in the meridional direction with respect to geomagnetic north.

Commensurability Effects in Superconducting Nb Films with Quasiperiodic Pinning Arrays

We study experimentally the critical depinning current I(c) versus applied magnetic field B in Nb thin films which contain 2D arrays of circular antidots placed on the nodes of quasiperiodic (QP) fivefold Penrose lattices. Close to the transition temperature T(c) we observe matching of the vortex lattice with the QP pinning array, confirming essential features in the I(c)(B) patterns as predicted by Misko et al. [Phys. Rev. Lett. 95, 177007 (2005)]. We find a significant enhancement in I(c)(B) for QP pinning arrays in comparison to I(c) in samples with randomly distributed antidots or no antidots.

Critical currents in superconductors with quasiperiodic pinning arrays: One-dimensional chains and two-dimensional Penrose lattices

We study the critical depinning current J_c, as a function of the applied magnetic flux Phi, for quasiperiodic (QP) pinning arrays, including one-dimensional (1D) chains and two-dimensional (2D) arrays of pinning centers placed on the nodes of a five-fold Penrose lattice. In 1D QP chains of pinning sites, the peaks in J_c(Phi) are shown to be determined by a sequence of harmonics of long and short periods of the chain. This sequence includes as a subset the sequence of successive Fibonacci numbers. We also analyze the evolution of J_c(Phi) while a continuous transition occurs from a periodic lattice of pinning centers to a QP one; the continuous transition is achieved by varying the ratio gamma = a_S/a_L of lengths of the short a_S and the long a_L segments, starting from gamma = 1 for a periodic sequence. We find that the peaks related to the Fibonacci sequence are most pronounced when gamma is equal to the golden mean. The critical current J_c(Phi) in QP lattice has a remarkable self-similarity. This effect is demonstrated both in real space and in reciprocal k-space. In 2D QP pinning arrays (e.g., Penrose lattices), the pinning of vortices is related to matching conditions between the vortex lattice and the QP lattice of pinning centers. Although more subtle to analyze than in 1D pinning chains, the structure in J_c(Phi) is determined by the presence of two different kinds of elements forming the 2D QP lattice. Indeed, we predict analytically and numerically the main features of J_c(Phi) for Penrose lattices. Comparing the J_c's for QP (Penrose), periodic (triangular) and random arrays of pinning sites, we have found that the QP lattice provides an unusually broad critical current J_c(Phi), that could be useful for practical applications demanding high J_c's over a wide range of fields.

Observation of three‐dimensional structures of quasi‐periodic echoes associated with mid‐latitude sporadic‐E layers by MU radar ultra‐multi‐channel system

Quasi‐periodic (QP) backscatter observed by VHF radars associated with the mid‐latitude Sporadic‐E (Es) layers is characterized by distinct striations on range‐time‐intensity (RTI) plots. Two competing models claim to explain the structure of unstable regions that scatter the radar waves: horizontally drifting patches at an almost constant altitude and unstable regions elongated in altitude along the geomagnetic field line. We have conducted interferometric imaging observations of QP radar echoes to investigate spatial structures of QP echoes, precisely. Kyoto University's newly developed ultra‐multi‐channel receiving system of middle and upper atmosphere (MU) radar was used. We used 19 independent channels for the radar imaging, and determined the three‐dimensional structure and the motion of the QP echoes. During the observation from 30 May to 02 June 2005, well‐defined QP echoes were observed on the nights of 31 May, 01 June, and 02 June 2005. Some of QP echoes were found at altitudes higher than 120 km and appeared to descend in altitude as they approached the radar. This result suggests that backscatter regions are developed along the geomagnetic field line from Es layer altitudes to as high as 130 km and that the fluctuations in plasma density and electric field observed by Pfaff et al. (2005) using in‐situ measurements form a part of QP echoes.

Quantum coherence of hard-core bosons: Extended, glassy, and Mott phases

Quantum phases of hard core bosons (HCBs) confined in a one-dimensional quasiperiodic (QP) potential are studied within the theoretical framework of Hanbury-Brown-Twiss interferometry. The QP potential induces a cascade of Mott-like band-insulator phases in the extended regime, in addition to the Mott insulator, Bose glass, and superfluid phases. The new phases are incompressible and have zero superfluid fraction. At critical filling factors, the appearance of these insulating phases is heralded by a peak to dip transition in the interferogram, which reflects the fermionic aspect of HCBs. In the localized phase, the interference pattern exhibits an hierarchy of peaks at the reciprocal lattice vectors of the system. Our study demonstrates that in contrast to the momentum distribution, HBTI provides an effective method to distinguish Mott and glassy phases.

Characteristics and implications of Doppler spectra of E region quasi‐periodic echoes observed by the multibeam middle and upper atmosphere radar

We present multibeam middle and upper atmosphere (MU) radar observations of quasi‐periodic (QP) echoes from 3.2‐m field‐aligned irregularities associated with plasma patches in the nighttime midlatitude sporadic E (Es) layers over Shigaraki, Japan, to give a cohesive view of the QP echo characteristics. Echo regions with zonal scales less than 50 km moved westward at altitudes near 105 km with trace velocities of 110–120 m s−1 and crossed the radar beams successively. Doppler spectra of the echoes changed in time, range, and azimuth and exhibited mostly type 2 echoes with Doppler velocities between −200 and +200 m s−1 and occasionally type 1 echoes due to the two‐stream instability (“the first type 1 echo detection by the MU radar”). We find that the spectra near the middle of the drifting echo region have two type 2 peaks, one at positive Doppler velocity (motion away from the radar) and the other at negative one (motion toward the radar) and that the spectra in the west and east parts of the echo region have single type 2 peak at positive and negative Doppler velocities, respectively. Interestingly, the type 1 echoes lasting for about 4 min on a fixed beam were located near the middle of the echo region. Their Doppler shifts between 250 and 320 m s−1 are very consistent with those from previous type 1 echo observations at other midlatitude locations. Occasionally, type 1 and type 2 echoes coexisted in the same range gates. On the basis of recent observations, theories, and simulations related to QP echoes, we propose a simple model of an Es plasma patch to explain the observations. In the model, polarization electric fields with opposite polarity inside and outside the patch, elongated along the NW‐SE direction, play a predominant role in exciting type 1 and type 2 echoes, and cause a variety of Doppler spectra.

Planetary wave trends in the lower thermosphere—Evidence for 22-year solar modulation of the quasi 5-day wave

Geomagnetic data from five northern hemisphere observatories at latitudes ranging from 22°N to 67°N have been used to investigate long-term trends in planetary wave activity in the lower thermosphere. The data extends back over eight 11-year solar cycles. At quasi 10- and 16-day periods, there is no long-term change in planetary wave activity through the 20th century; decadal-scale variability reaching 25% peak-to-peak is observed. At the quasi 5-day period there is a steady decrease in activity totalling 15% through the 20th century. Additionally, there is a Hale ‘22-year’ solar cycle modulation of ∼20% peak-to-peak in the 5-day planetary wave activity at 22°N which decreases in amplitude with increasing latitude. It is not clear what causative mechanism is responsible for this modulation which shows signs of breaking down at the very end of the century.

Rotationally driven quasi‐periodic radio emissions in the Jovian magnetosphere

The occurrence characteristics of Jovian multiple quasi‐periodic (QP) bursts are investigated based on Galileo spacecraft observations. Multiple QP bursts recurrently appeared in a group with a planetary spin period of 10 hours. Their appearance was synchronized with the planetary rotation, i.e., they were preferably excited when Jupiter had a particular spin phase angle with respect to the Sun (at a subsolar longitude of System III around 260–320°). We also found that the burst groups were activated during magnetospheric disturbances probably caused by solar wind pressure variations. The rotationally driven polar disturbances accompanying quasi‐periodic particle accelerations and radio wave bursts are discussed as being one of the daily unloading processes of planetary rotational energy.

Simultaneous observation of sporadic E with a rapid-run ionosonde and VHF coherent backscatter radar

Abstract. During the SEEK 2 rocket campaign, ionograms were recorded every minute at the Yamagawa Radio Observatory at about 90km west of the region monitored by a VHF (very high frequency) coherent backscatter radar. Sporadic E-layer parameters, which include the critical (foEs) and blanketing (fbEs) frequencies, the layer height (h'Es), and the width of the range spread of sporadic E-traces, were compared with RTI (range-time-intensity) plots of VHF quasi-periodic (QP) and continuous coherent backscatter echoes. A close relationship was found between the appearance of QP echoes in the RTI plots and the level of spatial inhomogeneity in sporadic E plasma, signified here by the difference between foEs and fbEs. During QP echo events, foEs increased while fbEs decreased, so that the difference foEs-fbEs was enhanced, indicating the development of strong spatial structuring in electron density within a sporadic E-layer. On the other hand, increases in sporadic E range spreading also correlated with the occurrence of QP echoes but the degree of correlation varied from event to event. Continuous radar echoes were observed in association with low altitude sporadic E-layers, located well below 100 km and at times as low as 90 km. During the continuous echo events, both foEs and fbEs were less variable, and the difference foEs-fbEs was small and not as dynamic as in the QP echoes. On the other hand, the Es-layer spread intensified during continuous echoes, which means that some patchiness or corrugation in those low altitude layers is also necessary for the continuous backscatter echoes to take place.