Aperiodic Variability Research Articles

Probing the Structure of Accreting Compact Sources through X‐Ray Time Lags and Spectra

We exhibit, by compiling all data sets we can acquire, that the Fourier-frequency-dependent, hard X-ray lags, first observed in the analysis of aperiodic variability of the light curves of the black hole candidate Cyg X-1, appear to be a property shared by several other accreting black hole candidate sources and also by the different spectral states of this source. We then present both analytic and numerical models of these time lags resulting by the process of Comptonization in a variety of hot electron configurations. We argue that, under the assumption that the observed spectra are due to Comptonization, the dependence of the lags on the Fourier period provides a means for mapping the spatial density profile of the hot electron plasma, while the period at which the lags eventually level off provides an estimate of the size of the scattering cloud. We further examine the influence of the location and spatial extent of the soft photon source on the form of the resulting lags for a variety of configurations; we conclude that the study of the X-ray hard lags can provide clues about these parameters of the Comptonization process, too. Fits of the existing data with our models indicate that the size of the Comptonizing clouds are quite large in extent (~1 lt-s) with inferred radial density profiles that are in many instances inconsistent with those of the standard dynamical models, whereas the extent of the source of soft photons appears to be much smaller than those of the hot electrons by roughly 2 orders of magnitude and its location consistent with the center of the hot electron corona.

A New Model for the Hard Time Lags in Black Hole X-Ray Binaries

The time-dependent Comptonized output of a cool soft X-ray source drifting inward through an inhomogeneous hot inner disk or corona is numerically simulated. We propose that this scenario can explain from first principles the observed trends in the hard time lags and power spectra of the rapid aperiodic variability of the X-ray emission of Galactic black hole candidates.

Strong Aperiodic X-Ray Variability and Quasi-Periodic Oscillationin X-Ray Nova XTE J1550−564

We report the discovery of strong aperiodic X-ray variability and quasi-periodic oscillation (QPO) in the Xray light curves of a new X-ray nova, XTE J15502564, and the evolution of the observed temporal properties during the rise of the recent X-ray outburst. The power spectral analysis of the first observation reveals strong aperiodic X-ray variability of the source (»28%) as well as the presence of a QPO at »82 mHz with fractional rms amplitude »14% over the 2‐60 keV energy range. Also apparent is the first harmonic of the QPO with the amplitude »9%. As the X-ray flux increases, the source tends to become less variable, and the QPO frequency increases rapidly, from 82 mHz to 4 Hz, over the flux (2‐50 keV) range of ergs cm 22 s 21 . 28 1.73‐5.75 # 10 The amplitude of the fundamental component of the QPO varies little, while that of the harmonic follows a decreasing trend. The fundamental component strengthens toward high energies, while its harmonic weakens. Initially, the power spectrum is roughly flat at low frequencies and turns into a power law at high frequencies, with the QPO harmonic sitting roughly at the break. In later observations, however, the high-frequency portion of the continuum can actually be better described by a broken power law (as opposed to a simple power law). This effect becomes more apparent at higher energies. The overall amplitude of the continuum shows a similar energy dependence to that of the fundamental component of the QPO. Strong rapid X-ray variability, as well as a hard energy spectrum, makes XTE J15502564 a good black hole candidate. We compare its temporal properties with those of other black hole candidates.

Short-term X-ray variability of black hole candidates: Ginga results

The superposition technique is a new analysis method to obtain average properties of certain kinds of aperiodic time variations. Real average profiles of X-ray shots in Cyg X-1, GX 339-4, GS 2023+338 and GS 1124-68 in the hard state are obtained in detail by applying the technique to rapid time variations of these sources observed by Ginga. The shot profiles obtained are consistent with structures of power spectral densities and phase (time) lags below a few Hz. Soft energy spectra of the X-ray shots are thought to be the origin of soft excess components in time-averaged energy spectra. The shot profiles imply that the X-ray shots arise from aperiodic mass accretion in the advection dominated disk. These results are briefly described.

Modeling the time variability of black hole candidates.

We present model light curves for accreting black hole candidates (BHC) based on a recently developed model of these sources. According to this model, the observed light curves and aperiodic variability of BHC are due to a series of soft photon injections at random (Poisson) intervals and the stochastic nature of the Comptonization process in converting these soft photons to the observed high energy radiation. The additional assumption of our model is that the Comptonization process takes place in an extended but non-uniform hot plasma corona surrounding the compact object. We compute the corresponding power spectral densities (PSD), autocorrelation functions, time skewness of the light curves, and time lags between the light curves of the sources at different photon energies and compare our results to observation. Our model reproduces the observed light curves well, in that it provides good fits to their overall morphology (as manifest by the autocorrelation and time skewness) and also to their PSDs and time lags, by producing most of the variability power at time scales approximately > a few seconds, while at the same time allowing for shots of a few msec in duration, in accordance with observation. We suggest that refinement of this type of model along with spectral and phase lag information can be used to probe the structure of this class of high energy sources.

The Broadband Power Spectra of X‐Ray Binaries

We analyze the rapid, aperiodic X-ray variability of different types of X-ray binaries (black hole candidates, atoll sources, the recently discovered millisecond X-ray pulsar, and Z sources) at their lowest inferred mass accretion rates. At these accretion rates, the power spectra of all sources are dominated by a strong band-limited noise component, which follows a power law with an index of roughly 1 at high frequencies and breaks at a frequency between 0.02 and 32 Hz, below which the spectrum is relatively flat. Superimposed on this, a broad bump (sometimes a quasi-periodic oscillation) is present, with a 0.2-67 Hz centroid frequency that varies in good correlation with the frequency of the break. The black hole candidates and the low-luminosity neutron star systems (including the millisecond X-ray pulsar) have the same relation between the frequency of the bump and the frequency of the break. These similar characteristics strongly suggest that in all those different types of sources, the band-limited noise and the bump are produced by the same physical mechanism. This mechanism cannot then depend on the presence or absence of either a small magnetosphere or a solid surface, so that it is most likely related to an instability in the flow in the accretion disk that modulates the accretion rate. The Z sources, which are more luminous than the other sources discussed here, follow a similar, but slightly shifted correlation between the break frequency and the frequency of the bump. The data suggest that the band-limited noise in Z sources is more complex than that in the other sources.

The Broadband Power Spectrum of SAX J1808.4−3658

We analyzed the rapid aperiodic X-ray variability of the recently discovered millisecond X-ray pulsar SAX J1808.4-3658. The power density spectrum is dominated by a strong band-limited noise component, which follows a power law with index 1.0-1.3 at high frequencies with a break that varies between 0.25 and 1.6 Hz, below which the spectrum is relatively flat. Superimposed on this, a broad bump is present with a centroid frequency that varies well correlated with the break frequency between 2.4 and 12.0 Hz. These characteristics are very similar to what is commonly seen in other low-luminosity low-mass X-ray binaries. Between 100 and 400 Hz, a third broad noise component is present similar to that recently reported in the low-luminosity low-mass X-ray binary 4U 1728-34 (Ford & van der Klis). We find a similar high-frequency noise component also in other low-luminosity neutron star systems. We conclude that at any given epoch, the rapid aperiodic X-ray variability of the millisecond X-ray pulsar is indistinguishable from that of other low-luminosity neutron star systems. However, contrary to what has been found in those sources, the break frequency of the band-limited noise does not have a strict correlation with mass accretion rate. With decreasing mass accretion rate, the break frequency first decreased, then increased again. This previously unobserved behavior could be typical for the very low mass accretion rates we observe near the end of the outburst of SAX J1808.4-3658, or it could be related to its being a pulsar.

A New Instability of Accretion Disks around Magnetic Compact Stars

Aperiodic variability and quasi-periodic oscillations (QPOs) are observed from accretion disks orbiting white dwarfs, neutron stars, and black holes, suggesting that the flow is universally broken up into discrete blobs. We consider the interaction of these blobs with the magnetic field of a compact, accreting star, where diamagnetic blobs suffer a drag. We show that when the magnetic moment is not aligned with the spin axis, the resulting force is pulsed, and this can lead to resonance with the oscillation of the blobs around the equatorial plane; a resonance condition where energy is effectively pumped into nonequatorial motions is then derived. We show that the same resonance condition applies for the quadrupolar component of the magnetic field. We discuss the conditions of applicability of this result, showing that they are quite wide. We also show that realistic complications, such as chaotic magnetic fields, buoyancy, radiation pressure, evaporation, Kelvin-Helmholtz instability, and shear stresses due to differential rotation do not affect our results. In accreting neutron stars with millisecond periods, we show that this instability leads to Lense-Thirring precession of the blobs and that damping by viscosity can be neglected.

Monsoons: Processes, predictability, and the prospects for prediction

The Tropical Ocean‐Global Atmosphere (TOGA) program sought to determine the predictability of the coupled ocean‐atmosphere system. The World Climate Research Programme's (WCRP) Global Ocean‐Atmosphere‐Land System (GOALS) program seeks to explore predictability of the global climate system through investigation of the major planetary heat sources and sinks, and interactions between them. The Asian‐Australian monsoon system, which undergoes aperiodic and high amplitude variations on intraseasonal, annual, biennial and interannual timescales is a major focus of GOALS. Empirical seasonal forecasts of the monsoon have been made with moderate success for over 100 years. More recent modeling efforts have not been successful. Even simulation of the mean structure of the Asian monsoon has proven elusive and the observed ENSO‐monsoon relationships has been difficult to replicate. Divergence in simulation skill occurs between integrations by different models or between members of ensembles of the same model. This degree of spread is surprising given the relative success of empirical forecast techniques. Two possible explanations are presented: difficulty in modeling the monsoon regions and nonlinear error growth due to regional hydrodynamical instabilities. It is argued that the reconciliation of these explanations is imperative for prediction of the monsoon to be improved. To this end, a thorough description of observed monsoon variability and the physical processes that are thought to be important is presented. Prospects of improving prediction and some strategies that may help achieve improvement are discussed.

Optical Variability in Active Galactic Nuclei: Starbursts or Disk Instabilities?

Aperiodic optical variability is a common property of active galactic nuclei (AGNs), though its physical origin is still open to question. To study the origin of the optical-ultraviolet variability in AGNs, we compare light curves of two models to observations of quasar 0957+561 in terms of a structure function analysis. In the starburst (SB) model, random superposition of supernovae in the nuclear starburst region produces aperiodic luminosity variations, while in the disk-instability (DI) model, variability is caused by instabilities in the accretion disk around a supermassive black hole. We calculate fluctuating light curves and structure functions, V(τ), by simple Monte Carlo simulations on the basis of the two models. Each resultant V(τ) possesses a power-law portion, [V(τ)]1/2 ∝ τβ, at short time lags (τ). The two models can be distinguished by the logarithmic slope β; β ~ 0.74-0.90 in the SB model and β ~ 0.41-0.49 in the DI model, while the observed light curves exhibit β ~ 0.35. Therefore, we conclude that the DI model is favored over the SB model in explaining the slopes of the observational structure function in the case of 0957+561, though this object is a radio-loud object and thus is not really a fair test for the SB model. In addition, we examine the time asymmetry of the light curves by calculating V(τ) separately for the brightening and the decaying phases. The two models exhibit opposite trends of time asymmetry to some extent, although the present observation is not long enough to test this prediction.

Comptonization Signatures in the Rapid Aperiodic Variability of Galactic Black Hole Candidates

We investigate the effect of Compton scattering of flares of soft radiation in different geometries of a hot, Comptonizing region and a colder accretion disk around a solar mass black hole. The photon energy-dependent light curves, their Fourier transforms, power spectra, and Fourier period-dependent time lags of hard photons with respect to softer photons are discussed. On the basis of a comparison with existing data we find arguments against Comptonization of external soft radiation as well as Comptonization in a homogeneous medium as dominant mechanisms for the rapid aperiodic variability in Galactic black hole candidates. Possible further observational tests for the influence of Comptonization on the rapid aperiodic variability of Galactic black hole candidates are suggested.

A survey of natural electromagnetic noise in the frequency range f = 1–10 kHz at Halley station, Antarctica: 1. Radio atmospherics from lightning

This paper presents results from the first systematic survey of VLF wave activity at Halley, Antarctica (76 °S, 27 °W, L = 4.3). Beginning in 1971, the peak, average and minimum (P, A, M) signal levels observed in four frequency bands centred on 0.75 kHz, 1.25 kHz, 3.2 kHz and 9.6 kHz have been recorded every 5 min. At these frequencies the observed radio noise is largely natural, the waves being generated either in the magnetosphere (e.g. chorus, hiss, etc.) or near the ground, the latter principally from lightning discharges (radio atmospherics, or “spherics”) which reach the receiver after propagating some distance in the Earth-ionosphere waveguide (lightning does not occur in the immediate vicinity of Halley). Here we analyse the observations for 1984, the first complete year for which we have data in digital form, in terms of thunderstorm regions, as a benchmark for more extended studies of possible long-term change in global lightning activity. The data are presented in compressed colour graphic format which facilitates the identification of periodic (diurnal and annual) and aperiodic variations. At 3.2 kHz, attenuation in the Earth-ionosphere waveguide is severe, and only relatively few spherics, from close lighting source regions, are observed. Thus, whilst the 3.2 M channel is insensitive to lightning, and responds mostly to magnetospheric emissions, the 3.2 P channel is dominated by spherics. The 3.2 P data show a marked diurnal and seasonal variation symmetrical about Halley local noon and about the solstices, consistent with nearby sources and attenuation rates for subionospheric propagation which are much greater during the day than at night. At 9.6 kHz, waveguide attenuation is much lower (and there is less difference between day and night), and the minimum channel is dominated by a continuum of spheric noise originating from globally distributed distant source regions, notably those in the tropics. Consequently, there is no control by the local dawn-dusk terminator; the diurnal and seasonal variation is not symmetrical about Halley local noon and the solstices but consists of a quasi-sinusoidal diurnal variation, in which the phases of the minimum and maximum vary during the year: ~07 LT (LT∼-UT−2h at Halley) and ~17 LT in December (summer) and ~10 LT and ~21 LT in June (winter). Agreement between the observations and the CCIR (1983) empirical model is poor. A somewhat better fit is given by a simple model in which thunderstorm regions consist of point sources having radiated powers which vary with local time and season, the total effect at Halley being modelled as the sum of contributions from these sources.

Optical Variability in AGNs; Disk Instability or Starbursts?

Aperiodic optical variability is a common property of Active Galactic Nuclei (AGN), though its physical origin is still open to question. We have compared light curves among the following two models and observation of quasar 0957+561A,B (Kundić et al. 1997) in terms of structure function analysis (§2).

ANTIOXIDATIVE PROTECTION OF PERIDINIUM GATUNENSE IN LAKE KINNERET: SEASONAL AND DAILY VARIATION1

ABSTRACTA comprehensive antioxidative mechanism was found in the freshwater dinoflagellate Peridinium gatunense Lemm. during the spring bloom in Lake Kinneret. Ascorbate was present throughout the bloom period and was responsible, together with catalase, for the elimination of photosynthetically produced H2O2. As glutathione concentrations and ascorbate regenerative enzymes were negligible during mid‐spring, ascorbate was presumably biosynthesized during the photosynthetically active period. Antioxidative activity increased overall at the end of the spring in conjunction with elevated ambient stress conditions, for example high light. Under such circumstances, ascorbate was regenerated. Ascorbate levels doubled when cells were exposed to an increase in irradiance from 60 to 600 μmol photons·m−2·s−1, and on addition of H2O2, concentrations increased a further 20‐fold. Significant antioxidative activity was also noted in the dark, although this was dependent on the presence of H2O2. Diurnal changes in antioxidants and their regenerative enzymes were observed. The activities of mono‐dehydroascorbate reductase, glutathione reductase, and ascorbate concentrations showed ultraradian periodicity and were completely in phase throughout the day/night period. Dehydroascorbate reductase activity and glutathione concentrations were also in phase but showed aperiodic variation, as did ascorbate peroxidase activity. Superoxide dismutase and catalase activities were generally out of phase during the 24‐h period but did show ultraradian periodicity. Lake samples entrained under constant light revealed an inate 12‐h rhythm for catalase activity, during at least 36 h.

16 Second Optical Quasi‐Periodic Oscillations in GX 339−4

We report the results of high-speed optical white-light photometry of the black hole candidate GX 339-4, obtained at the Cerro Tololo Inter-American Observatory during 1996 April. We searched the data for strictly periodic features, quasi-periodic oscillations (QPOs), and aperiodic variability (noise) using Fourier and wavelet techniques. (1) We found a QPO with amplitude 4.5%-6%, frequency f ~ 0.064 Hz, and width Δf/f ~ 0.3-0.5. The QPO was long-lived in that it was present in all segments of the data, even in sets acquired one day apart. (2) In addition to the 0.064 Hz QPO there are indications of low-amplitude QPOs at f = 0.02, 0.03, 0.08, 0.3, and 3 Hz. None of the features appeared in more than one day of data (except for the 0.08 Hz feature) and none of the features were as strong as the 0.064 Hz QPO, and so they should be taken with some caution. (3) We found power in excess of counting statistics out to frequencies of several hertz on both nights. The origin of this excess power is not clear.

Harmonic Coupling of the Red Noise in X‐Ray Pulsars

The power spectra of X-ray pulsars often show the presence of a red-noise component. This noise is produced by aperiodic variability believed to be associated with instabilities that seem to occur in accretion flows onto compact objects. In this paper we discuss how, independently of the details of the physical processes that generate these instabilities, a careful analysis of the power spectra can furnish some constraints on the distance from the stellar surface at which the sudden energy release associated with the instabilities occurs. In particular, any aperiodic variability coming from the accretion flow funneled toward the magnetic poles should be modulated at the pulsar spin period (coupling). We show how, in the power spectra, this coupling results in a broadening at the base of the harmonics. To investigate this effect, we have adopted a mathematical description of the noise in order to produce simulated light curves and the resulting power spectra. A comparison of power spectra from simulations with real data allows the detection or exclusion of the broadening effect. As an application of this method we have compared simulated power spectra with one obtained from a Ginga observation of the X-ray pulsar SMC X-1. For this source the coupling effect is evident.

On the Relationship between the Periodic and Aperiodic Variability of Accreting X‐Ray Pulsars

Besides the narrow peaks originating from the periodic signal, the power spectra of accreting X-ray pulsars display continuum components usually increasing toward low frequencies; these arise from the source aperiodic variability. Most studies up to the present have adopted the view that the periodic and aperiodic variations are independent. However, any aperiodic variability in the emission from the accretion column(s) toward the magnetic neutron star should be modulated at the X-ray pulsar period, by virtue of the same rotation-induced geometric effects that give rise to the periodic signal. We develop here a simple shot noise model to test the presence of a coupling between the periodic and aperiodic variability of X-ray pulsars. The model power spectrum is fitted to the power spectra of three X-ray pulsars, Vela X-1, 4U 1145-62, and Cen X-3, observed with EXOSAT. In the first two cases, we find that a highly significant coupling is present, as evidenced by a substantial broadening in the wings of the power spectrum peaks which are the result of the periodic modulation. We find also that these wings can mimic the presence of a knee in the continuum power spectrum components around the fundamental of the periodic modulation, therefore bringing into question the correlation reported by Takeshima between the X-ray pulsar frequency and the knee frequency, beyond which the continuum power spectral component steepens.

Growth and structural properties of [formula omitted] intentionally disordered superlattice structures grown by metalorganic vapor phase epitaxy

We present an X-ray diffraction (XRD) study of symmetrically strained superlattice structures with intentionally fluctuating individual layer thicknesses. Samples with periodic, aperiodic (random) and graded fluctuations have been realized by metalorganic vapor phase epitaxy (MOVPE). The precise structural properties of the disordered multilayer structures have been determined by high resolution XRD in combination with both kinematical as well as dynamical XRD theory. Periodic modulation of the compressive-strained (GaIn)As layers leads to peak splitting on the “tensile-strained” part of the XRD spectrum, while periodic variation of the tensile-strained Ga(PAs) layers results in splitting on the “compressive-strained” part. Aperiodic variation causes a broadening of the respective peaks. The study demonstrates the power of XRD to quantify fluctuations of layer thicknesses in the growth direction down to the monolayer level.

Some new methods of time series analysis: Applications to the AGB stars

Some supplementary methods of time series analysis are described which are used for study of periodic and aperiodic variability of the AGB stars. These are “multiharmonic fits” used to fit the periodic (asinusoidal) curve; running approximations – “parabolae”, “sines”; “asymptotic parabolae”; “linear fits”.

Long-term crustal deformations in NE Italy revealed by tilt-strain gauges

Tilt and strain gauge records are analysed to determine the long-term variations in space and time of the strain field in the Friuli area (NE Italy), located at the eastern boundary of the Adria plate. For such long-term variations a deep crustal origin is supposed. Spectral and cross-correlation analyses show that the strain energy involving a large area is polarized in two main directions, N20W and N70E, the first one characterized by aperiodic (or very long period, undetectable by the present analysis) variations, the second one by a period of about 8 years. The direction of the aperiodic motion (N20W) is normal to the Alpine chain, and parallel to the maximum horizontal compression acting in the Alpine area, whereas the direction of the 8 year period motion (N70E) is normal to the Dinaric Alps chain, and coincident with the maximum horizontal compression acting on the eastern boundary of the Adria plate, in the Balkans region. To explain such results, and particularly those related to the N70E direction, slow-travelling stress-strain waves are hypothesized.