Time Delay Spectrum Research Articles

Modelling photometric reverberation mapping data for the next generation of big data surveys. Quasar accretion discs sizes with the LSST

ABSTRACT Photometric reverberation mapping can detect the radial extent of the accretion disc (AD) in Active Galactic Nuclei by measuring the time delays between light curves observed in different continuum bands. Quantifying the constraints on the efficiency and accuracy of the delay measurements is important for recovering the AD size-luminosity relation, and potentially using quasars as standard candles. We have explored the possibility of determining the AD size of quasars using next-generation Big Data surveys. We focus on the Legacy Survey of Space and Time (LSST) at the Vera C. Rubin Observatory, which will observe several thousand quasars with the Deep Drilling Fields and up to 10 million quasars for the main survey in six broadband filter during its 10-yr operational lifetime. We have developed extensive simulations that take into account the characteristics of the LSST survey and the intrinsic properties of the quasars. The simulations are used to characterize the light curves from which AD sizes are determined using various algorithms. We find that the time delays can be recovered with an accuracy of 5 and 15 per cent for light curves with a time sampling of 2 and 5 d, respectively. The results depend strongly on the redshift of the source and the relative contribution of the emission lines to the bandpasses. Assuming an optically thick and geometrically thin AD, the recovered time-delay spectrum is consistent with black hole masses derived with 30 per cent uncertainty.

Experimental study on the effects of hygrothermal aging on the indentation creep behaviors of PMMA

The effects of aging time on indentation creep behaviors of Polymethyl methacrylate (PMMA) have been investigated and the creep displacement was found to increase with aging time. However, the rate of change weakens as aging time increases. The generalized Calvin model consists of three Voigt elements that provide a good description of the indentation creep behavior of PMMA during the holding stage. Each Voigt element represents the motions with different levels i.e. molecular chain, molecular chain segments, molecular chain linkage, and branched-chain. The motions of molecular chain with different levels always have different delay times. The mechanism on the effects of aging time on the indentation creep responses is analyzed by comparing the contributions of different deformation types to total deformation. The contribution of elastic deformation to the total deformation Rhe decreases initially and then increases as aging time increases, which indicates the weakening of elastic modulus results from hygrothermal aging. With the aging time increasing, the contribution of time-dependent deformation to the total deformation i.e. Rhev+Rhvfirst increases and then decreases, which can be attributed to the different aging mechanisms at different stages. Further studies on the effects of aging time on creep compliance and time delay spectrum of PMMA during the holding stage indicates different deformation levels of PMMA characterized by different Voigt elements.

Analysis and Modeling of UAV Near-ground Communication Channel

UAV communication platform has become a reliable solution to meet the needs of users in some unexpected situations. In this paper, the statistical method of small area modeling is used to model the UAV near-ground communication channel. The channel is abstracted into three factors: time-delay power spectrum, Doppler power spectrum and Rice factor. As a result, a new method of channel delay generation is introduced, and the time-varying Doppler frequency gain factor is adopted. The simulation results show that the UAV near-ground communication channel is a Rice channel whose amplitude obeys Rayleigh distribution and phase obeys uniform distribution. The bit-error-rate of UAV near-ground communication channel is much higher than that of AWGN channel. At the same time, the constellation chart also shows that the signal passing through the UAV near-ground communication channel will cause strong inter-symbol crosstalk.

Effect of SOIAIC on photoionization time delay of the Xe 3d subshell

SynopsisThe effect of spin-orbit activated interchannel coupling (SOIAIC) has been studied on the time delay spectrum of 3d photoionization in xenon. The relativistic-random-phase-approximation with relaxation (RRPA-R) has been used to calculate the Wigner-Eisenbud-Smith (WES) time delay in photoionization. SOIAIC has a strong influence on the WES time delay for all ejection angles of the photoelectrons with respect to the incident direction and the polarization vector of the photon absorbed.

Energy Spectra and Time Delays of Hard X-Rays of Solar Flares in Konus-Wind and RHESSI Experiments

This paper analyzes hard X-ray radiation of solar flares detected by Konus-Wind and RHESSI spacecrafts. X-ray quanta are measured using high energy, time (Konus-Wind), and spatial (RHESSI) resolution. Spectra of accelerated electrons are calculated from the spectra of hard X-rays both via the forward-fitting method and the Tikhonov reconstruction method. The X-ray data for the solar flare of July 8, 2013, 0120:24 UT are analyzed. In Konus-Wind data the irregularities and inversions are detected in the energy spectrum. According to RHESSI data, the hard X-ray (HXR) energy spectrum is smoother, although it contains breaks. The X-ray energy spectrum determined by the forward-fitting method is hard at the peak of the flux, and the spectrum index is γ ≈ 3.5. Over time, the HXR spectrum is significantly softened, and the spectrum index increases to γ ≈ 7.5. X-ray time delays are determined by a correlation analysis of the time series of different energies. The time delay spectrum is U-shaped with a 47-keV break in the energy for the Konus-Wind data and 57 keV for the RHESSI data.

Application Analysis of Space Time Channel Models in Smart Antenna System

Space–time channel models are very important in evaluation of smart antenna system. In this paper, several space–time channel models that are often used in smart antenna system are presented and the properties of each model are analyzed. At the same time, the situations for each model are discussed. DOA, time delay and Doppler spectrum are discussed specially, while computer simulation is also done. Clarke model is thought as a special one.

Wigner time delay and spin–orbit activated confinement resonances

A study of the photoionization of spin–orbit split subshells of high-Z atoms confined in C60 has been performed using the relativistic-random-phase approximation. Specifically, Hg@C60 5p, Rn@C60 6p and Ra@C60 5d were investigated and the near-threshold confinement resonances in the j = l − 1/2 channels were found to engender structures in the j = l + 1/2 cross sections via correlation in the form of interchannel coupling. These structures are termed spin–orbit induced confinement resonances and they are found to profoundly influence the Wigner time delay spectrum resulting in time delays of tens or hundreds of attoseconds along with dramatic swings in time delay over small energy intervals. Pronounced relativistic effects in time delay are also found. These structures, including their manifestation in time delay spectra, are expected to be general phenomena in the photoionization of spin–orbit doublets in confined high-Z atoms.

Sampled fiber grating for WDM signal queuing with picosecond time interval

A new sampled fiber grating scheme is investigated for WDM signal queuing in time domain. The queuing is aimed at achieving very short constant time intervals down to picoseconds (ps) for many wavelength channels by a single fiber device. Theory and modeling predict that a consistence of linear spectral profiles exists between reflectivity or transmission spectrum and time delay spectrum. A sampled fiber grating based on Gaussian beam writing is proposed to offer a large number of wavelength channels and time delays within an approximate linear profile. Results for simulating a Gaussian sampled fiber grating show that the time delay minima of 8 channels within a section of the spectral profile have an average time delay step of 2.83ps and standard deviation of 7.4% from linearity. Fiber gratings were successfully written by Gaussian laser beam in fabrication. A Gaussian sampled fiber grating was measured for time delay spectrum of 4 wavelength channels. An average time delay step of 9.67ps is obtained. Some issues and related analysis and discussion are given. Our preliminary experiment suggests that the sampled fiber grating scheme can achieve ps-time steps for WDM signal queuing such as high-resolution time delay lines.

Measurement of the interface characteristic base on ultrasonic longitudinal wave

Owing to the reason that multiple reflection waves are unable to separate, and transfer function and time-delay spectrum are developed, adhesion interface characteristic of the thin layer medium is difficult to detect. Acoustics characteristic of the thin layer is studied with the ultrasonic longitudinal testing method to solve this problem. The results show that the fundamental acoustic parameters of the thin layer, such as sound velocity, reflection coefficient and attenuation coefficient, are obtained based on transfer function and time-delay spectrum. The analytical results show that these parameters mentioned above can characterise adhesion interface quality of the thin layer. The maximum values of the transfer function are unchanged, and its curve is flatter with the increasing of the reflection coefficient. The maximum delayed time increases with the increasing of the reflection coefficient, and the maximum delayed time is more sensitive to the reflection time coefficient variation than the transfer function is.

Time Delay Model of Fractional Fourier Transform and the Application in Signal Filtering

Various frequency bands of noises are contained in the actual signal. And it's difficult to eliminate the noise portion, which has a time delay and same spectrum with the original signal with conventional filtering methods. Based on the time delay and the multiplication delay characteristics of the fractional Fourier transform (FRFT), we put forward a FRFT time delay model, which can increase distance between the signal and the noise component. Through corresponding Fractional Fourier Transform to noise-containing signal, the distance between the signal and common frequency noises can be constantly increased within the transform domain, thus easily separating the noise component. The algorithm of the model can be simply deduced, easy realized and converged fast. In the experiment, we simulated the separating characteristics of the transform, and used the method to de-noise the grating signal. Compared with other traditional methods, we find that the FRFT acquired a better result.

Tunable slow and fast light with large bandwidth in the band-edge of gain spectrum of the wide band fiber optical parametric amplifier

We propose a novel method theoretically to generate the slow and fast light with large bandwidth and low gains, which is based on the parametric process in fiber. In our scheme, the wide band fiber optical parametric amplifier is employed and the whole signal bandwidth should be located at a certain frequency range of the band-edge of gain spectrum, and then signal waves will be delayed or advanced with low signal gains because of the peculiar feature of signal gain and phase shift. By changing the pump power, the delay time is continuously-tunable optically. The ultimate delay bandwidth and the delay bandwidth product are constrained by the shape of time delay spectrum. Our simulation verifies that 22.4 ps delay or advanced time for the bandwidth of 10 GHz with little distortion can be obtained at certain wavelengths in the optical communication waveband, and their gains are nearly zero. The tunable range is from 0 ps to 22.4 ps for the signal bandwidth of 10 GHz, and it is from 0 ps to 15.6 ps for the bandwidth of 15 GHz. This type of slow and fast light in wide band FOPA has the potential capability to produce the tunable slow and fast light for large bandwidth with low signal gains in future.

Measurement of Attenuation of Ultrasonic Propagating through the Thin Layer Media with Time Delay Spectrum

The ultrasonic attenuation coefficient is one of the most important acoustic parameters to character the performance of a thin layer media, but it can not be measured due to mutual superposition of multiple reflected waves at the same interface in ultrasonic testing. Ultrasonic pulse echo and lamb wave to evaluate the thin layer media can not obtain attenuation coefficient at present. In this paper, analytical method was used to study the acoustics characteristic of thin layer media with the ultrasonic echo testing. Meanwhile, the process of ultrasonic attenuation measurement was presented. Simulation and experimental investigation is focused on a thin layer of rubber. Attenuation coefficient was introduced and evaluation mathematics model was established by the two echoes cross-correlation with and without the thin layer media based on the time delay spectrum. It involved the parameters related to the acoustic properties of the thin layer media. Through calculating the sound velocity and acoustic impedance with the evaluation model, it can deduce the relation between the attenuation coefficient and the frequency. Through analyzing the simulation results, it indicated that the attenuation coefficients were invariable with the varying of the frequency. However, the attenuation coefficients increased with the frequency increasing by ultrasonic testing the thin layer of rubber. The reason was that the attenuation factor was not taken into account during the simulation. This method overcomes shortcomings that the traditional ultrasonic testing can not evaluate the thin layer media whose thickness is less than motivation wavelength. It is a new solution to study the attenuation characteristic and on-line nondestructive evaluation in the thin layer media.

Propagation of UHE protons through a magnetized large scale structure

The propagation of ultra high energy cosmic rays is affected by the extragalactic magnetic fields (EGMFs) that were produced during the course of large scale structure formation of the universe. We adopt a novel model where the large scale EGMFs are estimated from local dynamic properties of the gas flows in hydrodynamic simulations of a concordance λCDM universe. Using the model EGMFs, we calculate the deflection angle, time delay and energy spectrum of protons with E > 1019 eV that are injected at cosmological sources and then travel through the large scale structure of the universe, losing the energy due to interactions with the cosmic background radiation. Implications of this study on the origin of UHECRs are discussed.

Indoor TDOA-AOA measurements at the 3G systems frequency band ­ A Simple Approach

This work reports the results of indoor joint AOATDOA measurements, using a frequency domain wideband propagation channel sounder (with a vector network analyzer). The whole survey has been carried out in 1.8 GHz, with a 200 MHz bandwidth. Since such kind of sounder only allows the measurement of the channel impulse response, that is, the timedelay spectrum, a synthetic aperture uniform linear antenna array was adopted in order to extend its use to perform also angle-of-arrival measurements. Spatial spectral estimation algorithms such as beamforming, Capon and MUSIC have been applied. The estimated results were compared to expected values, calculated from propagation analysis. Relative errors less than 10% and 35% have been observed in the time-delay and in the spatial domain, respectively. Thus, despite the simplicity, a reasonable performance has been achieved, validating the adopted joint sounding approach.

On group velocity delays in microwave millisecond oscillating events and radio spikes

Time delays between opposite polarizations, interpreted by a dierence between the ordinary and extraordinary modes, are studied. Two dierent density and magnetic field models are used, and three emission mechanisms generating radio waves in double upper-hybrid, plasma, and gyro frequencies are considered. It is found that time delays and their spectra can reach various values and forms depending on plasma parameters in the radio wave emission and propagation. For the emission in double upper-hybrid frequency the considerable decrease of the time delay is due to an increase of the electron plasma density in the radio wave generation. In a dense flare plasma the power index a of the time delay spectrum is negative. On the other hand, in a diluted plasma a becomes positive, but in this case it is much lower than observed for dm-pulsations (a= 3). Furthermore model values are compared with the time delays presented by Fleishman et al. (2002), and it is shown that the best agreement is with the model assuming the emission on double gyro-frequency. But, in this case the gyro-resonant absorption limits the angle for escape of radio waves to< 3-10. Finally, an eect of the dierence of the group and light velocities on the frequency drift is analyzed. It is shown that such an eect can be important for wave propagation along magnetic field lines at frequencies close to the plasma frequency.

Angle and time of arrival statistics for the Gaussian scatter density model

Starting from a Gaussian distribution of scatterers around a mobile station, expressions are provided for the probability density function (PDF) in the angle of arrival, the power azimuth spectrum, the PDF in the time of arrival, and the time delay spectrum, all as seen from a base station. Expressions are also provided for some of the quantities of practical interest such as the root-mean-square (RMS) angular spread, the RMS delay spread, and the spatial cross-correlation function. Results for the Gaussian scatter density model are compared with those for the circular scattering model and the elliptical scattering model as well as with experimental results available for outdoor and indoor environments. Comparison is shown for the PDFs as well as for the power spectra in angle and delay. It is shown that the present model, in contrast to the previous models, produces results that closely agree with experimental results. With an appropriate choice of the standard deviation of the scattering region, the Gaussian density model can be made suitable both for environments with very small angular spreads as well as those with very large angular spreads. Consequently, the results provided in the paper are applicable to both macrocellular as well as picocellular environments.

Detecting slow, long-duration slip of large earthquakes using very long-period orbital surface waves

SUMMARY In this paper, we present a simple yet effective technique for measuring the source time-delay spectrum from very long-period world-circling surface waves without relying on assumed earth models or calculation of synthetic seismograms. By using autocorrelation and self-convolution functions derived from whole seismograms of large earthquakes recorded at a seismic station it is possible to isolate the source effects and precisely (5 5 s) determine the source delay spectrum at periods as long as 600 s. Measurements at such long periods are necessary for detecting slow precursory slip and slow creep after the main ‘fast’ seismic phase of large earthquakes. We apply this technique to five recent large earthquakes: the 1989 Macquarie Ridge (M, 8.2), the 1992 Nicaragua (M, 7.6), the 1993 Mariana Islands (M, 7.8), and the 1994 deep events in Fiji (M, 7.6) and Bolivia (M, 8.3). Our results are largely in agreement with the results obtained by other investigators. The Mariana Islands, Fiji, and Bolivia earthquakes had a normal rupture duration. The Nicaragua earthquake exhibits a source time-delay spectrum with the delay increasing from 31 s at the 100 s period to 50 s at the 600 s period, indicating an unusually slow rupture process. The Macquarie Ridge earthquake is anomalous by the discrepancy between the Love-wave and Rayleighwave estimates. Love waves, in the period band of 150 to 600 s, show a normal, flat source-delay spectrum with a delay of about 28 s. The Rayleigh-wave spectrum agrees with the Love-wave spectrum for periods smaller than 200 s and greater than 400 s; however, it shows a significantly shorter source delay (about 15 s) in the 250 to 350 s period range. The unusual nature of the Macquarie Ridge earthquake’s surface wave spectrum was pointed out by Ihmli, Harabaglia & Jordan (1993) who also provided an explanation in terms of a slow, precursory moment release. This explanation is not consistent with the flat Love-wave spectrum determined here.

The time-delay spectrum of GX 5-1 in its horizontal branch

Using a cross-spectral technique we investigate time delays between intensity variations of GX 5-1 in 10 X-ray spectral channels. The data were taken during a 1989 Ginga observation during which the source was in its horizontal-branch spectral state. We develope a new method to measure 'time-delay spectra' in fixed Fourier frequency ranges and use it to determine the energy and intensity dependence of time delays in the low-frequency noise (nu less than 2 Hz), the horizontal branch quasi-periodic oscillations (QPO), and the QPO second harmonic. These are the first time-delay spectra of a Z-source in its horizontal branch, and the first detection of time delays in the second harmonic. We consider two mechanisms for the production of the time lags: Comptonization and evolving shots. We perform Monte Carlo simulations of Compton scattering in a homogeneous, isotropic, central corona and show that it qualitatively explain the observed energy and time-delay spectra, but that it cannot explain the differences in the QPO first and second harmomnic time-delay spectra, nor the observed dependence of the QPO fractional rms variability upon energy. We consider implications of our results for millisecond pulsar searches in low-mass X-ray binaries.

About some experimental aspects of double and triple coincidence techniques to study electron impact double ionizing processes

A critical discussion is given of some of the experimental parameters involved in measurement of the three-, four- and five-fold differential cross sections obtained in (e,2e), (e,(3-1)e) and (e,3e) experiments, respectively. Features observed in the triple coincidence time-delay spectrum are attributed to fully accidental, semi-accidental or true triple coincidence events. Formulas are given that allow computation of the true coincidence rate, the percentage statistical error and the signal-to-background ratio, and to optimize the gas density-electron beam current product for any given experiment. The effective coincidence energy resolutions and transmission functions are defined, and their importance clearly demonstrated.

Shot noise cross-correlation functions and cross spectra - Implications for models of QPO X-ray sources

The cross-correlation functions (CCFs) and cross spectra expected for quasi-periodic oscillation (QPO) shot noise models are calculated under various assumptions, and the results are compared to observations. Effects due to possible coherence of the QPO oscillations are included. General formulas for the cross spectrum, the cross-phase spectrum, and the time-delay spectrum for QPO shot models are calculated and discussed. It is shown that the CCFs, cross spectra, and power spectra observed for Cyg X-e2 imply that the spectrum of the shots evolves with time, with important implications for the interpretation of these functions as well as of observed average energy spectra. The possible origins for the observed hard lags are discussed, and some physical difficulties for the Comptonization model are described. Classes of physical models for QPO sources are briefly addressed, and it is concluded that models involving shot formation at the surface of neutron stars are favored by observation.