Non-zero Offsets Research Articles

Examining the self-interaction of dark matter through central cluster galaxy offsets

ABSTRACT While collisionless cold dark matter models have been largely successful in explaining a wide range of observational data, some tensions still exist, and it remains possible that dark matter possesses a non-negligible level of self-interactions. In this paper, we investigate a possible observable consequence of self-interacting dark matter: offsets between the central galaxy and the centre of mass of its parent halo. We examine 23 relaxed galaxy clusters in a redshift range of 0.1–0.3 drawn from clusters in the Dark Energy Survey and the Sloan Digital Sky Survey which have archival Chandra X-ray data of sufficient depth for centre and relaxation determination. We find that most clusters in our sample show non-zero offsets between the X-ray centre, taken to be the centroid within the cluster core, and the central galaxy position. All of the measured offsets are larger, typically by an order of magnitude, than the uncertainty in the X-ray position due to Poisson noise. In all but six clusters, the measured offsets are also larger than the estimated, combined astrometric uncertainties in the X-ray and optical positions. A more conservative cut on concentration to select relaxed clusters marginally reduces but does not eliminate the observed offset. With our more conservative sample, we find an estimated median X-ray to central galaxy offset of $\mu = 6.0 ^{+ 1.4}_{- 1.5}$ kpc. Comparing to recent simulations, this distribution of offsets is consistent with some level of dark matter self-interaction, though further simulation work is needed to place constraints.

Improved generalized moveout approximation with a novel parameterization

Reflection moveout approximations written as explicit functions of offset are convenient tools commonly applied in seismic data modeling, processing, and inversion. The generalized moveout approximation (GMA) has a flexible functional form that accommodates various causes of moveout nonhyperbolicity. We have developed a novel parameterization for the existing six-parameter version of GMA by changing the definition of one of its parameters. Three parameters of the proposed GMA approximation are obtained by fitting the traveltime, the normal moveout velocity, and the fourth-order Taylor series coefficient of traveltime squared at zero offset. The remaining three parameters are obtained by fitting kinematic properties at another two nonzero reference offsets. Through numerical tests on linear sloth models, circular reflector models, and acoustic transversely isotropic media with a vertical symmetry axis (acoustic VTI media), the proposed new GMA method is compared with other published GMA methods in terms of their accuracy. Numerical tests on the three models demonstrate that, when the far reference offset is fixed, the location of the middle reference offset has a considerable impact on the proposed GMA accuracy. We propose a moveout inversion method in a layer-stripping fashion for layered acoustic VTI media based on the proposed and other published GMA methods.

Correlation-Based Interferometry Method to Enhance Near-Surface Reflection Signals in Surface Active Seismic Exploration

In surface active seismic exploration, the near-surface imaging based on reflection signals provides valuable information for mineral exploration and deeper resources. However, the strong noises at near surface and stretching effects at nonzero offsets with conventional seismic data processing method severely damage the seismic reflection signals with low fold at near surface. The innovative method for the enhancement of seismic signals is the seismic interferometry method. However, due to the one-sided illumination, the seismic interferometry is still not suitable for the retrieval of reflection signals in most practical situations. In this article, we present the correlation-based interferometry (CI) method to enhance the reflection signals for near-surface imaging using the first cross correlation to construct virtual reflection signals and the second cross correlation to achieve the interferometric normal move-out (INMO) correction of the seismic reflection events with no stretching, which keeps the high resolution of near-surface reflection signals. Meanwhile, because of constructive interference stack, the CI method can significantly improve the signal-to-noise ratio (SNR) of reflection signals. The synthetic and field seismic data examples illustrate that the data-driven CI method performs better than the conventional stack (CS) method based on the velocity model for the near-surface reflection imaging.

An SNR Estimation Technique Based on Deep Learning

Signal-to-noise ratio (SNR) is a priori information necessary for many signal processing algorithms or techniques. However, there are many problems exsisting in conventional SNR estimation techniques, such as limited application range of modulation types, narrow effective estimation range of signal-to-noise ratio, and poor ability to accommodate non-zero timing offsets and frequency offsets. In this paper, an SNR estimation technique based on deep learning (DL) is proposed, which is a non-data-aid (NDA) technique. Second and forth moment (M2M4) estimator is used as a benchmark, and experimental results show that the performance and robustness of the proposed method are better, and the applied ranges of modulation types is wider. At the same time, the proposed method is not only applicable to the baseband signal and the incoherent signal, but can also estimate the SNR of the intermediate frequency signal.

Active phase correction of high resolution silicon photonic arrayed waveguide gratings.

Arrayed waveguide gratings provide flexible spectral filtering functionality for integrated photonic applications. Achieving narrow channel spacing requires long optical path lengths which can greatly increase the footprint of devices. High index contrast waveguides, such as those fabricated in silicon-on-insulator wafers, allow tight waveguide bends which can be used to create much more compact designs. Both the long optical path lengths and the high index contrast contribute to significant optical phase error as light propagates through the device. Therefore, silicon photonic arrayed waveguide gratings require active or passive phase correction following fabrication. Here we present the design and fabrication of compact silicon photonic arrayed waveguide gratings with channel spacings of 50, 10 and 1 GHz. The largest device, with 11 channels of 1 GHz spacing, has a footprint of only 1.1 cm2. Using integrated thermo-optic phase shifters, the phase error is actively corrected. We present two methods of phase error correction and demonstrate state-of-the-art cross-talk performance for high index contrast arrayed waveguide gratings. As a demonstration of possible applications, we perform RF channelization with 1 GHz resolution. Additionally, we generate unique spectral filters by applying non-zero phase offsets calculated by the Gerchberg Saxton algorithm.

Cascade-Like Modular Tracking Controller for non-Standard N-Trailers

A general trajectory-tracking control solution is proposed for truly N-trailer robots comprising a unicycle-like tractor and arbitrary number of passive trailers interconnected by sign-homogeneous nonzero hitching offsets. The solution permits either backward or forward motion strategy of a vehicle, preserving location of a guidance point on the last trailer. The presented control law is modular and highly scalable with respect to a number of trailers thanks to employing a cascade-like control structure. Stability and robustness analysis of the closed-loop system provides sufficient conditions for asymptotic and practical tracking for a wide set of the so-called segment-platooning reference trajectories containing both constant-curvature and varying-curvature motion profiles. Efficiency of the controller and its robustness to parametric uncertainty have been illustrated by experimental results obtained for a three-trailer vehicle.

COSMIC-RAY POSITRONS FROM MILLISECOND PULSARS

Observations by the Fermi Large Area Telescope of gamma-ray millisecond pulsar light curves imply copious pair production in their magnetospheres, and not exclusively in those of younger pulsars. Such pair cascades may be a primary source of Galactic electrons and positrons, contributing to the observed enhancement in positron flux above ~10 GeV. Fermi has also uncovered many new millisecond pulsars, impacting Galactic stellar population models. We investigate the contribution of Galactic millisecond pulsars to the flux of terrestrial cosmic-ray electrons and positrons. Our population synthesis code predicts the source properties of present-day millisecond pulsars. We simulate their pair spectra invoking an offset-dipole magnetic field. We also consider positrons and electrons that have been further accelerated to energies of several TeV by strong intrabinary shocks in black widow and redback systems. Since millisecond pulsars are not surrounded by pulsar wind nebulae or supernova shells, we assume that the pairs freely escape and undergo losses only in the intergalactic medium. We compute the transported pair spectra at Earth, following their diffusion and energy loss through the Galaxy. The predicted particle flux increases for non-zero offsets of the magnetic polar caps. Pair cascades from the magnetospheres of millisecond pulsars are only modest contributors around a few tens of GeV to the lepton fluxes measured by AMS-02, PAMELA, and Fermi, after which this component cuts off. The contribution by black widows and redbacks may, however, reach levels of a few tens of percent at tens of TeV, depending on model parameters.

Lateral wave-field stacking of seismic Fresnel zones for the generalized-offset case

To unify different seismic geometries, the concept of generalized offset is defined and the expressions for Fresnel zones of different order on a plane are presented. Based on wave theory, the equation of the lateral wave-field stacking for generalized-offset Fresnel zones is derived. For zero and nonzero offsets, the lateral stacking amplitude of diffraction bins of different sizes is analyzed by referring to the shape of the Fresnel zones of different order. The results suggest the following. First, the contribution of diffraction bins to wave-field stacking is related to the offset, surface relief, interface dip, the depth of the shot point to the reflection interface, the observational geometry, and the size of the interference stacking region. Second, the first-order Fresnel zone is the main constructive interference, and its contribution to the reflection amplitude is slightly smaller than half the contribution of all Fresnel zones. Finally, when the size of the diffraction bin is smaller than the first-order Fresnel zone, the larger the size of the diffraction bin, the larger is the amplitude of the receiver, even in the nonzero offset-case.

The contribution of millisecond pulsars to the Galactic cosmic-ray lepton spectrum

Pulsars are believed to be sources of relativistic electrons and positrons. The abundance of detections of γ-ray millisecond pulsars by Fermi Large Area Telescope coupled with their light curve characteristics that imply copious pair production in their magnetospheres, motivated us to investigate this old pulsar population as a source of Galactic electrons and positrons and their contribution to the enhancement in cosmic-ray positron flux at GeV energies. We use a population synthesis code to predict the source properties (number, position, and power) of the present-day Galactic millisecond pulsars, taking into account the latest Fermi and radio observations to calibrate the model output. Next, we simulate pair cascade spectra from these pulsars using a model that invokes an offset-dipole magnetic field. We assume free escape of the pairs from the pulsar environment. We then compute the cumulative spectrum of transported electrons and positrons at Earth, following their diffusion and energy losses as they propagate through the Galaxy. Our results indicate that the predicted particle flux increases for non-zero offsets of the magnetic polar caps. Comparing our predicted local interstellar spectrum and positron fraction to measurements by AMS-02, PAMELA, and Fermi, we find that millisecond pulsars are only modest contributors at a few tens of GeV, after which this leptonic spectral component cuts off. The positron fraction is therefore only slightly enhanced above 10GeV relative to a background flux model. This implies that alternative sources such as young, nearby pulsars and supernova remnants should contribute additional primary positrons within the astrophysical scenario.

Tunable evolutions of wave modes and bandgaps in quasi-1D cylindrical phononic crystals

We investigate the tunable characteristics of mechanical waves propagating in quasi-1D phononic crystals composed of horizontally stacked short cylinders at various contact angles and offsets. According to the Hertzian contact theory, elastic compression of laterally-touching cylindrical bodies exhibits a various range of contact stiffness depending on their alignment angles. In this study, we first assemble cylindrical particles in various combinations of inclination angles and systematically examine their forming mechanisms of frequency bandgaps. We also investigate the effect of the rattling motions of cylindrical particles by introducing asymmetric center-of-mass offsets with respect to their contact points. We find that the frequency responses of these quasi-1D phononic crystals evolve into multiple band structures as we employ higher deviations of contact angles and offsets. We calculate the dispersive behavior of propagating waves using a discrete particle model for simple zero-offset cases, while we use a finite element method for simulating the rattling motions of particles under non-zero offsets. We report branching behavior of frequency band structures and the evolution of their vibration modes as we manipulate the contact angles and offsets of the phononic crystals. This study implies that we can leverage the versatile wave filtering characteristics of quasi-1D phononic crystals to construct tunable wave filtering devices for engineering applications.

Shift-Invariant Protocol Sequences for the Collision Channel Without Feedback

The authors consider collision channel without feedback in which collided packets are considered unrecoverable. For each user, the transmission of packets follows a specific periodical pattern, called the protocol sequence. Due to the lack of feedback, the beginning of the protocol sequences cannot be synchronized and nonzero relative offsets are inevitable. It results in variation of throughput. In this paper, we investigate optimal protocol sequence sets, in the sense that the throughput variance is zero. Such protocol sequences are said to be shift-invariant (SI). The characterizing properties of SI protocol sequences are presented. We also prove that SI sequences are identifiable, meaning that the receiver is able to determine the sender of each successfully received packet without any packet header. A general construction of SI sequences that meets the lower bound on sequence length is given. Besides, we study the least periods of SI sequences, and show that the least periods must be distinct in some cases. The throughput performance is compared numerically with other protocol sequences.

Intercomparison of oxygenated volatile organic compound measurements at the SAPHIR atmosphere simulation chamber

This paper presents results from the first large‐scale in situ intercomparison of oxygenated volatile organic compound (OVOC) measurements. The intercomparison was conducted blind at the large (270 m3) simulation chamber, Simulation of Atmospheric Photochemistry in a Large Reaction Chamber (SAPHIR), in Jülich, Germany. Fifteen analytical instruments, representing a wide range of techniques, were challenged with measuring atmospherically relevant OVOC species and toluene (14 species, C1 to C7) in the approximate range of 0.5–10 ppbv under three different conditions: (1) OVOCs with no humidity or ozone, (2) OVOCs with humidity added (r.h. ≈ 50%), and (3) OVOCs with ozone (≈60 ppbv) and humidity (r.h. ≈ 50%). The SAPHIR chamber proved to be an excellent facility for conducting this experiment. Measurements from individual instruments were compared to mixing ratios calculated from the chamber volume and the known amount of OVOC injected into the chamber. Benzaldehyde and 1‐butanol, compounds with the lowest vapor pressure of those studied, presented the most overall difficulty because of a less than quantitative transfer through some of the participants' analytical systems. The performance of each individual instrument is evaluated with respect to reference values in terms of time series and correlation plots for each compound under the three measurement conditions. A few of the instruments performed very well, closely matching the reference values, and all techniques demonstrated the potential for quantitative OVOC measurements. However, this study showed that nonzero offsets are present for specific compounds in a number of instruments and overall improvements are necessary for the majority of the techniques evaluated here.

Residual stereotomographic inversion

Depth migration requires highly accurate knowledge of the subsurface velocity field. Different traveltime tomographic methods are used for this purpose. Stereotomography is a tomographic method that uses local dip estimates in addition to traveltimes for velocity model estimation. We present a new methodology for velocity model updating. It combines poststack stereotomography and residual moveout velocity inversion. The former is used for initial model construction and the latter for updating the velocity model. Residual inversion is a kind of stereotomographic inversion applied to common reflection point (CRP) gathers after model-based moveout correction. Velocity analysis can be made more efficient by preselecting the traces that contribute to a series of CRP gathers and using only these traces for inversion. The algorithm is defined in a two-step procedure. First, ray tracing from the reflection point for nonzero reflection offsets defines the source and receiver locations of the data traces in the CRS gather. Then these traces are moveout corrected according to the calculated traveltimes and residual moveout is estimated. The interval velocity model is updated by fitting the velocity that minimizes estimated residuals. Application of the proposed technique demonstrates its robustness and reliability for fast and automatic velocity model estimation.

VLT/UVES constraints on the cosmological variability of the fine-structure constant

We propose a new methodology for probing the cosmological variability of α from pairs of Fe  lines (SIDAM, single ion differential α measurement) observed in individual exposures from a high resolution spectrograph. By this we avoid the influence of the spectral shifts due to (i) ionization inhomogeneities in the absorbers; and (ii) non-zero offsets between different exposures. Applied to the Fe  lines of the metal absorption line system at zabs = 1.839 in the spectrum of Q 1101-264 obtained by means of the UV-Visual Echelle Spectrograph (UVES) at the ESO Very Large Telescope (VLT), SIDAM provides ∆α/α = (2.4 ± 3.8stat) × 10 −6 .T hezabs = 1.15 Fe  system toward HE 0515-4414 has been re-analyzed by this method thus obtaining for the combined sample ∆α/α = (0.4 ± 1.5stat) × 10 −6 . These values are shifted with respect to the Keck/HIRES mean ∆α/α = (−5.7 ± 1.1stat) × 10 −6 (Murphy et al. 2004) at very high confidence level (95%). The fundamental photon noise limitation in the ∆α/α measurement with the VLT/UVES is discussed to figure the prospects for future observations. It is suggested that with a spectrograph of ∼10 times the UVES dispersion coupled to a 100 m class telescope the present Oklo level (∆α/α ≥ 4.5 × 10 −8 )

VLT/UVES shows no cosmological variability of $\alpha$

The cosmological variability of alpha is probed from individual observations of pairs of FeII lines. This procedure allows a better control of the systematics and avoids the influence of the spectral shifts due to ionization inhomogeneities in the absorbers and/or non-zero offsets between different exposures. Applied to the FeII lines of the metal absorption systems at zabs = 1.839 in Q1101--264 and at zabs = 1.15 in HE0515--4414 observed by means of UVES at the ESO-VLT, it provides da/a = 0.4 (+/- 1.5 stat)x10^{-6}. The result is shifted with respect to the Keck/HIRES mean da/a = -5.7(+/- 1.1 stat})x10^{-6} (Murphy et al. 2004) at a high confidence level (95%). Full details of this work are given in Levshakov et al (2005)

Does osteoporosis classification using heel BMD agree across manufacturers?

The lack of standardization in bone mineral density (BMD) measurements is known. Several studies have been carried out to cross-calibrate the axial dual X-ray absorptiometry (DXA) devices. Recently, a number of peripheral DXA (pDXA) densitometers have been introduced. In this study we evaluated the agreement between two heel DXA devices on BMD and T-scores. A total of 99 females aged 21-78 years (ca. 16 per decade) had their non-dominant heel BMD measured using the PIXI (Lunar Inc.) and the Apollo (Norland Medical) pDXA scanners. The mean BMD values were 0.492 and 0.607 g/cm(2) and the mean T-scores using manufacturers' specified reference data were -0.07 and -0.25 for the PIXI and Apollo, respectively. Both the BMD and T-score intermachine relationships were highly correlated but showed significant nonidentity slopes and non-zero offsets. The diagnostic comparison on T-scores resulted in 86% agreement between the instruments (weighted kappa score of 0.550). Normalizing the reference peaks and SDs using this study's young adult population BMD results removed the systematic T-score disagreement. We found that PIXI and Apollo are highly correlated. Differences in BMD values are mainly due to different region of interest (ROI) definitions and additional T-score disagreement reflects the difference in normative databases.

Antialiasing methods in Kirchhoff migration

This paper addresses some practical aspects of antialiasing in Kirchhoff migration. We show that the effective trace spacing related to the aliasing‐frequency limits for directions between the in‐line and cross‐line directions is smaller than values previously used and that the frequency limits for antialiasing may be increased. We present a corrected expression for the migration operator spatial derivative for 3-D time migration that avoids the overfiltering at nonzero offsets produced by a previous approximation. Simple operator truncation is shown to reduce aliasing noise; it is inexpensive but may lead to errors in interpretation. Triangle filtering and Gray’s method both produce similar results, but the relative costs of these two methods will vary with the size of the migration and the computational environment. Gray’s method takes more setup time for each input trace than does triangle filtering, but Gray’s method might be more cost effective when each input trace contributes to a large number of output traces.

Classification of 5R closed kinematic chains with self mobility

In this paper we shall classify and explicitly describe all 5-parametric closed kinematic chains consisting of rotational pairs which have one degree of freedom. We shall prove that beside the so-called Goldberg linkages we obtain another class of 5R closed kinematic chains with one degree of freedom. Similarly as in the case of Goldberg linkages the new class is also closely connected with the Bennett's mechanism, but linkages have non-zero offsets. Because of the complexity of the problem, all computations were performed on a work station using Mathematica. For one example from the new class of movable 5R closed kinematic chains we give explicit formulas for joint angles that allow verification of the movability of the closed kinematic chain on a computer.

Offset dependent amplitudes: Fault curvature effects and dip move-out corrections

Equations have been presented previously which predict that reflector curvature can affect significantly seismic reflection amplitudes at both zero and nonzero source-receiver offsets. Here the fact that faults are generally concave-upward is used to examine the curvature effect for compaction-driven faults which the sediments have both exponential and logarithmic porosity decreases with increasing depth.

Inverting shear-wave polarizations for anisotropy using three-component offset VSPs: synthetic seismograms

Summary A numerical algorithm is presented for interpreting shear-wave polarization directions, from offset VSP data, as subsurface anisotropy. The technique compares observed (synthetic) horizontal polarization angles with expected values for a range of physically plausible anisotropic models with different orientations. The comparison is implemented in a database inversion scheme. The anisotropic models are formed by combining the effective anisotropy of parallel, aligned, vertical cracks with matrix anisotropy due to thin-layering or lithology. Different models are defined by varying the crack and matrix properties. The technique is applied to synthetic offset VSP data for a multilayered anisotropic structure, generated by multi-azimuth sources. Analysis of the results indicates that polarization measurements from non-zero offsets with at least two and preferably three different azimuths are required in order to reasonably constrain the final inversion solutions. The parameters which are resolved depend upon the depth of the subsurface zone of interest. Crack strike is well resolved in the deeper layers for which directions of propagation are sub-vertical, whereas other model parameters such as aspect ratio and crack content can only be resolved for wider ranges of incidence angle. The key to a successful inversion appears to lie in the selection of propagation paths through the relatively unique patterns of disruption created by singularities.