Discovery of a 552 Hz burst oscillation in the low-mass X-ray binary EXO 0748-676

We report the detection of pulsations at 552 Hz in the rising phase of two type-I (thermonuclear) X-ray bursts observed from the accreting neutron star EXO 0748-676 in 2007 January and December, by the Rossi X-ray Timing Explorer. The fractional amplitude was 15% (rms). The dynamic power density spectrum for each burst revealed an increase in frequency of approx. 1-2 Hz while the oscillation was present. The frequency drift, the high significance of the detections and the almost identical signal frequencies measured in two bursts separated by 11 months, confirms this signal as a burst oscillation similar to those found in 13 other sources to date. We thus conclude that the spin frequency in EXO 0748-676 is within a few Hz of 552 Hz, rather than 45 Hz as was suggested from an earlier signal detection by Villarreal & Strohmayer (2004). Consequently, Doppler broadening must significantly affect spectral features arising from the neutron star surface, so that the narrow absorption features previously reported from an XMM-Newton spectrum could not have arisen there. The origin of both the previously reported 45 Hz oscillation and the X-ray absorption lines is now uncertain.Comment: 5 pages, 2 figures, accepted for Astrophysical Journal Letters. Minor changes following referees repor

Wasserstein Principal Component Analysis for Circular Measures

We consider the 2-Wasserstein space of probability measures supported on the unit-circle, and propose a framework for Principal Component Analysis (PCA) for data living in such a space. We build on a detailed investigation of the optimal transportation problem for measures on the unit-circle which might be of independent interest. In particular, we derive an expression for optimal transport maps in (almost) closed form and propose an alternative definition of the tangent space at an absolutely continuous probability measure, together with the associated exponential and logarithmic maps. PCA is performed by mapping data on the tangent space at the Wasserstein barycentre, which we approximate via an iterative scheme, and for which we establish a sufficient a posteriori condition to assess its convergence. Our methodology is illustrated on several simulated scenarios and a real data analysis of measurements of optical nerve thickness

Dynamic similarity promotes interpersonal coordination in joint-action

Human movement has been studied for decades and dynamic laws of motion that are common to all humans have been derived. Yet, every individual moves differently from everyone else (faster/slower, harder/smoother etc). We propose here an index of such variability, namely an individual motor signature (IMS) able to capture the subtle differences in the way each of us moves. We show that the IMS of a person is time-invariant and that it significantly differs from those of other individuals. This allows us to quantify the dynamic similarity, a measure of rapport between dynamics of different individuals' movements, and demonstrate that it facilitates coordination during interaction. We use our measure to confirm a key prediction of the theory of similarity that coordination between two individuals performing a joint-action task is higher if their motions share similar dynamic features. Furthermore, we use a virtual avatar driven by an interactive cognitive architecture based on feedback control theory to explore the effects of different kinematic features of the avatar motion on the coordination with human players

Characterisation of Dynamic Process Systems by Use of Recurrence Texture Analysis

This thesis proposes a method to analyse the dynamic behaviour of process systems using sets of textural features extracted from distance matrices obtained from time series data. Algorithms based on the use of grey level co-occurrence matrices, wavelet transforms, local binary patterns, textons, and the pretrained convolutional neural networks (AlexNet and VGG16) were used to extract features. The method was demonstrated to effectively capture the dynamics of mineral process systems and could outperform competing approaches

Indications for a slow rotator in the Rapid Burster from its thermonuclear bursting behaviour

We perform time-resolved spectroscopy of all the type I bursts from the Rapid Burster (MXB 1730-335) detected with the Rossi X-ray Timing Explorer. Type I bursts are detected at high accretion rates, up to \sim 45% of the Eddington luminosity. We find evidence that bursts lacking the canonical cooling in their time-resolved spectra are, none the less, thermonuclear in nature. The type I bursting rate keeps increasing with the persistent luminosity, well above the threshold at which it is known to abruptly drop in other bursting low-mass X-ray binaries. The only other known source in which the bursting rate keeps increasing over such a large range of mass accretion rates is the 11 Hz pulsar IGR J17480$-$2446. This may indicate a similarly slow spin for the neutron star in the Rapid Burster

Spatio-temporal analysis of GRACE gravity field variations using the principal component analysis

Gravity Recovery and Climate Experiment (GRACE) mission has amplified the knowledge of both static and time-variable part of the Earth’s gravity field. Currently, GRACE maps the Earth’s gravity field with a near-global coverage and over a five year period, which makes it possible to apply statistical analysis techniques to the data. The objective of this study is to analyse the most dominant spatial and temporal variability of the Earth’s gravity field observed by GRACE using a combination of analytical and statistical methods such as Harmonic Analysis (HA) and Principal Component Analysis (PCA). The HA is used to gain general information of the variability whereas the PCA is used to find the most dominant spatial and temporal variability components without having to introduce any presetting. The latter is an important property that allows for the detection of anomalous or a-periodic behaviour that will be useful for the study of various geophysical processes such as the effect from earthquakes. The analyses are performed for the whole globe as well as for the regional areas of: Sumatra- Andaman, Australia, Africa, Antarctica, South America, Arctic, Greenland, South Asia, North America and Central Europe. On a global scale the most dominant temporal variation is an annual signal followed by a linear trend. Similar results mostly associated to changing land hydrology and/or snow cover are obtained for most regional areas except over the Arctic and Antarctic where the secular trend is the prevailing temporal variability.Apart from these well-known signals, this contribution also demonstrates that the PCA is able to reveal longer periodic and a-periodic signal. A prominent example for the latter is the gravity signal of the Sumatra-Andaman earthquake in late 2004. In an attempt to isolate these signals, linear trend and annual signal are removed from the original data and the PCA is once again applied to the reduced data. For a complete overview of these results the most dominant PCA modes for the global and regional gravity field solutions are presented and discussed

A probabilistic approach to analyse Blade Tip Timing data of non-synchronous vibrations under constant rotor speed

Blades are among the most critical components of turbomachines, their monitoring and characterization undergoing working conditions are fundamental for the insiders, both for preventing eventual breakage and for optimising future development. Two approaches are possible for monitoring rotor blade vibrations: a traditional one based on the use of strain gauges and another one called Blade Tip Timing (BTT). BTT is an indirect, non-intrusive simple and robust measurement method, but the processing of such data is not easy because they are often subsampled with respect to the Nyquist limit and the ordering of the samples is not unique. In this work the focus is on multi component non-synchronous vibrations, typical for example of flutter, measured at constant rotor speed by a BTT system. These data are organized into batches of fixed length called snapshots and they are interpreted as members of a random vector. When the signal contains only one harmonic component the frequency can be determined using a method here described and called Harmonic Matching (HM). While for the analyses of multi harmonic component vibrations a probabilistic approach capable of separating and identify the components using Principal Component Analysis (PCA) and Independent Component Analysis (ICA) is proposed. For the development of data processing methods, the possibility of having controllable and repeatable data is fundamental, for this reason two test rigs of increasing complexity have been developed and are here described

Nonlinear Dimensionality Reduction Methods in Climate Data Analysis

Linear dimensionality reduction techniques, notably principal component analysis, are widely used in climate data analysis as a means to aid in the interpretation of datasets of high dimensionality. These linear methods may not be appropriate for the analysis of data arising from nonlinear processes occurring in the climate system. Numerous techniques for nonlinear dimensionality reduction have been developed recently that may provide a potentially useful tool for the identification of low-dimensional manifolds in climate data sets arising from nonlinear dynamics. In this thesis I apply three such techniques to the study of El Nino/Southern Oscillation variability in tropical Pacific sea surface temperatures and thermocline depth, comparing observational data with simulations from coupled atmosphere-ocean general circulation models from the CMIP3 multi-model ensemble. The three methods used here are a nonlinear principal component analysis (NLPCA) approach based on neural networks, the Isomap isometric mapping algorithm, and Hessian locally linear embedding. I use these three methods to examine El Nino variability in the different data sets and assess the suitability of these nonlinear dimensionality reduction approaches for climate data analysis. I conclude that although, for the application presented here, analysis using NLPCA, Isomap and Hessian locally linear embedding does not provide additional information beyond that already provided by principal component analysis, these methods are effective tools for exploratory data analysis.Comment: 273 pages, 76 figures; University of Bristol Ph.D. thesis; version with high-resolution figures available from http://www.skybluetrades.net/thesis/ian-ross-thesis.pdf (52Mb download

Astrophysical parameters and orbital solution of the peculiar X-ray transient IGR J00370+6122

BD+6073 is the optical counterpart of the X-ray source IGR J00370+6122, a probable accretion-powered X-ray pulsar. The X-ray light curve of this binary system shows clear periodicity at 15.7 d, which has been interpreted as repeated outbursts around the periastron of an eccentric orbit. We obtained high-resolution spectra of BD+6073 at different epochs. We used the FASTWind code to generate a stellar atmosphere model to fit the observed spectrum and obtain physical magnitudes. The synthetic spectrum was used as a template for cross-correlation with the observed spectra to measure radial velocities. The radial velocity curve provided an orbital solution for the system. We have also analysed the RXTE/ASM and Swift/BAT light curves to confirm the stability of the periodicity. BD +6073 is a BN0.7 Ib low-luminosity supergiant located at an approximate distance of 3.1 kpc, in the CasOB4 association. We derive Teff=24000 K and log gc=3.0, and chemical abundances consistent with a moderately high level of evolution. The spectroscopic and evolutionary masses are consistent at the 1 sigma level with a mass of 15 solar masses. The recurrence time of the X-ray flares is the orbital period of the system. The NS is in a high eccentricity (e=0.56) orbit, and the X-ray emission is strongly peaked around orbital phase 0.2, though the observations are consistent with some level of X-ray activity happening at all orbital phases. The X-ray behaviour of IGR J00370+6122 is reminiscent of intermediate SFXTs, though its peak luminosity is rather low. The orbit is somewhat wider than those of classical persistent supergiant X-ray binaries, which, combined with the low luminosity of the mass donor, explains the low X-ray luminosity. IGR J00370+6122 will likely evolve towards a persistent supergiant system, highlighting the evolutionary connection between different classes of wind-accreting X-ray sources.Comment: Accepted for publication in A&