Derivation of an observer model adapted to irregular signals based on convolution channels.

Anthropomorphic model observers are mathe- matical algorithms which are applied to images with the ultimate goal of predicting human signal detection and classification accuracy across varieties of backgrounds, image acquisitions and display conditions. A limitation of current channelized model observers is their inability to handle irregularly-shaped signals, which are common in clinical images, without a high number of directional channels. Here, we derive a new linear model observer based on convolution channels which we refer to as the "Filtered Channel observer" (FCO), as an extension of the channelized Hotelling observer (CHO) and the nonprewhitening with an eye filter (NPWE) observer. In analogy to the CHO, this linear model observer can take the form of a single template with an external noise term. To compare with human observers, we tested signals with irregular and asymmetrical shapes spanning the size of lesions down to those of microcalfications in 4-AFC breast tomosynthesis detection tasks, with three different contrasts for each case. Whereas humans uniformly outperformed conventional CHOs, the FCO observer outperformed humans for every signal with only one exception. Additive internal noise in the models allowed us to degrade model performance and match human performance. We could not match all the human performances with a model with a single internal noise component for all signal shape, size and contrast conditions. This suggests that either the internal noise might vary across signals or that the model cannot entirely capture the human detection strategy. However, the FCO model offers an efficient way to apprehend human observer performance for a non-symmetric signal

Detecting Sunyaev-Zel'dovich clusters with PLANCK: I. Construction of all-sky thermal and kinetic SZ-maps

All-sky thermal and kinetic Sunyaev-Zel'dovich (SZ) maps are presented for assessing how well the PLANCK-mission can find and characterise clusters of galaxies, especially in the presence of primary anisotropies of the cosmic microwave background (CMB) and various galactic and ecliptic foregrounds. The maps have been constructed from numerical simulations of structure formation in a standard LCDM cosmology and contain all clusters out to redshifts of z = 1.46 with masses exceeding 5e13 M_solar/h. By construction, the maps properly account for the evolution of cosmic structure, the halo-halo correlation function, the evolving mass function, halo substructure and adiabatic gas physics. The velocities in the kinetic map correspond to the actual density environment at the cluster positions. We characterise the SZ-cluster sample by measuring the distribution of angular sizes, the integrated thermal and kinetic Comptonisations, the source counts in the three relevant PLANCK-channels, and give the angular power-spectra of the SZ-sky. While our results are broadly consistent with simple estimates based on scaling relations and spherically symmetric cluster models, some significant differences are seen which may affect the number of cluster detectable by PLANCK.Comment: 14 pages, 16 figures, 3 tables, submitted to MNRAS, 05.Jul.200

A High-Resolution Radio Continuum Study of the Dwarf Irregular Galaxy IC10

Dwarf galaxies are a significant contributor to the current star formation rate for the Universe as a whole, yet little is understood of what drives star formation in these low gas density environments. IC10 is a nearby dwarf irregular galaxy that is currently in a starburst phase making it an ideal place to study star formation and its effects on the interstellar medium in a low density setting. We present new high resolution (~1 pc) radio continuum maps of IC10 at λ=20cm (ν = 1.5GHz) taken with the e-MERLIN array. Maps were produced by reducing and combining data taken in two observational epochs, one in February and the other in November 2013. Inspection of the final maps reveal 8 compact sources coincident with extended emission, 4 of which are classified as HII regions and 4 as supernova remnants (SNR). We summarise some characteristic parameters for these candidates. Due to the lack of short baselines we detect much less flux than single dish observations, however the star formation derived from counting the individual SNR reveals a star formation rate consistent with the literature. Nearly all spatially resolved sources are coincident with regions of H-alpha emission, suggesting that they are the sites of current star formation. Interestingly, no compact sources were detected within the star forming non-thermal superbubble, either supporting that it is indeed a hypernova remnant or suggesting that SNRs within this region are too dim to be detected. Further information such as the spectral index of each observed source as well as higher resolution images will be required for further analysis

Methods for detecting and characterising clusters of galaxies

The main theme of this PhD-thesis is the observation of clusters of galaxies at submillimetric wavelengths. The Sunyaev-Zel'dovich (SZ) effect due to interaction of cosmic microwave background (CMB) photons with electrons of the hot intra-cluster medium causes a distinct modulation in the spectrum of the CMB and is a very promising tool for detecting clusters out to very large distances. Especially the European PLANCK-mission, a satellite dedicated to the mapping of CMB anisotropies, will be the first experiment to routinely detect clusters of galaxies by their SZ-signature. This thesis presents an extensive simulation of PLANCK's SZ-capabilities, that combines all-sky maps of the SZ-effect with a realisation of the fluctuating CMB and submillimetric emission components of the Milky Way and of the Solar system, and takes instrumental issues such as the satellite's point-spread function, the frequency response, scan paths and detector noise of the receivers into account. For isolating the weak SZ-signal in the presence of overwhelming spurious components with complicated correlation properties across PLANCK's channels, multifrequency filters based on matched and scale-adaptive filtering have been extended to spherical topologies and applied to simulated data. These filters were shown to efficiently amplify and extract the SZ-signal by combining spatial band-filtering and linear combination of observations at different frequencies, where the filter shapes and the linear combination coefficients follow from the cross- and autocorrelation properties of the sky maps, the anticipated profile of SZ clusters and the known SZ spectral dependence. The characterisation of the resulting SZ-sample yielded a total number of 6000 detections above a statistical significance of 3 sigma and the distribution of detected clusters in mass, redshift, and position on the sky. In a related project, a method of constructing morphological distance estimators for resolved SZ cluster images is proposed. This method measures a cluster's SZ-morphology by wavelet decomposition. It was shown that the spectrum of wavelet moments can be modeled by elementary functions and has characteristic properties that are non-degenerate and indicative of cluster distance. Distance accuracies following from a maximum likelihood approach yielded values as good as 5% for the relative deviation, and deteriorate only slightly when noise components such as instrumental noise or CMB fluctuations were added. Other complications like cool cores of clusters and finite instrumental resolution were shown not to affect the wavelet distance estimation method significantly. Another line of research is the Rees-Sciama (RS) effect, which is due to gravitational interaction of CMB photons with non-stationary potential wells. This effect was shown to be a second order gravitational lensing effect arising in the post-Newtonian expansion of general relativity and measures the divergence of gravitomagnetic potentials integrated along the line-of-sight. The spatial autocorrelation function of the Rees-Sciama effect was derived in perturbation theory and projected to yield the angular autocorrelation function while taking care of the differing time evolution of the various terms emerging in the perturbation expansion. The RS-effect was shown to be detectable by PLANCK as a correction to the primordial CMB power spectrum at low multipoles. Within the same perturbative formalism, the gravitomagnetic corrections to the autocorrelation function of weak gravitational lensing observables such as cosmic shear could be determined. It was shown that those corrections are most important on the largest scales beyond 1~Gpc, which are difficult to access observationally. For contemporary weak lensing surveys, gravitomagnetic corrections were confirmed not play a significant role. A byproduct of the simulation of CMB fluctuations on the basis of Gaussian random fields was a new way of generating coded mask patterns for X-ray and gamma-ray imaging. Coded mask cameras observe a source by recording the shadow cast by a mask onto a position-sensitive detector. The distribution of sources can be reconstructed from this shadowgram by correlation techniques. By using Gaussian random fields, coded mask patterns can be specifically tailored for a predefined point-spread function which yields significant advantages with respect to sensitivity in the observation of extended sources while providing a moderate performance compared to traditional mask generation schemes in the observation of point sources. Coded mask patterns encoding Gaussian point-spread functions have been subjected to extensive ray-tracing studies where their performance has been evaluated

High-Fidelity Gravity Modeling Applied to Spacecraft Trajectories and Lunar Interior Analysis

As the complexity and boldness of emerging mission proposals increase, and with the rapid evolution of the available computational capabilities, high-accuracy and high-resolution gravity models and the tools to exploit such models are increasingly attractive within the context of spaceflight mechanics, mission design and analysis, and planetary science in general. First, in trajectory design applications, a gravity representation for the bodies of interest is, in general, assumed and exploited to determine the motion of a spacecraft in any given system. The focus is the exploration of trajectories in the vicinity of a system comprised of two small irregular bodies. Within this context, the primary bodies are initially modeled as massive ellipsoids and tools to construct third-body trajectories are developed. However, these dynamical models are idealized representations of the actual dynamical regime and do not account for any perturbing effects. Thus, a robust strategy to maintain a spacecraft near reference third-body trajectories is constructed. Further, it is important to assess the perturbing effect that dominates the dynamics of the spacecraft in such a region as a function of the baseline orbit. Alternatively, the motion of the spacecraft around a given body may be known to extreme precision enabling the derivation of a very high-accuracy gravity field for that body. Such knowledge can subsequently be exploited to gain insight into specific properties of the body. The success of the NASA\u27s GRAIL mission ensures that the highest resolution and most accurate gravity data for the Moon is now available. In the GRAIL investigation, the focus is on the specific task of detecting the presence and extent of subsurface features, such as empty lava tubes beneath the mare surface. In addition to their importance for understanding the emplacement of the mare flood basalts, open lava tubes are of interest as possible habitation sites safe from cosmic radiation and micrometeorite impacts. Tools are developed to best exploit the rich gravity data toward the numerical detection of such small features

On Floating Breakwaters Efficiency - a 2DV Parametric Based Analysis

In long term evolution numerical models, the interactions of a floating barrier with the wave field is then deputed to some parametrized transfer functions, which mimic wave energy transmission and dissipation in the frequency domain. This thesis provide, as final result, two transfer functions (one for incident waves, one for reflected ones) for a particular class of compact shaped floating breakwaters. These functions are based on three main parameters, which have been derived on physical model results. The first one (x) is the ratio between the incoming wave frequency and an approximation of FB heave natural frequency, based on principal FB cross section dimensions. Wave steepness has been considered to be the second variable which helps in describing the amount of dissipated energy. An FB draft to water depth ratio has been identified. Available algorithms for the decomposition into incident and reflected waves of flume records are mostly Stokes-FFT based. Therefore they suffer some limitations for relatively high wave steepness (Ch. 4). Since the latter is considered as a crucial parameter, a lot of effort has been drawn in solving some conundrums of actual methods. Two algorithms are proposed. The first one (Ch. 5), based on empirical mode decomposition, did not give satisfactory results. The second one (Ch. 6) is based on linear waves superposition, but, getting rid of linear dispersion relation, detects automatically each phase celerity. The proposed algorithm appears to be effective for relatively shallow water waves, for which the phase modulation approach is more consistent than Stokes formulations. A Stokes 2nd order algorithm has also been implemented. In Ch. 7 the experimental set up is presented. A second order analysis of transmission and reflection processes is also introduced.Results are given (and discussed) in Ch. 8. Linear transmission and reflection transfer functions are derived, based on experimental data fitting. These are finally validated with irregular wave test measurements.It is found that the transmission process mainly depends on frequency (x) and on FB relative draft. The last parameter does not enter the reflection process, which basically described by x and wave steepness. In particular, steeper waves loose more energy, and are less reflected. For transmitted waves only, a significant amount of energy transfer from primary to secondary harmonics is observed

Probing the Interstellar Medium and Dark Matter with Pulsars

Pulsars are rapidly rotating, highly magnetised neutron stars which emit electromagnetic radiation from their magnetic poles in the form of highly collimated beams. Pulsars are known as a powerful tool to probe the interstellar medium (ISM) and its constituents in the Miky Way. In this thesis we focus on probing the non-baryonic entities in the Milky Way, namely interstellar magnetic fields and dark matter. The first part of the thesis is dedicated to the investigation of Galactic magnetic fields, which are a major agent in the dynamics and energy balance of the ISM, and general evolution of the Galaxy. Small-scale turbulent magnetic fields in the Milky Way can be probed by monitoring variations in the Faraday rotation of linearly polarised radiation of pulsars. Following this idea, we use high-cadence, low-frequency observations from a set of selected pulsars carried out with German LOw-Frequency ARray (LOFAR) stations. The method that is used to determine the Faraday rotation measures (RMs) of pulsar signals is the Bayesian generalised Lomb-Scargle periodogram technique, developed in this thesis. We find that measured RMs are strongly affected by the highly time-variable terrestrial ionosphere. We have mitigated the ionospheric contribution assuming a thin-layer model of the ionosphere. We conclude that within this approximation the ionospheric RM corrections are accurate to ~ 0.06 - 0.07 rad/m2, which defines our sensitivity towards long-term astrophysical RM variations. Following these results, we investigate the sensitivity to the turbulence in the magnetised ISM between the pulsar and observer. No astrophysically credible signal has been detected. We discuss implications of the non-detection and analyse the possibilities for future investigations. The second part of this thesis deals with dark matter - a matter which accounts for about a quarter of the energy density of the Universe, and the nature of which is still under debate. The ultralight scalar field dark matter is one of the compelling dark matter candidates, which leaves characteristic imprints in the times of arrival of radio pulses from pulsars. We search for traces of ultralight scalar-field dark matter in the Galaxy using the latest Parkes Pulsar Timing Array dataset that contains the times of arrival of 26 pulsars. No statistically significant signal has been detected. Therefore, we set an upper limit on the local dark matter density. The most stringent constraints are still one order of magnitude above the local dark-matter density inferred from kinematics of stars in the Milky Way. We conclude by discussing the prospects of detecting the fuzzy dark matter with future radio astronomical facilities