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

The 25th Annual Precise Time and Time Interval (PTTI) Applications and Planning Meeting

Papers in the following categories are presented: recent developments in rubidium, cesium, and hydrogen-based frequency standards, and in cryogenic and trapped-ion technology; international and transnational applications of precise time and time interval (PTTI) technology with emphasis on satellite laser tracking networks, GLONASS timing, intercomparison of national time scales and international telecommunication; applications of PTTI technology to the telecommunications, power distribution, platform positioning, and geophysical survey industries; application of PTTI technology to evolving military communications and navigation systems; and dissemination of precise time and frequency by means of GPS, GLONASS, MILSTAR, LORAN, and synchronous communications satellites

Elevation and Deformation Extraction from TomoSAR

3D SAR tomography (TomoSAR) and 4D SAR differential tomography (Diff-TomoSAR) exploit multi-baseline SAR data stacks to provide an essential innovation of SAR Interferometry for many applications, sensing complex scenes with multiple scatterers mapped into the same SAR pixel cell. However, these are still influenced by DEM uncertainty, temporal decorrelation, orbital, tropospheric and ionospheric phase distortion and height blurring. In this thesis, these techniques are explored. As part of this exploration, the systematic procedures for DEM generation, DEM quality assessment, DEM quality improvement and DEM applications are first studied. Besides, this thesis focuses on the whole cycle of systematic methods for 3D & 4D TomoSAR imaging for height and deformation retrieval, from the problem formation phase, through the development of methods to testing on real SAR data. After DEM generation introduction from spaceborne bistatic InSAR (TanDEM-X) and airborne photogrammetry (Bluesky), a new DEM co-registration method with line feature validation (river network line, ridgeline, valley line, crater boundary feature and so on) is developed and demonstrated to assist the study of a wide area DEM data quality. This DEM co-registration method aligns two DEMs irrespective of the linear distortion model, which improves the quality of DEM vertical comparison accuracy significantly and is suitable and helpful for DEM quality assessment. A systematic TomoSAR algorithm and method have been established, tested, analysed and demonstrated for various applications (urban buildings, bridges, dams) to achieve better 3D & 4D tomographic SAR imaging results. These include applying Cosmo-Skymed X band single-polarisation data over the Zipingpu dam, Dujiangyan, Sichuan, China, to map topography; and using ALOS L band data in the San Francisco Bay region to map urban building and bridge. A new ionospheric correction method based on the tile method employing IGS TEC data, a split-spectrum and an ionospheric model via least squares are developed to correct ionospheric distortion to improve the accuracy of 3D & 4D tomographic SAR imaging. Meanwhile, a pixel by pixel orbit baseline estimation method is developed to address the research gaps of baseline estimation for 3D & 4D spaceborne SAR tomography imaging. Moreover, a SAR tomography imaging algorithm and a differential tomography four-dimensional SAR imaging algorithm based on compressive sensing, SAR interferometry phase (InSAR) calibration reference to DEM with DEM error correction, a new phase error calibration and compensation algorithm, based on PS, SVD, PGA, weighted least squares and minimum entropy, are developed to obtain accurate 3D & 4D tomographic SAR imaging results. The new baseline estimation method and consequent TomoSAR processing results showed that an accurate baseline estimation is essential to build up the TomoSAR model. After baseline estimation, phase calibration experiments (via FFT and Capon method) indicate that a phase calibration step is indispensable for TomoSAR imaging, which eventually influences the inversion results. A super-resolution reconstruction CS based study demonstrates X band data with the CS method does not fit for forest reconstruction but works for reconstruction of large civil engineering structures such as dams and urban buildings. Meanwhile, the L band data with FFT, Capon and the CS method are shown to work for the reconstruction of large manmade structures (such as bridges) and urban buildings

The 26th Annual Precise Time and Time Interval (PTTI) Applications and Planning Meeting

This document is a compilation of technical papers presented at the 26th Annual PTTI Applications and Planning Meeting. Papers are in the following categories: (1) Recent developments in rubidium, cesium, and hydrogen-based frequency standards, and in cryogenic and trapped-ion technology; (2) International and transnational applications of Precise Time and Time Interval technology with emphasis on satellite laser tracking, GLONASS timing, intercomparison of national time scales and international telecommunications; (3) Applications of Precise Time and Time Interval technology to the telecommunications, power distribution, platform positioning, and geophysical survey industries; (4) Applications of PTTI technology to evolving military communications and navigation systems; and (5) Dissemination of precise time and frequency by means of GPS, GLONASS, MILSTAR, LORAN, and synchronous communications satellites

Cross-correlation searches for persistent gravitational waves with Advanced LIGO and noise studies for current and future ground-based gravitational-wave detectors.

University of Minnesota Ph.D. dissertation. 2018. Major: Physics. Advisor: Vuk Mandic. 1 computer file (PDF); 241 pages.Over the last three years, the Advanced Laser Interferometer Gravitational-wave Observatory (LIGO) has detected signals from colliding black holes and a signal from colliding neutron stars. These detections ushered in a new era of gravitational-wave (GW) astrophysics and multimessenger astronomy that allows us to probe new regions of the universe. One of the next frontiers for gravitational-wave astronomy is the detection and characterization of the stochastic GW background (SGWB). A measurement of the SGWB from unresolved compact binary systems could come as Advanced LIGO reaches design sensitivity, and future detectors will be important for digging beyond that astrophysical background towards trying to measure signals from relic gravitational waves produced in the early universe. In this dissertation, I present cross-correlation-based searches for a SGWB and other persistent sources of GWs. I introduce and use a new method for setting limits on the strain amplitude of a potential source of GWs in the directions of Scorpius X-1, the galactic center, and Supernova 1987a in the frequency band from 20-1726 Hz. I also set limits on persistent, broadband point sources of GWs across the whole sky. Finally, I show how we can implement data analysis techniques to improve the Advanced LIGO detector sensitivity to persistent sources of GWs. Improving sensitivity of current detectors and planning for future detectors is vital to the effort to measure and understand the SGWB. This will requires a better understanding of the noise sources that limit sensitivity, especially at lower frequencies. To this end, I outline a method for estimating and modeling correlated magnetic noise between spatially separated GW detectors. I also present results from a 3D seismometer array deployed at the Homestake Mine, aimed at characterizing seismic and Newtonian noise for future GW detectors. I estimate the fundamental Rayleigh-wave eigenfunction, and then use it in a seismic radiometer algorithm to separate different components of the seismic field that contribute differently to the Newtonian noise. Finally, I present estimates of the Newtonian noise as a function of depth in the frequency band from 0.5-5 Hz based on results from the seismic radiometer