Browsing by master's degree program "Master's Programme in Particle Physics and Astrophysical Sciences"

Mercury is the smallest planet in the Solar System. The planet has a tenuous atmosphere, which means that it can be modeled as an atmosphereless object. Such Solar System bodies are covered in loose material called regolith, which affects how the object scatters light from the Sun. Photometry is a type of measurement that records the intensity of scattered light as a function of the viewing geometry, which is defined using angles from the surface towards the Sun and towards the observing instrument. Mercury was studied in 2011–2015 by the MESSENGER (MErcury Surface, Space ENvironment, GEochemistry and Ranging) mission of the United States’ National Aeronautics and Space Admin- istration, NASA. The present thesis uses spectrophotometric data, i.e., brightness as a function of wavelength, from the spacecraft’s Mercury Dual Imaging System (MDIS) instrument. Two theo- retical models, the Lommel–Seeliger (LS) and particulate medium (PM) models, are fitted to the observed reflectance using the least-squares method. The PM model is the more complicated of the two, and it includes a shadowing correction that de- pends on three model parameters. The parameters describe the properties of a particulate medium, i.e., regolith. The most important of the parameters is the packing density of the regolith, which is defined as the ratio of the volume of the particles to the total volume. The other two parameters describe the medium’s surface roughness in horizontal and vertical directions. The PM model is fitted to the observed reflectance for various different combinations of the model parameters. Ini- tially, only discrete and predetermined parameter values are used, but the parameter values are extended to arbitrary values using interpolation. Trilinear interpolation is utilized using several methods, followed by Markov chain Monte Carlo (MCMC) sampling for the final results. Most of the methods agree with one another, and fall within the uncertainties of the best solution, which allows to form an argumented conclusion about the best parameter values of the PM model. The best parameter values correspond to a densely-packed regolith with horizontally smooth surface and large height variations. The results of the present study can be used in the BepiColombo mission to Mercury, which is planned to begin its science operations in early 2026.

Gravitational waves predicted by the theory of general relativity are providing us with an opportunity to study cosmological processes well beyond the horizon for electromagnetic observations. One such process is a first-order phase transition as the universe cools down, which could generate gravitational waves observable in the millihertz range today. The upcoming Laser Interferometer Space Antenna (LISA) is a space-based gravitational wave observatory with peak sensitivity in the millihertz range, so it has the potential to observe a sufficiently strong phase transition signal. The LISA Consortium have developed the LISA simulation pipeline for the purposes of creating mock data, which can then be used to test data analysis methods in preparation for the real LISA data analysis. Our aim is to first test the simulation pipeline by injecting a phase transition signal, with added instrument noises and galactic binary confusion noise, which we expect to be present in the real observations as well. Second, we will attempt to recover the injected signal to see whether it is detectable based on the deviance information criterion (DIC). We will do this for 25 different parameter combinations in order to chart the detectability of signals from different phase transition scenarios. Our results show better detectability for phase transition signals with higher amplitudes and frequencies centered around the mHz range, which is where the expected peak sensitivity of LISA lies. The confusion noise appears to be less of a distraction to our observations than the instrument noises, which set limits in the extremes of the LISA frequency range.

The primordial perturbations created by inflation in the early Universe are known to be able to produce significant amount of primordial black holes and gravitational waves with large amplitudes in some inflationary models. Primordial black holes are produced by primordial scalar perturbations and gravitational waves are partly primordial tensor perturbations and partly produced by scalar perturbations. In this thesis we review some of the current literature on the subject and discuss a few inflationary models that are capable of producing primordial scalar perturbations large enough to create a significant amount of primordial black holes. The main focus is on ultra-slow roll inflation with a concrete example potential illustrating the dynamics of the scenario followed by a briefer treatment of some of the alternative models. We start by explaining the necessary background theory for the understanding of the subject at hand. Then we move on to the inflationary models covered in this thesis. After that we explain the production of the primordial black holes and gravitational waves from scalar perturbations. Then we consider primordial black holes as a dark matter candidate and go through the most significant known restrictions on the existence of primordial black holes with different masses. We discuss some of the possible future constraints for the remaining possible mass window for which primordial black holes could explain all of dark matter. We then briefly discuss two planned space-based gravitational wave detectors that may be able to detect gravitational waves created by inflation.

Coronal mass ejections (CMEs) are large eruptions of magnetized plasma from the solar corona. Fast CMEs can drive shock waves which are capable of accelerating charged particles to high energies. These accelerated particles emit electromagnetic radiation, including radio emission. Studying radio emission associated with CME-driven shocks offers a way to remotely investigate shock-accelerated electrons as well the shock itself. Solar radio bursts are transient events where the radio emission of the Sun rises above the background level. A classical division based on their appearance in a dynamic spectrum divides solar radio bursts into five categories: types I-V. Of these five different types, type II and type IV radio bursts are most commonly associated with CMEs. Occasionally, type II radio bursts exhibit a bursty fine structure known as herringbones. These are regarded as signatures of individual electron beams accelerated by CME-driven shocks. This thesis studies the radio emission associated with a CME that erupted on 1 September 2014. White-light imaging of the CME revealed a prominent shock wave. Simultaneously, the dynamic spectrum exhibited spike-like radio emission resembling herringbones. The aim of the study presented in this thesis is to find the source location of this radio emission relative to a three dimensional reconstruction of the shock. The source location of the radio emission can be used to conclude the likely origin of the electrons responsible for it. Additionally, in situ electron flux measurements are investigated in an attempt to connect the remote and in situ detections of energetic electrons. Using interferometric radio observations of the Sun and reconstructing the CME shock in three dimension revealed the location of the radio emission to be at the flank of the CME-driven shock. Such location suggests that the spike-like radio emission observed in the dynamic spectrum originates from shock-accelerated electrons. The location of the radio emission at the flanks of the CME shock was also used to get an estimation of the lateral expansion of the CME. Although the in situ electron flux measurements detected high-energy electrons, their inferred release time at the Sun did not coincide with observed radio emission.

I use hydrodynamic cosmological N-body simulations to study the effect that a secondary period of inflation, driven by a spectator field, would have on the Local Group substructures. Simulations of the Local Group have been widely adopted for studying the nonlinear structure formation on small scales. This is essentially because detailed observations of faint dwarf galaxies are mostly limited to within the Local Group and its immediate surroundings. In particular, the ∼ 100 dwarf galaxies, discovered out to a radius of 3 Mpc from the Sun, constitute a sample that has the potential to discriminate between different cosmological models on small scales, when compared to simulations. The two-period inflaton-curvaton inflation model is one such example, since it gives rise to a small-scale cut-off in the ΛCDM primordial power spectrum, compared to the power spectrum of the ΛCDM model with single field power-law inflation. I investigate the substructures that form in a simulated analogue of the Local Group, with initial conditions that incorporate such a modified power spectrum. The most striking deviation, from the standard power-law inflation, is the reduction of the total number of subhalos, with v_max > 10 km/s, by a factor of ∼ 10 for isolated subhalos and by a factor of ∼ 6 for satellites. However, the reduction is mostly in the number of non-star-forming subhalos, and the studied model thus remains a viable candidate, taking into account the uncertainty in the Local Group total mass estimate. The formation of the first galaxies is also delayed, and the central densities of galaxies with v_max < 50 km/s are lowered: their circular velocities at 1 kpc from the centre are decreased and the radii of maximum circular velocity are increased. As for the stellar mass-metallicity and the stellar mass-halo mass relations, or the selection effects from tidal disruption, I find no significant differences between the models.

Over the last thirty years more than five thousand exoplanets have been discovered around a wide variety of stellar objects. Most exoplanets have been discovered using the transit method, which relies on observing the periodic brightness changes of stars as their planet transits in front of them. The discovery efficiency of these planets has been strongly enhanced with the advent of space telescopes dedicated to the discovery of planets using the transit method. Planetary signals in the photometric data of active stars can be challenging to find, as the surface features of the stars combined with their rotation might produce signals which are orders of magnitude stronger than those caused by the planetary transit. The question of what statistical methods should be applied to account for the innate variability of stars in order to identify the transits of exoplanets in the lightcurves of active stars is being investigated in this thesis. I test a number of statistical methods in order to combat stellar activity and to identify planetary transit signals. The rotation period of the star is investigated using the Lomb-Scargle and likelihood ratio periodograms. Starspot induced variability is approximated with a number of sinusoids, with periods based on the star's rotation period. Additional stellar activity is filtered out using autoregressive and moving average models. Model fittings are performed with least squares fitting, and using samples generated by the Adaptive Metropolis algorithm. After the lightcurve has been detrended for stellar activity, the likelihoods of planetary transit signals are assessed with a box-fitting algorithm. Models are compared with the Bayesian and Akaike information criteria. Planetary characteristics are then estimated by modeling the shape of the transit lightcurve. These methods are tested and performed on the lightcurve of HD~110082, a highly active young star with one confirmed planetary companion, based on the observations of the TESS space telescope. I find that stellar activity is sufficiently filtered out with a model containing four sinusoid signals. The signal corresponding to the planet is confirmed by the box fitting algorithm, agreeing with results available in scientific literature.

The thesis discusses the observations of linear polarization occurring on the surface of near-Sun asteroid (NSA) (3200) Phaethon. As part of the research, new observational data from the Nordic Optical Telescope (NOT) are analyzed. The data were obtained in 2019, a few weeks after the perihelion passage. The scientific goal of these observations was to explain the partially conflicting results of linear polarization in earlier literature. The conflicting degrees of linear polarization in large phase angles (the Sun - the object - the observer) covered different hemispheres of Phaethon, which may be explained by the differences in surface regolith size distribution. A new pipeline was created to analyze the data. The pipeline was used to calculate the Stokes parameters of Phaethon based on the fluxes in the data frames obtained by using the ALFOSC (Alhambra Faint Object Spectrograph and Camera). The pipeline is directly applicable to other optical linear polarization data observed using ALFOSC. However, it is also applicable to sidereal observations or non-sidereal observations conducted with other instruments with relatively minor modifications. Features of the pipeline include the estimation of the uncertainty and validity of the observations by classical error propagation, as well as the possible dependence of the results based on the diameter of the circular aperture. The results of the analysis are compared to the prior observations of Phaethon presented in the literature. First, the variation of linear polarization is modeled as a function of phase angle using the Lumme-Muinonen Function (LMF) with different initial conditions. The analysis results are in line with previous results, although the data points in large phase angles have relatively large uncertainty estimates. Second, the existence of latitudinal correlation is evaluated, as such analysis is enabled by the sufficiently large total number of observations and a pole solution presented in earlier literature. No clear evidence of latitudinal correlation was found. Third, the sufficiently stable sub-observer latitude during the NOT observations allows the search for a correlation between linear polarization and rotational phase. There are consistent variation features as a function of the rotational phase, but they are classified as hints of detection in terms of statistical uncertainties (<5σ). In addition to analyzing the variation of linear polarization, a statistical summary of the orbital parameters of near-Sun asteroids is presented. To complete the thesis, a credibility analysis of all the reported results is performed.

This thesis studies gas-rich galaxy mergers at redshifts of z ∼ 1-2 using numerical simulations, with a particular focus on the effect of feedback from active galactic nuclei (AGNs). In total, 16 galaxy mergers at redshifts z = 1 and z = 2 were modeled using the simulation codes KETJU and GADGET-3. The simulations were performed on the supercomputer Mahti located at the Finnish IT Centre for Science (CSC). AGN feedback can be described as the radiative and mechanical energy released through accretion, which act to heat and disperse the remaining gaseous material surrounding the central supermassive black hole (SMBH). The feedback mechanisms include, for example, photoionization heating due to high-energy photons and winds and jets driven by the AGN. Numerically, AGN feedback was implemented using two models in this thesis: thermal and kinetic AGN feedback, in which the gas particles are either heated or ‘kicked’, respectively. In addition to AGN feedback, the simulations included metal-dependent gas cooling, stochastic star formation, and stellar feedback. The simulated progenitor galaxies were gas-rich spirals consistent with observed galaxies at redshifts z = 1 and z = 2. The virial masses of the progenitors were set to correspond to typical massive galaxies at their redshifts using the Press-Schechter mass function, while the initial masses for the central SMBHs were set using observed MBH-M⋆ and MBH-σ⋆ relations. The gas fractions and metal abundances of the progenitors were calibrated using observational data at their respective redshifts. The KETJU and GADGET-3 simulations produced very similar results for the overall evolution of a given merger configuration. Consistent with earlier studies, the kinetic feedback was observed to be significantly more effective at removing gas from the galaxies than the thermal feedback. The combined effect of AGN and stellar feedback was observed to strongly suppress star formation, with the star formation of one merger being almost completely shut down. The thermal and kinetic feedback models caused noticeable differences in the orbital evolution of the SMBH binaries. Merger timescales were significantly longer for the SMBHs in the KETJU simulations with kinetic feedback. In general, the merger timescales increased with decreasing initial eccentricity for the SMBH binary. The merger remnants were compared to observed MBH-σ⋆, R1/2-M⋆, fgas-M⋆, and mass-metallicity relations. Overall, the remnants were reasonably consistent with the observed relations. Hence, we can conclude that AGN feedback plays a crucial role in galaxy evolution and that both the thermal and kinetic feedback models are able to produce realistic high-redshift galaxies.

The high luminosity upgrade of the Large Hadron Collider (LHC) will result in higher collision rates and current equipment is not up to par with this future era of operations. Identification and reconstruction of hard interactions may be hampered by the spatial overlapping of particle tracks and energy deposits from additional collisions and this often leads to false triggers. In addition, current particle detectors suffer from radiation damage that severely affects the accuracy of our results. The new minimum ionizing particle (MIP) timing detector will be equipped with low gain avalanche detectors which have a comparably small timing resolution that helps with the track reconstruction and their thin design limits the radiation damage over time. In this thesis, we build an experimental set-up in order to study the timing resolution of these detectors closely. In order to find the timing resolution, we take the time difference between the signals from two detectors and put it in a histogram to which we apply a Gaussian fit. The standard deviation of this Gaussian is called the time spread from which we can estimate the timing resolution. We first build, characterize and improve our experimental set-up using reference samples with known timing resolution until our set-up is able to reproduce the reference value. Then we repeat the measurements with irradiated samples in order to study how radiation damage impacts timing. We were able to adjust our setup with reference samples until we measured a timing resolution of 33$\pm$2~ps. We use this result to calculate the timing resolution of an irradiated sample ($8.0 \times 10^{14}$ proton fluence) and we found a timing resolution of 62$\pm$2~ps. This thesis also discusses the analysis of the data and how the data can be re-analyzed to try to improve the final result.