<p dir="ltr">Understanding the acceleration of particles responsible for high-energy non-thermal phenomena in astrophysical jets is a ubiquitous pursuit. A possible culprit for non-thermal particle acceleration is turbulence. Specifically in this thesis, I investigate highly magne- tized or relativistic turbulence, where the magnetic energy to enthalpy ratio of the plasma is much greater than one, as a possible high-energy accelerator inside relativistic jets. I do this through three distinct projects. </p><p dir="ltr">My first project [1] (discussed in Section 3) was built upon a recent study of relativistic turbulence from [2], which found that a non-thermal particle equilibrium can be achieved when a plasma is heated via turbulence but allowed to cool radiatively. I extrapolated these results from PIC (Particle-in-Cell) simulations to larger scales and magnetizations, allowing me to encode key microphysical results of PIC simulations into a Fokker-Planck formalism. Combining these results with a single zone model for a blazar jet, I successfully define the underlying particle distribution with the global parameters of the emission region. To test this model, I fit data from 12 sources and successfully constrain key blazar parameters such as magnetization, bulk Lorentz factor, emission region size, and distance from the central engine. </p><p dir="ltr">My second project covers the development and testing of the open-source toolkit Tleco. This code base was used to evolve the Fokker-Planck equation and solve the resultant emission in my first project. Tleco offers efficient algorithms for evolving particle distributions and solving the resultant emission. It is meant to be user-friendly and easily customizable. </p><p dir="ltr">My third project attempts to enhance our understanding of coherent structures in relativistic turbulence. I employ intermittency analysis to establish a link between statistical fluctuations within the plasma and regions of high-energy dissipation. To achieve this, we used first-principle turbulent PIC simulations across a range of magnetizations and fluctuating magnetic field values. By utilizing the statistical fluctuations to determine the fractal dimension of the structures, I then examine their filling fraction and its dependence on magnetization and the fluctuating magnetic field.</p>

This master thesis discuss about ionizing radiation, interaction with matter and effects on her. In the work is discussed differend types of interactions of directly ionizing radiation and indirectly ionizing radiation. Below is an overview of methods of shielding from all of types of ionizing radiation. Second part of this thesis discuss about single event effects in semiconductors which are cause by ionizing radiation. At the end is described design of measuring instrument included FPGA chips. This design is discused both from point of wiew hardware and software too.

This master thesis discuss about ionizing radiation, interaction with matter and effects on her. In the work is discussed differend types of interactions of directly ionizing radiation and indirectly ionizing radiation. Below is an overview of methods of shielding from all of types of ionizing radiation. Second part of this thesis discuss about single event effects in semiconductors which are cause by ionizing radiation. At the end is described design of measuring instrument included FPGA chips. This design is discused both from point of wiew hardware and software too.

<p>     </p> <p>This article-based dissertation provides a review of the broad subject of Neutron Star- their emission properties, plasmoids ejection events, and their proposed physical mechanisms. The primary purpose of this dissertation is to provide an extensive description of the research projects I undertook during my tenure as a Graduate Research Assistant, under the guidance of my advisor Prof. Maxim Lyutikov. </p> <p><br></p> <p>Chapter 1 provides a broad overview of the Neutron stars, their classification, proposed emission models, and a summary of magnetars and associated observed phenomena. </p> <p><br></p> <p>In Chapter 2, I present a version of the research article published in the <em>Monthly Notices of the Royal Astronomical Society</em>. The work is titled "Rotating Neutron Stars Without Light Cylinders" and discusses twisted and differentially rotating neutron star magnetospheres that do not have a light cylinder, generate no wind, and thus do not spin down. The magnetosphere of such neutron stars is composed of embedded differentially rotating flux surfaces, with the angular velocity decreasing as Ω ∼ 1/r. It was found, both analytically and using numerical simulations, that for spin parameters larger than some critical value, the light cylinder appears, the magnetosphere opens up, and the wind is generated. </p> <p>In Chapter 3, I present a version of the research article published in <em>The Astrophysical Journal</em>. The work is titled "Relativistic Magnetic Explosions" and was undertaken under the supervision of Dr. Maxim Barkov, in collaboration with Dr. Konstantinos N. Gourgou- liatos and Dr. Lyutikov2 Barkov. It discusses the dynamics of magnetically driven explosive astrophysical events, like magnetar bursts and flares. We model a relativistic expansion of highly magnetized and highly magnetically over-pressurized clouds. We observe that the corresponding dynamics are qualitatively different from fluid explosions due to the topological constraint of the conservation of the magnetic flux. Using analytical, relativistic MHD as well as force-free calculations, we find that the creation of a relativistically expanding, causally disconnected flow obeys a threshold condition: it requires sufficiently high initial over-pressure and sufficiently quick decrease of the pressure in the external medium (the pre-explosion wind). In the subcritical case, the magnetic cloud just puffs up" and quietly expands with the pre-flare wind. We also find a compact analytical solution to the Prendergast problem - the expansion of force-free plasma into the vacuum. </p> <p><br></p> <p>Chapter 4 is the extension of the work in Chapter 3 and focuses on the dynamics of relativistic Coronal Mass Ejections (CMEs), from launching by shearing of foot-points (either slowly or suddenly), to propagation in the preceding magnetar wind. The work has been accepted to be published in <em>Monthly Notices of the Royal Astronomical Society</em>. For slow shear, we find that most of the energy injected into the CME is first spent on the work done on breaking through the over-laying magnetic field. At later stages, sufficiently powerful CMEs may lead to the detonation of a CME and opening of the magnetosphere beyond some equipartition radius req, where the decreasing energy of the CME becomes larger than the decreasing external magnetospheric energy. Post-CME magnetosphere relaxes via the formation of a plasmoid-mediated current sheet, initially at req, and slowly reaching the light cylinder. Both the location of the foot-point shear and the global magnetospheric configuration affect the frequent/weak versus rare/powerful CME dichotomy - to produce powerful flares the slow shear should be limited to field lines that close in near the star.  After the creation of a topologically disconnected flux tube, the tube quickly (at ∼ the light cylinder) comes into force-balance with the preceding wind and is passively advected/frozen in the wind afterward. </p> <p>For fast shear case, the shearing of foot-points leads to the generation of Alfvén wave and the pressure of such Alfvén leads to the opening of the magnetosphere. At distances much larger than the light cylinder, the resulting shear Alfvén waves propagate through the wind non-dissipatively. </p> <p><br></p> <p>In Chapter 5, I switch gears and study the optical polarization of Crab pulsar. I start by deriving a general relation for the polarization direction of the electric dipole-type radiation produced by a particle moving in an arbitrary electromagnetic field. The derived relations are then applied to reproduce optical polarization swings in Crab pulsar assuming a Michel- Bogovalov solution for the current sheet. With this, I was able to reproduce down to intricate details the spin-phase trajectory of the position angle (PA) in the Stokes parameters U-Q plane. This chapter however remains a work in progress. We still don't fully understand the physical mechanism behind the polarization characteristics of the Crab, especially the origin of the point where the inner loop connects with the bigger outer loop. I plan to fully answer these questions before sending our findings for publication. </p> <p><br></p> <p>Chapter 6 summarizes the main results and conclusions of the research projects and mentions the prospects. References are compiled after the appendices so that they are first cited, followed by a CV and a list of publications. </p>

<p dir="ltr">Neutron star mergers generate powerful gravitational waves and various types of electromagnetic signals, including gamma-ray bursts (GRB), kilonovae, and their afterglows. Observing and modeling these signals help us understand the physical processes of the merger events. Radiation from mergers can also serve as probes to study nuclear physics and cosmology. In this report, I focus on two types of signals: the GRB afterglow and the kilonova. GRB afterglows are non-thermal radiation produced by the interaction of relativistic jets and circumburst material, where the jets are launched perpendicular to the merger plane. Kilonovae are the thermal radiation emitted from the hot materials ejected during the merger. Besides the modeling of these objects, I also investigate their application in multimessenger astrophysics, especially the constraint on the expansion rate of the Universe. </p><p dir="ltr">First, I developed a GRB afterglow model to account for the off-axis observation of a structured jet. Using a jet structure derived from a three-dimensional general relativistic magnetohydrodynamic simulation, we performed a joint analysis of the multimessenger data of the neutron star merger event GW170817, including the gravitational wave data and GRB afterglow data in the radio band. We have tightly constrained the observing angle of GW170817 and broken the degeneracy between the inclination angle and luminosity distance measured in gravitational waves. With a better constrained distance, we improved the standard siren measurement of the Hubble constant to $H_0 = 69.5\pm 4\ \mathrm{km\ s^{-1}\ Mpc^{-1}}$. The error bar has been reduced by a factor of 2. This work demonstrates that the modeling of off-axis GRB afterglow can significantly improve the standard siren method, provided that we have a reliable jet structure.</p><p dir="ltr">Second, I upgrade the GRB afterglow model in the first work, extending it to the late time where lateral spreading of the GRB jet becomes important. In this model, the ultra-relativistic blastwave is approximated by an infinitely thin two-dimensional surface. With this approximation, the hydrodynamic equations can be analytically integrated over the radius. Further assuming axial symmetry, the three-dimensional hydrodynamic simulation can be reduced to one dimension, which significantly increases the computational efficiency. We have compared our method to full numerical simulations and existing GRB afterglow modeling tools. The comparison shows good agreement and verifies our approach. Compared to these tools, our model has better flexibility and is applicable in a broader context. This method has been developed into a numerical code, \texttt{jetsimpy}, which we have provided to the community. It will serve as a powerful tool in the era of multimessenger astrophysics.</p><p dir="ltr">Finally, I investigate the possibility of long-lived massive neutron stars as neutron star merger remnants. A long-lived massive neutron star can inject a significant amount of energy into the merger ejecta, boosting the luminosity of kilonova by several orders of magnitude. However, this type of event has not yet been observed in optical sky surveys. We developed a boosted kilonova model with a detailed calculation of the photoionization process to better describe the efficiency of energy injection from spin down power to the ejecta. Our study found that boosted kilonovae, if commonly occurring, they should have already been observed given the accumulated time in sky surveys. As a result, the absence of detection implies that long-lived massive neutron stars as neutron star merger remnants are likely to be rare in the Universe.</p>