No description available.

<p>Astrophysical jets are collimated streams of magnetized plasma launched from compact objects, such as neutron stars or black holes. These jets, powered by the accretion of surrounding gas onto the compact object, can accelerate particles to extreme energies and produce powerful radiation.</p> <p><br></p> <p>In this report, I investigate energy dissipation and particle acceleration in two key regions in jets: (i) external shocks which form where jets interact with ambient gas and (ii) internally in the jet where particles are likely to be energized through the process of magnetic reconnection.</p> <p><br></p> <p>First, I explore inverse Compton scatterings of electrons accelerated at the external shock as a candidate for the high energy emissions from gamma-ray burst afterglows. I consider two sources of seed photons for scattering: synchrotron photons from the blast wave (synchrotron self-Compton) and photon fields external to the shock (external Compton) from the star-forming region in the host galaxy. I develop an analytical model to predict the high-energy spectra from these blasts and reproduce the observed spectra and lightcurves of GRB~190114C. The model implies that inverse Compton can dominate the sub-TeV/TeV emission in this event.</p> <p><br></p> <p>Second, I study the particle acceleration mechanism of magnetic reconnection internally in astrophysical jets. I employ particle-in-cell (PIC) simulations of 3D relativistic magnetic reconnection. My analysis reveals a novel acceleration mechanism that only operates in 3D that results in faster particle acceleration. Unlike in 2D simulations where particles are trapped in the reconnected plasma and stop being accelerated, a fraction of particles in 3D can escape from this region (along the third direction) and be further accelerated. The escaped particles are characterized by a harder energy spectrum with a higher cutoff compared to those found in previous studies. Based on the PIC simulation findings, I build an analytical model for the particle kinetics, which divides particles into two groups --- one undergoing fast energization in the reconnection upstream region and the other residing in the reconnected plasma without energy change. The model predicts a power-law spectra for both groups of particles. PIC simulations reveal a universal magnetization-independent spectra with $dN/d\gamma\propto \gamma^{-2}$ for the overall particle population. The results demonstrate that relativistic reconnection in jets may be a promising mechanism for generating Ultra-High-energy Cosmic Rays. </p>

Despite many years of research dedicated to elucidating the conditions in which cosmic rays (CRs) are accelerated, there is still great uncertainty about exactly how such particles are accelerated up to energies of 1 TeV (1 TeV= 10¹² eV) and well beyond. Additionally, there is also great uncertainty about the structure and amplitude of the Galactic magnetic field which necessarily has a great impact upon the movement and interaction of CRs in the Galaxy. This thesis deals with a number of ways in which Gigahertz (GHz) frequency radio continuum observations can be used with GeV–TeV γ-ray observations to explore (i) the CR spectrum and (ii) the magnetic field amplitude in the Galaxy. An accurate knowledge of the CR spectrum and amplitude of the magnetic field has important consequences for a wide range of phenomena, such as particle acceleration and even star formation within the Galaxy. We present a simple static, single-zone model of secondary electron and positron production from CR protons and heavier nuclei interacting with ambient matter. We then apply this model, assuming a local CR spectrum, to predict the synchrotron emission from two cold, dense, massive molecular cores which are relatively nearby using a prescription for the magnetic field which scales as the (approximate) square-root of the hydrogen number density. Radio continuum observations with the Australia Telescope Compact Array (ATCA) are then used to search for this emission and, due to the lack of detection, upper limits to the magnetic fields within these cores are obtained. We find that these limits are not inconsistent with the prescription used in the theoretical modeling. We also present observations of a giant molecular cloud located in the Galactic centre (GC) region, Sagittarius B2 (Sgr B2), chosen because of the expectation of a higher CR flux (than that observed at the top of the earth’s atmosphere). Based on previous work, the simple model presented in this thesis is then extended to include effects of CR diffusion into the Sgr B2 cloud parameterized by a “diffusion transport suppression” factor (and based on a molecular distribution – obtained from NH₃ spectral line emission studies – that can modeled as a three-dimensional Gaussian distribution). Our results show that the complex nature of the environment severely hampers the separation of the thermal and non-thermal emission so that no spectral, polarized or morphological evidence is found for non-thermal emission due to secondary electrons and positrons. Analysis of the radial brightness distribution from the centre of the main complex of Sgr B2 allowed us to place limits on the diffusion of GeV energy CRs into the cloud. This leads to a relative deficit of CRs at the centre of the cloud and a morphology which is reminiscent of a ‘limb-brightening’ of synchrotron emission from secondary electrons and positrons. This is in contrast to to the TeV energy γ-rays from which a good correlation with molecular matter in the GC region is observed. This is interpreted by us as evidence of the exclusion of GeV energy CRs from the densest molecular environments in this region, whilst the TeV (or higher) CRs are able to freely penetrate these regions leading to the γ-ray -molecular line emission correlation observed by the HESS telescopes. Serendipitously, observations of this region uncovered evidence of non-thermal emission from a source to the south of the main complex of emission within Sgr B2. Analysis of archival XMM-Newton X-ray observations revealed an X-ray source located approximately 20” from the non-thermal radio source whose spectrum is strongly suggestive of a SNR. The non-thermal radio spectrum, X-ray source and spectrum were then used in concert with NH3 line emission to argue that this source is a SNR of approximately 3000 years of age which had exploded in this dense region. A large gradient in the NH₃ line emission towards the X-ray source suggests that any SNR shell would expand towards this region of lower density. Analysis of higher resolution 1720 MHz ATCA data revealed a weak source whose extension is coincident with the X-ray source. Finally, the observations of the Sgr B2 region were then expanded to explore the nature of the magnetic field amplitude on large scales in the region, of which there is a two orders of- magnitude uncertainty. Based on earlier work, which showed a large (6° x 2°) region of synchrotron emission at the GC, we assembled single-dish and interferometric observations of this region. The objective of this was to explore the possibility that a ‘spectral downturn’ existed at GHz frequencies, which is due to the gradual dominance towards lower energies of the bremsstrahlung cooling rate over the synchrotron cooling rate. After the removal of appropriate background and the consideration of limitations at GeV and TeV energies, we found significant statistical evidence for a spectral break at ~ 2 GHz, which implies a magnetic field amplitude of 100 μG in a density of ~ 100 cm ⁻³. An amplitude this high, on such large scales will have a large impact on processes such as particle acceleration, star-formation and gas-dynamics in the region. / http://proxy.library.adelaide.edu.au/login?url= http://library.adelaide.edu.au/cgi-bin/Pwebrecon.cgi?BBID=1456598 / Thesis (Ph.D.) -- University of Adelaide, School of Chemistry and Physics, 2009

Synchrotron radiation and inverse Compton (IC) scattering are the two most essential radiation mechanisms in high energy astrophysics. Synchrotron radiation typically dominates lower energy emission, up to GeV, and IC scattering dominates higher energy gamma-ray emission. In this work, radiation codes are developed to calculate broadband synchrotron and IC spectra for relativistic astrophysical sources: Pulsar Wind Nebulae (PWNe) and Gamma-Ray Bursts (GRBs). Our robust radiation code takes into account varying intrinsic plasma properties (e.g., magnetic field evolution), various inverse Compton processes (synchrotron self-Compton and external Compton) while accounting for Klein-Nishina effects, as well as relativistic bulk motion of the emitting plasma.

The cosmic X-ray background was discovered at the dawn of the X-ray astronomy: during the first successful rocket flight launched to study the X-ray emission from the Moon, the presence of a residual diffuse emission was also "serendipitously" revealed. In the intervening decades, observations with improving angular and spectral resolution have enhanced our understanding of the components that make up this background. Above 1 keV, the emission is highly isotropic on large angular scales, has extragalactic origin, and about ~80 percent has been resolved into discrete sources (Mushotzky et al. 2000, Hasinger et al. 1998). Our current interpretation of the diffuse X-ray emission below 1 keV uses a combination of 5 components, solar wind charge exchange, Local Bubble, Galactic halo, intergalactic gas, and unresolved point sources. Resolving the different components is made particularly difficult by the similar spectral emission of most components, X-ray lines of heavily ionized metals, which are poorly resolved by the energy resolution of CCD cameras onboard current X-ray satellites with typical observing times. The goal of this investigation is to assess the integral emission of the major components of the diffuse Soft X-Ray Background. In the first part of my project, I analyzed the shadow observations performed with XMM-Newton and Suzaku X-ray observatories. Shadow observations offer a tool to separate the fore ground component, due to the Local Bubble and, possibly, charge exchange within the solar system, from the background component, due primarily to the Galactic Halo and unidentified point sources. In the second part of my project, I studied the contribution of unresolved point sources and intergalactic medium to the diffuse Soft X-ray Background.

Gamma-ray Bursts (GRBs) are powerful explosions that emit most of their energy, as their name suggests, in gamma-rays of typical energies of about 1 MeV. This emission lasts for about two minutes or less and it is called the prompt emission. The isotropic energy radiated in GRBs is equivalent to the energy that the Sun will radiate in its entire lifetime. After decades of studying this cosmological phenomenon, we have come to learn that it involves a collimated and relativistic jet. Also, we know that they radiate energy in the X-ray, optical and radio bands for days, weeks and years, respectively, which is called the afterglow. Recently, NASA's Fermi Satellite was launched and, in addition to MeV photons, it detected GeV photons from these astrophysical sources. We show that these GeV photons are produced when the GRB jet interacts with the medium that surrounds it: the external forward shock model. We arrive at this conclusion not only by studying the GeV emission, but also by studying the afterglow observations (Chapter 2). We corroborate this model by studying the electron acceleration in the external forward shock model and find that electrons can radiate at the maximum observed energy of ~ 10 GeV (Chapter 3). We also provide an extensive analysis of the most recent afterglow observations of GRB 090902B within the same framework of an external forward shock origin. We find that the data for this burst requires a small deviation from the traditionally used power-law electron energy distribution, however, our previous results remain unchanged (Chapter 4). To conclude, we use the end of the prompt emission phase, which exhibits a steep X-ray temporal decay, to constrain the behavior of the central engine responsible for launching the relativistic jet (Chapter 5). / text

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京都大学 / 新制・課程博士 / 博士(理学) / 甲第24777号 / 理博第4971号 / 新制||理||1710(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 鶴 剛, 准教授 細川 隆史, 教授 田島 治 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

We utilize a Poisson likelihood in a maximum likelihood statistical analysis to analyze X-ray spectragraphic data. Specifically, we examine four extragalactic supernova remnants (SNR). IKT 5 (SNR 0047-73.5), IKT 25 (SNR 0104-72.3), and DEM S 128 (SNR 0103-72.4) which are designated as Type Ia in the literature due to their spectra and morphology. This is troublesome because of their asymmetry, a trait not usually associated with young Type Ia remnants. We present \emph{Chandra X-ray Observatory} data on these three remnants, and perform a maximum likelihood analysis on their spectra. We find that the X-ray emission is dominated by interactions with the interstellar medium. In spite of this, we find a significant Fe overabundance in all three remnants. Through examination of radio, optical, and infrared data, we conclude that these three remnants are likely not "classical" Type Ia SNR, but may be examples of so-called "prompt" Type Ia SNR. We detect potential point sources that may be members of the progenitor systems of both DEM S 128 and IKT 5, which could suggest a new subclass of prompt Type Ia SNR, Fe-rich CC remnants. In addition, we examine IKT 18. This remnant is positionally coincident with the X-ray point source HD 5980. Due to an outburst in 1994, in which its brightness changed by 3 magnitudes (corrsponding to an increase in luminosity by a factor of 16) HD 5980 was classified as a luminous blue variable star. We examine this point source and the remnant IKT 18 in the X-ray, and find that its non-thermal photon index has decreased from 2002 to 2013, corresponding to a larger proportion of more energetic X-rays, which is unexpected.

The main results of this work are the contribution to find the following results: <br />• The most general GRB is made by an early emission (P-GRB or Proper-GRB), with a time-scale not larger than 1 or 2 seconds and an afterglow, whose light curve is characterized by an increasing phase followed by a peak and a decreasing phase. This peak has been identified with the long GRBs prompt emission. In this scenario short GRBs are not but P-GRB, while long GRBs present both a peak and a decreasing late time emission, which is the observed afterglow.<br />• A possible GRB/SuperNova connection is based on the process of induced gravitational collapse of a companion star of the black hole originating the GRB. <br />• A thermal distribution in the comoving frame of the expanding system is assumed for X and γ bands of the spectrum. This assumption leads to a natural bending of the late time light curves making not necessary the hypothesis of a beamed emission within a collimated jet from the inner engine, which has been introduced in literature essentially to reduce the energy requirements.