| 1. |
- Puschnig, Johannes, 1980-
(författare)
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How ISM properties drive Lyman Continuum Escape
- 2016
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The thesis introduces physical processes that are at work in astrophysical plasmas and reviews the current state of research related to the emission of ionizing photons, i.e. Lyman continuum (LyC). Star forming galaxies and active galactic nuclei are discussed as sources of LyC. Observations of LyC leakage at all redshifts are summarized and escape fractions are brought into a cosmological context, i.e. its implications for the reionization of the Universe, one of the major gas phase changes that was completed already after ∼1Gyr after the Big Bang at redshift z∼6.The main work focuses on observations of the local LyC leaking galaxy Tololo 1247-232. Physical properties of the interstellar medium, its porosity and neutral medium column density, could be derived using newly obtained Hubble Space Telescope (HST) data. The work is based on spectroscopy obtained with the Cosmic Origins Spectrograph (COS), as well as optical and ultraviolet multi-band imaging with the Wide Field Camera 3 (WFC3). An improved COS data reduction procedure was adopted. The recent detection of ionizing radiation emerging from Tololo 1247-232 could be confirmed. A LyC escape fraction of 6.6% was derived, in agreement with previous results. We used FUV absorption lines of Si II and Si IV as a probe of the neutral and ionized interstellar medium and find that most of the ISM gas is ionized, likely facilitating LyC escape from density bounded regions. Neutral gas covering as a function of line-of-sight velocity is derived using the apparent optical depth method. The ISM is found to be sufficiently clumpy, supporting the direct escape of LyC photons. We further report on broadband UV and optical continuum imaging as well as narrowband imaging of Lyα, Hα and Hβ. We also performed VLA 21cm imaging. The hydrogen hyperfine transition was not detected, but a deep upper limit atomic gas mass of 10^9 Mo could be derived. The upper limit gas fraction is only 20 percent. Evidence is found that the H I gas halo is relatively small compared to other Lyman Alpha emitters.
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| 2. |
- Georgiev, Ivelin, 1995-
(författare)
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Understanding the large-scale structure of the the21-cm signal originating from the Epoch of Reionisation
- 2022
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The first billion years from the beginning of the Universe is the focus of multiple astronomical facilities in the upcoming decade. This unique era is marked by the formation of the first stars and galaxies, which release ionising radiation into the intergalactic medium(IGM). As a result, these sources initiate a period during which the cold and dense IGM, primarily consisting of neutral hydrogen (HI ), is heated and ionised. We refer to this era as the Epoch of Reionisation (EoR). How the EoR transpired hence depends on the properties of these ionising sources, and this forms a vital piece to the puzzle of understanding the early Universe. This licentiate thesis aims to educate the reader on the power spectrum (PS) statistic of the 21-cm signal from HI during the EoR. The PS is a prospective observable by radio interferometers, such as the Low-Frequency Array (LOFAR) and the future Square Kilometre Array (SKA). It is an essential stepping stone in comprehending the dominant physical processes affecting the IGM at a given length scale during the EoR.In Paper I, we analyse the decomposition of the 21-cm PS from Lidz et al. (2007) (eq. 2)and study the evolution of its terms. We conduct our investigation for a set of C 2 -Ray and 21cmFAST simulations with volumes of (714 cMpc)3, concentrating on large-scales (k < 0.3 Mpc−1 ) as the signal-to-noise ratio of observing these scales will be high. We find that the 21-cm PS tracks the PS of neutral hydrogen fluctuations, which itself past a certain scale tracks the matter PS after a global ionisation fraction of x̄HII∼ 0.1. Hence, the 21-cm PS possesses a two-regime form for which the large-scale PS is a biased version of the cosmological density field and the small-scale PS depends on the astrophysics of the EoR. We construct a bias parameter to explore whether the 21-cm PS can be used as a probe of cosmology on large k-scales. We discover a transition feature for both simulations, following the ktrans ≈ 2/λMFP empirical formula. The transition scale between the scale-independent and scale-dependent bias regimes is directly related to the value of the mean free path of ionising photons (λMFP ).
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| 3. |
- Warnecke, Jörn, 1982-
(författare)
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Flux emergence: flares and coronal mass ejections driven by dynamo action underneath the solar surface
- 2011
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Helically shaped magnetic field structuresknown as coronal mass ejections (CMEs) are closely related to so-called eruptive flares. On the one hand, these events are broadly believed to be due tothe buoyant rise of magnetic flux tubes from the bottom of the convection zone to the photosphere where they form structures such as sunspots. On the other hand, models of eruptive flares and CMEs have no connection to the convection zone and the magnetic field generated bydynamo action. It is well known that a dynamo can produce helical structures and twisted magnetic fields as observed in the Sun. In this work we ask, how a dynamo-generated magnetic field appears above the surface without buoyancy force and how this field evolves inthe outer atmosphere of the Sun. We apply a new approach of a two layer model, where the lower one represents the convection zone and the upper one the solar corona. The two layers are included in one single simulation domain. In the lower layer, we use a helical forcing function added to the momentum equation to create a turbulent dynamo. Due to dynamo action, a large-scale field is formed. As a first step we use a Cartesian cube. We solve the equations of the so-called force-free model in the upper layer to create nearly force-free coronal magnetic fields. In a second step we use a spherical wedge, which extends radially from 0.7 to 2 solar radii. We include density stratification due to gravity in anisothermal domain. The wedge includes both hemispheres of the Sun and we apply a helicalforcing with different signs in each hemisphere. As a result, a large-scale field is generated by a turbulent dynamo acting underneath the surface. Due to the latitudinal variation of the helicity produced by the helical forcing, the dynamo is oscillating in the spherical wedge. Twisted magnetic fields emerge above the surface and form arch-like structures with strong current sheets. Plasmoids and CME-like structures are ejected recurrently into the outerlayers. In the spherical simulations we find that the magnetic helicity changes sign in the exterior, which is in agreement with recent analysis of the solar wind data.
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| 4. |
- Handirk, Rebekka, 1994
(författare)
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Legacy fostering the twins. Connecting the S/X and VGOS telescope network generations
- 2024
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- With the VLBI Global Observing System (VGOS) being the next step in the development of geodetic Very Long Baseline Interferometry (VLBI), it is necessary to connect the new VGOS network to existing legacy S/X telescopes. Specially designed short-baseline interferometry sessions aim to obtain local-tie vectors between these telescope generations at observatories that have both legacy S/X and new generation VGOS telescopes. At the Onsala Space Observatory (OSO), this is being done by short-baseline interferometry between the VGOS Onsala Twin Telescopes ONSA13SW and ONSA13NE, and the legacy antenna ONSALA60. Similarly, short-baseline interferometry sessions referred to as NYTIE have been performed at Ny-Ålesund, involving the VGOS telescope NYALE13S and the legacy S/X antenna NYALES20. In both cases, these dedicated experiments yielded station coordinates and baselines with mm or even sub-mm accuracy. This thesis focuses on exploring the possibilities of connecting co-located radio telescopes with short-baseline interferometric measurements, specially in the cases of the observatories in Onsala and Ny-Ålesund.
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| 5. |
- Otto, Ernst, 1971
(författare)
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Direct Write Technology of SIN Tunnel Junctions for Thermometry and Microwave Devices
- 2008
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis concerns development of SIN junction technology for thermometry and microwave detectors, as well as technology of a planar phase switch as a part of integratedmicrowave receiver.Cold-Electron Bolometer (CEB) is a sensitive detector of a microwave radiation, in which SIN tunnel junctions are used for temperature measurement of a nanoscale absorber in thesame way as in SIN thermometers. A novel direct-write trilayer technology and a Ti-based technology were developed for the CEB and thermometry.The trilayer technology is based on in-situ evaporation of the superconductive electrode followed by the oxidation and the normal counter-electrode as a first step and deposition of a normal-metal absorber as a second one. The junctions were characterized as temperature sensors, and a thermometer responsivity of 52 mV/K was observed for an array of 100junctions in series with a temperature resolution of ± 5μK.A novel Ti-based direct-write technology for fabrication of Ti-TiO2-Al tunnel junctions has another advantages related to thickness of the absorber. In particular, a very thin andcompletely flat absorber can be created with no bending parts. The key point of this technology is deposition of Ti film as a base electrode and deposition of Al electrode after oxidation of Ti.The junctions are to be used for fabrication of microwave receivers for sensitive measurements in new generation of telescopes, e.g. CLOVER project including polarizationCosmic Microwave Background (CMB) radiation measurements, BOOMERANG and OLIMPO balloon telescope project which is dedicated to measuring the Sunyaev-Zeldovich effect in clusters of Galaxies.A planar phase switch operating at 97-225 GHz has been developed, fabricated, and tested. The system makes it possible to modulate the microwave signal or switch it between two branches of a circuit with low losses and high speed. The proposed fully planar design allowsthe switch to be integrated into the detector circuit.The phase modulation scheme will be used in high-precision astrophysical experiments such as the investigation of the polarization of the cosmic microwave background.Keywords: SIN tunnel junction, direct write technology, Cold-Electron Bolometer, cryogenicthermometry, phase switch, microwave device, noise equivalent power
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| 6. |
- Valtonen-Mattila, Nora
(författare)
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Extending the observational reach of core-collapse supernovae for IceCube using high-energy neutrinos
- 2022
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Neutrino telescopes such as IceCube monitor for low-energy neutrinos O(10 MeV) produced in nuclear processes during core-collapse in supernovae. The detection horizon to the neutrino burst is 50 kpc, the distance to the Magellanic Clouds. However, this limits the number of supernovae accessible through low-energy neutrino detection, as the Galactic rate is only 2 per century. Some models predict the production of high-energy O(>GeV) neutrinos through acceleration mechanisms, such as the ejecta colliding with the circumstellar material or relativistic jets in the stellar envelope. This thesis examines how these high-energy neutrinos could be exploited with neutrino telescopes like IceCube to extend the detection horizon to core-collapse supernovae past the Magellanic clouds. To examine the detection horizon for IceCube, we use two data samples, one utilizing muon tracks which provide good sensitivity in the northern sky, and the other all flavor starting events, which provide good sensitivity in the southern sky. We demonstrate that extending the reach past 50 kpc and well into the 10s of Mpc is possible, where the expected rate is more than two core-collapse supernovae per year.
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| 7. |
- Ejdetjärn, Timmy
(författare)
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Exploring the nature of ISM turbulencein disc galaxies
- 2024
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Galaxy formation is a continuous process that started only a few hundred million yearsafter the Big Bang. The first galaxies were very volatile, with bursts of star formationand disorganised gas motions. However, even as these galaxies evolved to have orderlyrotating gas discs, the gas within the disc, referred to as the interstellar medium (ISM),still remained highly turbulent. In fact, the ISM is supersonically turbulent, meaning thatthe disorganised gas motion exceeds the speed of sound in the medium. This supersonicturbulence has been connected to several crucial properties related to galaxy evolution; forexample, increasing (and decreasing in some regions) the ISM gas density, star formation,and gas mixing.Many observation have shown that all of the gas phases in the ISM experience su-personic levels of turbulence, with line widths (an observational method to quantify theamount of turbulence) as high as σg ≲ 100 km s−1 in high-redshift (younger) disc galaxies,while local quiescent discs have σg ≲ 40 km s−1 . However, the ISM contains a variety ofgas phases that cover a wide range of temperatures and densities, which exhibit differentlevels of turbulence. For example, the warm ionised gas phase represents the upper limitsquoted above, while colder denser gas only reaches σg ≲ 40 km s−1 and σg ≲ 15 km s−1 inhigh-redshift and local galaxies, respectively.The physical processes driving this turbulence are not fully understood, but a combi-nation of stellar feedback (e.g. supernova) and gravitational instability (e.g. during cloudcollapse) have been suggested to provide a majority of the turbulent energy. In particular,stellar feedback is crucial in the formation of warm ionised gas and may therefore have asignificant contribution on the turbulence within ionised gas. Furthermore, heterogeneousdata of widely different galaxies (in terms of e.g. mass and size) at different resolutions(which causes artificial line broadening) complicates understanding the underlying cause.A commonly used tracer of ionised gas is the Hα emission line and has been usedextensively in high-redshift surveys. However, the contribution of the Hα signal comesfrom two primary sources: the radiatively ionised regions around massive newborn starsembedded in molecular gas (called H II regions) and diffuse ionised gas (DIG) filling theentire galactic disc. Observations have found that these two sources contribute, on average,roughly the same amount to the Hα signal (although with a large spread), but the levelsof turbulence is starkly different; with the DIG being roughly 2-3 times more turbulethan the gas in H II regions.Numerical simulations have come a long way and are now able to simulate entire discgalaxies at parsec-scale resolution (in regions of interest). Furthermore, galaxy simulationshave been able to reproduce the level of turbulence observed in local and high-redshiftgalaxies. Direct comparisons between numerical and observational studies are crucial tounderstand the relevant physics driving observed correlations. However, numerical andobservational work have different data available and the reduction/analysis varies betweenauthors, and so diligence is required to perform qualitative comparisons.In this work, I perform numerical simulations to investigate ISM turbulence in differentgas phases. My simulations model a Milky Way-like galaxy at two different redshifts(using gas fraction as a proxy for redshift) and with/without stellar feedback physics, toevaluate its impact. I perform mock observations to explore the relation between the starformation rate and turbulence, and investigate what is driving this relation. Additionally, Ianalyse the Hα emission line and compare the contribution in intensity and line broadening(turbulence) from H II regions and DIG.
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| 8. |
- Samuelsson, Filip
(författare)
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Multi-messenger emission from gamma-ray bursts
- 2020
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Multi-messenger astronomy is a very hot topic in the astrophysical community. A messenger is something that carries information. Different astrophysical messenger types are photons, cosmic rays, neutrinos, and gravitational waves. They all carry unique and complementary information to one another. The idea with multi-messenger astronomy is that the more different types of messengers one can obtain from the same event, the more complete the physical picture becomes.In this thesis I study the multi-messenger emission from gamma-ray bursts (GRBs), the most luminous events known in the Universe. Specifically, I study the connection of GRBs to extremely energetic particles called ultra-high-energy cosmic rays (UHECRs). UHECRs have unknown origin despite extensive research. GRBs have long been one of the best candidates for the acceleration of these particles but a firm connection is yet to be made. In Paper I and Paper II, we study the possible GRB-UHECR connection by looking at the electromagnetic radiation from electrons that would also be accelerated together with the UHECR. My conclusion is that the signal from these electrons does not match current GRB observation, disfavoring that a majority of UHECRs comes from GRBs.
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| 9. |
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| 10. |
- Alp, Dennis, MSc, 1992-
(författare)
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Core-collapse Supernovae : Theory vs. Observations
- 2019
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A core-collapse supernova (CCSN) is an astronomical explosion that indicates the death of a massive star. The iron core of the star collapses into either a neutron star or a black hole while the rest of the material is expelled at high velocities. Supernovae (SNe) are important for the chemical evolution of the Universe because a large fraction of the heavier elements such as oxygen, silicon, and iron are liberated by CCSN explosions. Another important role of SNe is that the ejected material seed the next generation of stars and planets. From observations, it is clear that a large fraction of all massive stars undergoes SN explosions, but describing how SNe explode has remained a challenge for many decades.The attached papers focus on comparing theoretical predictions with observations, primarily observations of SN 1987A. The compact remnant in SN 1987A has not yet been detected and we have investigated how a compact object can remain hidden in the ejecta (Paper I and II). Because of the high opacity of the metal-rich ejecta, the direct X-ray observations are not very constraining even for potentially favorable viewing angles. However, the combined observations still strongly constrain fallback accretion and put a limit on possible pulsar wind activity. The thermal surface emission from a neutron star is consistent with the observations if our line of sight is dust-obscured, and only marginally consistent otherwise. Future observations provide promising opportunities for detecting the compact object.We have also compared the most recent three-dimensional neutrino-driven SN models that are based on explosion simulations with early X-ray and gamma-ray observations of SN 1987A (Paper III). The models that are designed to match SN 1987A fit the data well, but not all tensions can be explained by choosing a suitable viewing angle. More generally, the asymmetries do not affect the early emission qualitatively and different progenitors of the same class result in similar early emission. We also find that the progenitor metallicity is important for the low-energy X-ray cuto↵. Current instruments should be able to detect this emission from SNe at distances of 3–10 Mpc, which correspond to distances slightly beyond the Local Group.
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