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| 4. |
- Ylinen, Tomi, 1982-
(författare)
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Search for Gamma-ray Lines from Dark Matter with the Fermi Large Area Telescope
- 2010
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Dark matter (DM) constitutes one of the most intriguing but so far unresolved issues in physics. In many extensions of the Standard Model of particle physics, the existence of a stable Weakly Interacting Massive Particle (WIMP) is predicted. The WIMP is an excellent DM particle candidate. One of the most interesting scenarios is the creation of monochromatic gamma-rays from the annihilation or decay of these particles. This type of signal would represent a “smoking gun” for DM, since no other known astrophysical process should be able to produce it. In this thesis, the search for spectral lines with the Large Area Telescope (LAT) onboard the Fermi Gamma-ray Space Telescope (Fermi) is presented. The satellite was successfully launched from Cape Canaveral in Florida, USA, on 11 June, 2008. The energy resolution and performance of the detector are both key factors in the search and are investigated here using beam test data, taken at CERN in 2006 with a scaled-down version of the Fermi-LAT instrument. A variety of statistical methods, based on both hypothesis tests and confidence interval calculations, are then reviewed and tested in terms of their statistical power and coverage. A selection of the statistical methods are further developed into peak finding algorithms and applied to a simulated data set called obssim2, which corresponds to one year of observations with the Fermi-LAT instrument, and to almost one year of Fermi-LAT data in the energy range 20–300 GeV. The analysis on Fermi-LAT data yielded no detection of spectral lines, so limits are placed on the velocity-averaged cross-section, , and the decay lifetime, , and theoretical implications are discussed.
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| 5. |
- Asayama, Shinichiro, et al.
(författare)
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Three institutional pathways to envision the future of the IPCC
- 2023
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Ingår i: Nature Climate Change. - : Nature Portfolio. - 1758-678X .- 1758-6798. ; 13:9, s. 877-880
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Tidskriftsartikel (refereegranskat)abstract
- The IPCC has been successful at building its scientific authority, but it will require institutional reform for staying relevant to new and changing political contexts. Exploring a range of alternative future pathways for the IPCC can help guide crucial decisions about redefining its purpose.
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| 6. |
- Davour, Anna, 1975-
(författare)
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Search for low mass WIMPs with the AMANDA neutrino telescope
- 2007
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Recent measurements show that dark matter makes up at least one fifth of the total energy density of the Universe. The nature of the dark matter is one of the biggest mysteries in current particle physics and cosmology.Big Bang nucleosynthesis limits the amount of baryonic matter that can exist, and shows that the dark matter has to be non-baryonic. Particle physics provides some candidates for non-baryonic matter that could solve the dark-matter problem, weakly interacting massiveparticles (WIMPs) being the most popular. If these particles were created in the early Universe a substatial relic abundance would exist today. WIMPs in our galactic halo could be gravitationally bound in the Solar System and accumulate inside heavy bodies like the Earth. Supersymmetric extensions to the Standard Model give a viable WIMP dark matter candidate in the form of the lightest neutralino. This thesis describes an indirect search for WIMPs by the neutrino signature from neutralino annihilation at the core of the Earth using the AMANDA detector. As opposed to previous dark matter searches with AMANDA, this work focuses on the hypothesis of a relatively light WIMP particle with mass of 50-250GeV/c2The AMANDA neutrino telescope is an array of photomultiplier tubes installed in the clear glacier ice at the South Pole which is used as Cherenkov medium. Data taken with AMANDA during the period 2001-2003 is analyzed. The energy threshold of the detector is lowered by the use of a local correlation trigger, and the analysis is taylored to select vertically upgoing low energy events. No excess above the expected atmospheric neutrino background is found. New limits on the flux of muons from WIMP annihilations in the center of the Earth are calculated.
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| 7. |
- Hofverberg, Petter, 1979-
(författare)
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Imaging the high energy cosmic ray sky
- 2006
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The Stockholm Educational Air Shower Array (SEASA) project is deploying an array of plastic scintillator detector stations on school roofs in the Stockholm area. Signals from GPS satellites are used to time synchronise signals from the widely separated detector stations, allowing cosmic ray air showers to be identified and studied. A low-cost and highly scalable data acquisition system has been produced using embedded Linux processors which communicate station data to a central server. Air shower data can be visualised in real-time using a Java-applet client. The design and performance of the first three detector stations located at the AlbaNova University Centre are presented. The detectors have been running since the beginning of October 2005 and the data from this period is analysed to assess the stability and performance of the detector array. A total of 503 showers with a primary particle energy above 1016 eV, hitting all three detector stations simultaneously, have been detected during this period. The read out and data-base system used to collect the data are described together with a quicklook tool for ensuring the integrity of the data. A preliminary study of the acceptance of the detector array as a function of weather conditions, to be used in future studies of cosmic ray anisotropy, is presented. The acceptance of the single detector stations is found to decrease with increasing atmospheric pressure and to stay constant over a large range of temperatures. The acceptance of the entire array of detector stations is found to have a stronger continuous dependence on temperature than single stations. The dependence of the array acceptance on pressure is inconclusive. The ability of the array to reconstruct the primary cosmic ray direction is assessed with simulations. A critical feature for the reconstruction is the time resolution of the system. The performance of the GPS system is therefore tested, and the time resolution is found to be better than 15 ns for all tested GPS units. The angular resolution of the array for this time resolution is found to be (7.0\pm0.3)^{\circ}. As the time resolution is expected to decrease for a larger array of detectors, the dependency of the time resolution on the angular resolution is derived. The measured distribution of the primary cosmic ray arrival direction is derived and compared to the expected distribution to check the performance of the system. The agreement between the distributions is good and the GPS timing system can therefore be concluded to work well. The simulations also show that the energy threshold of the array is slightly above 1016 eV. A preliminary study of the cosmic ray anisotropy is presented. The hypothesis of an isotropic flux of cosmic rays was tested using a two point correlation function. The probability that the observed flux is a random sampling from an isotropic flux was checked with a Kolmogorov test and it was found to be 82%. The hypothesis of an isotropic flux is therefore supported.
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| 8. |
- Kiss, Mózsi
(författare)
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Pre-Flight Development of the PoGOLite Pathfinder
- 2011
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The Polarized Gamma-ray Observer (PoGOLite) is a balloon-borne instrument that will measure gamma-ray polarization in the energy range 25-80 keV from astronomical sources such as pulsars, accretion discs and jets from active galacticnuclei. The two additional parameters provided by such observations, polarizationangle and degree, will allow these objects to be studied in a new way, providing information about their emission mechanisms and geometries.The instrument measures azimuthal scattering angles of photons within a closepacked array of phoswich detector cells (PDCs) based on coincident detection of Compton scattering and photoelectric absorption. Each PDC comprises three different scintillating components and combines photon detection, active collimation and bottom anticoincidence into one single unit. The three parts are viewed by a photomultiplier tube (PMT) and pulse shape discrimination is used to identify signals from dierent parts. Surrounding the detector array is a segmented side anticoincidence shield (SAS) made of BGO crystals.The detector elements of the instrument (PDCs, SAS units, PMTs) have been characterized, resulting in a placement scheme which details where within the detector array each element should be placed in order to maximize the instrument sensitivity and response uniformity. Suitable operating parameters for flight, suchas threshold settings and PMT voltages, have also been dened.Geant4 Monte Carlo simulations have shown that a polyethylene shield is needed around the detector array in order to sufficiently reduce the background from atmospheric neutrons. To validate these simulations, a simple detector array with four plastic scintillators and three BGO crystals shielded with polyethylene was irradiated with 14 MeV neutrons. Measured results were accurately recreated i nsimulations, demonstrating that the treatment of neutron interactions in Geant4 is reliable.A Pathnder version of the PoGOLite instrument has been constructed and tested with unpolarized and polarized photon beams, and results have been compared with simulations. The Pathnder is being prepared for a maiden flight from northern Sweden in mid-2011. A circumnavigation is foreseen at an altitude of up to 40 km, whereby the instrument travels westwards over Greenland and Canada and returns over Russia after a period of about 20 days. The main observational targets for this flight will be the Crab system and Cygnus X-1.
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| 9. |
- Kole, Merlin, 1986-
(författare)
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Background Studies for the Balloon-Borne Hard X-ray Polarimeter PoGOLite
- 2014
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The polarisation degree and angle of the X-ray flux emitted by astrophysical objects holds valuable information on the responsible emission mechanisms and on the emission environments. PoGOLite is a balloon-borne hard X-ray polarimeter designed to measure polarisation using a segmented plastic scintillator array. The instrument was launched for its first scientific, near-circumpolar, flight in July 2013 from the Esrange Space Centre in Northern Sweden. The primary observation target for this flight, the Crab, was observed during the first 2 days of flight. One of the main challenges for PoGOLite is the relatively high measurement background, predicted to be induced by atmospheric neutrons. No measurement data on the neutron environment for the flight conditions of PoGOLite is however available, making exact predictions impossible. This environment was therefore studied in detail. A Monte Carlo based model of the atmospheric neutron flux was developed. This model is capable of providing differential neutron energy spectra for all altitudes, latitudes and solar activities. The predictions of this model were found to be in good agreement both with measurement data, measured by high altitude aircraft, and with predictions by location and time specific models. The results from the model were verified with data recorded by a purpose-build balloon-borne neutron detector, PoGOLino. The PoGOLino instrument uses novel neutron sensitive LiCAF scintillators sandwiched between BGO crystals which serve as an anti-coincidence system. PoGOLino was launched from the Esrange Space Centre to an altitude of 31 km on March 20th 2013 and performed the first successful measurements of the neutron flux for the PoGOLite flight conditions. Using the developed model the background as measured by the PoGOLite mission in 2013 was studied. Monte Carlo simulations were used to confirm that the PoGOLite background during flight is dominated by neutrons. The simulated neutron induced signal rate and its variations with time were furthermore found to be in good agreement with measurements. Based on these results the implications of the background on the polarisation measurements of the Crab were studied. Lastly, based on the acquired knowledge of the background, changes to the instrument geometry for potential future flight of PoGOLite were studied along with the expected achievable improvement in performance for such flights.
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| 10. |
- Marini Bettolo, Cecilia, 1978-
(författare)
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Performance Studies and Star Tracking for PoGOLite
- 2010
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- PoGOLite is a balloon-borne experiment, which will study polarized soft γ-ray emissionfrom astrophysical targets in the 25-80 keV energy range by applying well-typephoswich detector technology. Polarized γ-rays are expected from a wide variety of sources including rotation-powered pulsars, accreting black holes and neutron stars,and jet-dominated active galaxies. Polarization measurements provide a powerfulprobe of the γ-ray emission mechanism and the distribution of magnetic and radiation fields around the source. The polarization is determined using Compton scattering and photoelectric absorption in an array of 217 plastic scintillators. The sensitive detector is surrounded by a segmented Bismuth Germanium Oxide (BGO) anticoincidence shield. The function of this shield is to reduce backgrounds from charged cosmic rays, primary and atmospheric γ-rays, and atmospheric and instrumenta lneutrons. The anticoincidence shield consists of 427 BGO crystals with three different geometries. The characteristics of the BGO crystals of the bottom anticoincidence shield have been studied with particular focus on the light yield.The maiden flight of PoGOLite will be with a reduced detector volume “pathfinder” instrument. The flight, lasting about 24 hours, is foreseen from Esrange, Sweden in August 2010. The performance of the pathfinder has been studied using computer simulations. The effect of atmospheric attenuation, both on the signal of theastronomical target and on the background, are studied. These allow an observationstrategy to be developed for the forthcoming flight. A polarization analysis method is described and applied to an observation example. The method sets anupper limit on the accuracy with which the polarimeter will be able to detect polarization the angle and degree. The PoGOLite polarimeter has a relatively small field of view (2.4◦×2.4◦) which must be kept aligned to objects of interest on the sky. A star tracker forms part of the attitude control system. The star trackersystem comprises a CCD camera, a lens, and a baffle system. Preliminary studiesof the star identification performance are presented and are found to be compatible with the environment around the Crab, which is the main observational target for the first flight.
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| 11. |
- Marini Bettolo, Cecilia, 1978-
(författare)
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PoGOLite : The Polarised Gamma-ray Observer
- 2008
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- PoGOLite is a balloon-borne experiment which will study polarised soft gamma-ray emission from astrophysical targets in the 25 keV – 80 keV energy range by applying well-type phoswich detector technology. Polarised gamma-rays are expected from a wide variety of sources including rotation-powered pulsars, accreting black holes and neutron stars, and jet-dominated active galaxies. Polarisation measurements provide a powerful probe of the gamma-ray emission mechanism and the distribution of magnetic and radiation fields around the source. The polarisation is determined using Compton scattering and photoelectric absorption in an array of 217 plastic scintillators. The sensitive detector is surrounded by a segmented Bismuth Germanium Oxide (BGO) anticoincidence shield. The function of this shield is to reduce backgrounds from charged cosmic rays, primary and atmospheric gamma-rays, and atmospheric and instrumental neutrons. The anticoincidence shield consists of 427 BGO crystals with three different geometries. The characteristics of the BGO crystals of the bottom anticoincidence shield have been studied with particular focus on the light yield. The PoGOLite polarimeter has a field of view of 2.4° x 2.4° and must be kept aligned to objects of interest on the sky. A star tracker forms part of the attitude control system. The star tracker system comprises a CCD camera, lens, and a baffle system. Preliminary studies have been made concerning optimization of the focus, flat field correction, map of hot pixel and CCD response. An estimate of the star magnitude limit is also derived and found to be compatible with the environment around the Crab, which is the first observational target. These studies pave the way toward an autonomous star tracking device which together with the other attitude control devices will reconstruct the pointing solution.
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| 12. |
- Mi, Wujun, 1986-, et al.
(författare)
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A stacked prism lens concept for next-generation hard X-ray telescopes
- 2019
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Ingår i: Nature Astronomy. - United Kingdom : Springer Science and Business Media LLC. - 2397-3366.
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Tidskriftsartikel (refereegranskat)abstract
- Effective collecting area, angular resolution, field of view and energy response are fundamental attributes of X-ray telescopes. The performance of state-of-the-art telescopes is currently restricted by Wolter optics, especially for hard X-rays. Here we report the development of a stacked prism lens (SPL), which is lightweight and modular and has the potential for a significant improvement in effective area, while retaining high angular resolution. The proposed optics are built by stacking disks embedded with prismatic rings, created with photoresist by focused ultraviolet lithography. We demonstrate the SPL approach using a prototype lens that was manufactured and characterized at a synchrotron radiation facility. The design of a potential satellite-borne X-ray telescope is outlined and the performance is compared with contemporary missions.
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| 15. |
- Wu, Juan, 1982-
(författare)
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Measurements of cosmic ray antiprotons with PAMELA and studies of propagation models
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Studying the acceleration and propagation mechanisms of Galactic cosmic rays can provide information regarding astrophysical sources, the properties of our Galaxy, and possible exotic sources such as dark matter. To understand cosmic ray acceleration and propagation mechanisms, accurate measurements of different cosmic ray elements over a wide energy range are needed. The PAMELA experiment is a satellite-borne apparatus which allows different cosmic ray species to be identified over background. Measurements of the cosmic ray antiproton flux and the antiproton-to-proton flux ratio from 1.5 GeV to 180 GeV are presented in this thesis, employing the data collected between June 2006 and December 2008. Compared to previous experiments, PAMELA extends the energy range of antiproton measurements and provides significantly higher statistics. During about 800 days of data collection, PAMELA identified approximately 1300 antiprotons including 61 above 31.7 GeV. A dramatic improvement of statistics is evident since only 2 events above 30 GeV are reported by previous experiments. The derived antiproton flux and antiproton-to-proton flux ratio are consistent with previous measurements and generally considered to be produced as secondary products when cosmic ray protons and helium nuclei interact with the interstellar medium. To constrain cosmic ray acceleration and propagation models, the antiproton data measured by PAMELA were further used together with the proton spectrum reported by PAMELA, as well as the B/C data provided by other experiments. Statistical tools were interfaced with the cosmic ray propagation package GALPROP to perform the constraining analyses. Different diffusion models were studied. It was shown in this work that only current PAMELA data, i.e. the antiproton-to-proton ratio and the proton flux, are not able to place strong constraints on propagation parameters. Diffusion models with a linear diffusion coefficient and modified diffusion models with a low energy dependence of the diffusion coefficient were studied in the $\chi^{2}$ study. Uncertainties on the parameters and the goodness of fit of each model were given. Some models are further studied using the Bayesian inference. Posterior means and errors of the parameters base on our prior knowledge on them were obtained in the Bayesian framework. This method also allowed us to understand the correlation between parameters and compare models. Since the B/C ratio used in this analysis is from experiments other than PAMELA, future PAMELA secondary-to-primary ratios (B/C, $^{2}$H/$^{4}$He and $^{3}$He/$^{4}$He) can be used to avoid the data sets inconsistencies between different experiments and to minimize uncertainties on the solar modulation parameters. More robust and tighter constraints are expected. The statistical techniques have been demonstrated useful to constrain models and can be extended to other observations, e.g. electrons, positrons, gamma rays etc. Using these channels, exotic contributions from, for example, dark matter will be further investigated in future.
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| 16. |
- Ylinen, Tomi
(författare)
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Towards detecting lines from dark matter annihilations with GLAST
- 2008
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Dark matter (DM) constitutes one of the most intriguing but so far unresolved issues in physics. In many extensions of the Standard Model of particle physics, the existence of a stable Weakly Interacting Massive Particle (WIMP) is predicted. The WIMP is an excellent DM particle candidate and one of the most interesting scenarios include an annihilation of two WIMPs into two gamma-rays. If the WIMPs are assumed to be non-relativistic, the resulting photons will both have an energy equal to the mass of the WIMP and will manifest themselves as a monochromatic spectral line in the energy spectrum. This type of signal would represent a “smoking gun” for DM, since no other known astrophysical process should be able to produce it. When searching for a line, the energy resolution and performance of the calorimeter are key factors. In this thesis, these are investigated using beam test data, taken at CERN in 2006. Four statistical methods that can be used to search for DM spectral lines are, then, studied in terms of their power and coverage. The methods are based on both hypothesis tests and confidence interval calculations. Two peak finding methods are also tested on a simulated data set representing one year of realistic data, obtained with the Large Area Telescope (LAT) on-board the Gamma-ray Large Area Space Telescope (GLAST). The data set is called Service Challenge 2 (SC2) and contains a variety of gamma-ray sources, including different DM components. Finally, an upper limit on < σν > γγ, based on SC2, is calculated.
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