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Abstract: The Fermi Gamma-Ray Space Telescope was launched in June 2008 and the onboard Large Area Telescope (LAT) has been collecting data since August of that same year. The LAT is currently being used to study a wide range of science topics in high-energy astrophysics, one of which is the study of high-energy cosmic rays. The LAT has recently demonstrated its ability to measure cosmic-ray electrons, and the Fermi LAT Collaboration has published a measurement of the high-energy cosmic-ray electron spectrum in the 20 GeV to 1 TeV energy range. Further cosmic-ray studies with the LAT involve measuring the cosmic-ray proton energy spectrum. A method for performing this measurement of proton energy spectrum will be presented. The event selections will be described, and the instrument response for protons will be characterized. An emphasis will be on unfolding the measured proton energy to overcome the instrument's poor energy resolution, and this procedure will be detailed. Finally the spectrum will be calculated and systematic errors will be estimated.
We report on measurements of the cosmic-ray induced?-ray emission of Earth's atmosphere by the Large Area Telescope onboard the Fermi Gamma-ray Space Telescope. The LAT has observed the Earth during its commissioning phase and with a dedicated Earth-limb following observation in September 2008. These measurements yielded H"6.4 x 106 photons with energies> 100 MeV and H"250 hours total livetime for the highest quality data selection. This allows the study of the spatial and spectral distributions of these photons with unprecedented detail. The spectrum of the emission - often referred to as Earth albedo gamma-ray emission - has a power-law shape up to 500 GeV with spectral index? = 2.79 ± 0.06.
Provides information and explores theories behind such phenomena as eclipses, black holes, gamma ray bursts, star births, and quasars
Supernova remnants (SNRs) are the only class of sources known in our Galaxy capable of providing the energy necessary to power the bulk of the Galactic cosmic-rays (CRs) below the `knee' (~ 3 PeV). They are observable across the entire frequency spectrum from radio to TeV gamma-rays, and are known to exhibit a rich variety of complex morphologies in multi-wavelength. Non-thermal emissions from SNRs in X-ray and gamma-ray arise from interaction between particles accelerated by the SNR blast wave and the surrounding medium, and are hence one of the most useful probe for the Galactic CR production process. In this thesis, we will try to obtain a fuller understanding of the origin of Galactic CRs through studying non-thermal emissions from SNRs and modelling CR injection from their astrophysical accelerators. In the first part of the thesis, we will develop a robust tool to simulate time and space-resolved broadband emission from young shell-type SNRs using coupled hydrodynamic and diffusive shock acceleration (DSA) calculations. Usually, the DSA process is expected to be highly non-linear for young SNRs due to a number of postulated coupling phenomena, which leads to the inter-correlation of the emission spectra and morphology at different wavelengths. Therefore, to gain the full picture, it is important to combine multi-wavelength observations and the relevant physical processes into a self-consistent and flexible calculation framework. By taking into account particle transport, escape, interaction and various radiative processes, our tool can predict photon emissivity in full three-dimension and multi-wavelength for any given SNR model and surrounding environment, such as in the presence of a nearby molecular cloud. Through illustrations using a few typical models for Type Ia SNR, we will demonstrate its capability of calculating results directly comparable to observations, as well as to pinpoint the gamma-ray emission mechanism, namely the leptonic and hadronic scenarios. In the second part, we will study the gamma-ray emission from a middle-aged SNR IC 443 (G189.1+3.0) using the Fermi Large Area Telescope (LAT). IC 443 has been extensively studied in the past few decades through radio to TeV gamma-ray, but high quality data in the sub-GeV to sub-TeV band, the most crucial window for constraining the origin of the high-energy emission, has still been missing. We will fill in this gap by analyzing LAT data from 200 MeV to 50 GeV using the 1st year of LAT data. Equipped with the high photon statistics available, and the excellent resolution, sensitivity and low background rate of LAT, we are able to probe the gamma-ray emission from IC 443 with minimal confusion with the backgrounds. We discovered spatially extended emission from IC 443 in the 1 - 50 GeV band for the first time, which eliminates the pulsar wind nebula (PWN) as the dominating gamma-ray emitter. We found good spatial correlation of the GeV mission with the TeV source recently detected by VERITAS, as well as a known group of ambient and shocked molecular clouds (MC). The sub-GeV to TeV broadband spectrum can be described by a power-law with a smooth break at a few GeV, the same feature also observed from several other LAT-detected middle-aged SNRs interacting with MCs. We will argue that the gamma-ray emission is most naturally explained by a neutral pion decay dominated origin, and the leptonic scenarios are disfavored. Finally, we will also discuss the major discoveries from LAT observations of other gamma-ray bright Galactic SNRs during the first 2 years of operation of Fermi. In the last part, we will construct a model of Galactic CR injection using constraints from most recent GeV and TeV observation data and CR measurements, which can provide a natural explanation for the enhanced positron flux above 10 GeV recently observed by PAMELA as compared to previous measurements. Without making speculation on `additional' positron contribution from any special nearby objects or resorting to exotic phenomena, we will look at a steady-state picture of our Galaxy in which the ensembles of SNRs and PWNe steadily inject CRs into the interstellar space. Using the GALPROP CR propagation code, the CR spectra and ratios at Earth are calculated and compared with data. Without tweaking the model parameters specifically to fit the positron data other than using observation and astrophysics-based assumptions, we will show that this steady-state model can satisfactorily reproduce the positron enhancement and other CR measurement results. Assisted by recent observations of middle-aged SNRs interacting with MCs by Fermi LAT, we are also able to set an upper-limit on the total number of these systems residing in our Galaxy. Finally, using this consistent model, we will estimate the energy budgets of the major species of Galactic CRs.
The Fermi-LAT (Large Area Telescope) gamma-ray space observatory was launched in June 2008 and has been continuously operating since. By far the brightest gamma-ray source in the sky for Fermi is the Earth. This emission is produced by the interactions between cosmic-ray (CR) particles and the Earth's atmosphere. Various properties of this emission have been measured with unprecedented details. Its energy spectrum is used to infer the spectrum of CR proton. The correlations between the thickness of the atmosphere and the solar cycle are tested by observing the time variation of its profile shape. Also, Fermi has demonstrated an excellent capacity to detect electrons and positrons. This enables the measurements of separate CR electrons and positrons spectra between 20 - 200 GeV, using the geomagnetic field to differentiate the charge sign. The result shows that the positron fraction is increasing with energy in this energy range, which strongly contradicts our standard models of CR productions and propagations. The interpretation of the excess positrons is still at the frontier of current CR physics research. It may be a sign of new phenomena, such as dark matter annihilation signal, or normal astrophysical sources in the local universe that we have to better understand.
The analysis presented in this paper incorporated photon events received during the full run time of the Fermi Gamma Space Telescope (FGST) Large Area Telescope (LAT) to date. By studying the [gamma]y emission of the supernova remnant (SNR) Kes 41 for the energy range ~ 200MeV-200GeV, the [gamma]-ray morphology and spectrum were measured. These measurements required the use of reduced log likelihood statistics mediated by the Fermi Science Tools toolkit, developed for LAT analysis. The spatial analysis of the [gamma]-ray emission was measured at 5[sigma] for the area within and around the contours established during radio measurements [25]. It also resembles Kes 41's observed, centrally bright, X-ray emission [18, 25]. Spectral analysis was also carried out and the resulting [gamma]-ray spectrum was successfully fit to a power-law model of emission consistent with [pi]0-decay, a form of non-thermal emission caused by cosmic ray acceleration. An overall approximation of the [gamma]-ray luminosity was then measured as L[gamma] = 1.94 x 1035 erg/s using a measure of the total [gamma]-ray flux. A calculation also measured the particle density associated with material interacting with Kes 41 emission as n = 0.15 particles/cm-3. This value resembles that from other calculations involving SNR-Molecular cloud interaction [22]. This interaction serves to constrain [gamma]-ray emission to the [pi]0-decay channel, so evidence of a similar density value may be evidence that the significant [gamma]-ray emission observed, was due to the acceleration of cosmic rays.
This book reports on the extraordinary observation of TeV gamma rays from the Crab Pulsar, the most energetic light ever detected from this type of object. It presents detailed information on the painstaking analysis of the unprecedentedly large dataset from the MAGIC telescopes, and comprehensively discusses the implications of pulsed TeV gamma rays for state-of-the-art pulsar emission models. Using these results, the book subsequently explores new testing methodologies for Lorentz Invariance Violation, in terms of a wavelength-dependent speed of light. The book also covers an updated search for Very-High-Energy (VHE), >100 GeV, emissions from millisecond pulsars using the Large Area Telescope on board the Fermi satellite, as well as a study on the promising Pulsar Wind Nebula candidate PSR J0631. The observation of VHE gamma rays is essential to studying the non-thermal sources of radiation in our Universe. Rotating neutron stars, also known as pulsars, are an extreme source class known to emit VHE gamma rays. However, to date only two pulsars have been detected with emissions above 100 GeV, and our understanding of their emission mechanism is still lacking.