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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.
Recent accurate measurements of cosmic-ray (CR) species by ATIC-2, CREAM, and PAMELA reveal an unexpected hardening in the proton and He spectra above a few hundred GeV, a gradual softening of the spectra just below a few hundred GeV, and a harder spectrum of He compared to that of protons. These newly-discovered features may offer a clue to the origin of high-energy CRs. We use the ${\it Fermi}$ Large Area Telescope observations of the $\gamma$-ray emission from the Earth's limb for an indirect measurement of the local spectrum of CR protons in the energy range $\sim 90~$GeV-$6~$TeV (derived from a photon energy range $15~$GeV-$1~$TeV). Our analysis shows that single power law and broken power law spectra fit the data equally well and yield a proton spectrum with index $2.68 \pm 0.04$ and $2.61 \pm 0.08$ above $\sim 200~$GeV, respectively.
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.
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.
The Fermi-LAT experiment recently reported high precision measurements of the spectrum of cosmic-ray electrons-plus-positrons (CRE) between 20 GeV and 1 TeV. The spectrum shows no prominent spectral features, and is significantly harder than that inferred from several previous experiments. Here we discuss several interpretations of the Fermi results based either on a single large scale Galactic CRE component or by invoking additional electron-positron primary sources, e.g. nearby pulsars or particle Dark Matter annihilation. We show that while the reported Fermi-LAT data alone can be interpreted in terms of a single component scenario, when combined with other complementary experimental results, specifically the CRE spectrum measured by H.E.S.S. and especially the positron fraction reported by PAMELA between 1 and 100 GeV, that class of models fails to provide a consistent interpretation. Rather, we find that several combinations of parameters, involving both the pulsar and dark matter scenarios, allow a consistent description of those results. We also briefly discuss the possibility of discriminating between the pulsar and dark matter interpretations by looking for a possible anisotropy in the CRE flux.
The diffuse galactic?-ray emission is produced by cosmic rays (CRs) interacting with the interstellar gas and radiation field. Measurements by the Energetic Gamma-Ray Experiment Telescope (EGRET) instrument on the Compton Gamma-Ray Observatory indicated excess?-ray emission ≥1 GeV relative to diffuse galactic?-ray emission models consistent with directly measured CR spectra (the so-called 'EGRET GeV excess'). The Large Area Telescope (LAT) instrument on the Fermi Gamma-Ray Space Telescope has measured the diffuse?-ray emission with improved sensitivity and resolution compared to EGRET. We report on LAT measurements for energies 100 MeV to 10 GeV and galactic latitudes 10{sup o} ≤.
Offers an accessible text and reference (a cosmic-ray manual) for graduate students entering the field and high-energy astrophysicists will find this an accessible cosmic-ray manual Easy to read for the general astronomer, the first part describes the standard model of cosmic rays based on our understanding of modern particle physics. Presents the acceleration scenario in some detail in supernovae explosions as well as in the passage of cosmic rays through the Galaxy. Compares experimental data in the atmosphere as well as underground are compared with theoretical models
Real breakthrough during last 1-1.5 years in cosmic ray electrons: ATIC, HESS, Pamela, and finally Fermi-LAT. New quality data have made it possible to start quantitative modeling. With the new data more puzzles than before on CR electrons origin. Need "multi-messenger" campaign: electrons, positrons, gammas, X-ray, radio, neutrino... It is viable that we are dealing with at least two distinct mechanisms of "primary" electron (both signs) production: a softer spectrum of negative electrons, and a harder spectrum of both e(+)+e(-). Exotic (e.g. DM) origin is not ruled out. Upper limits on CR electrons anisotropy are set. Good perspectives to have the Fermi LAT results on proton spectrum and positron fraction.
In the first part, the book gives an up-to-date summary of the observational data. In the second part, it deals with the kinetic description of cosmic ray plasma. The underlying diffusion-convection transport equation, which governs the coupling between cosmic rays and the background plasma, is derived and analyzed in detail. In the third part, several applications of the solutions of the transport equation are presented and how key observations in cosmic ray physics can be accounted for is demonstrated.
This revised edition provides an up-to-date summary of the field of ultra-high energy cosmic rays, dealing with their origin, propagation, and composition,. The authors reflect the enormous strides made since the first edition in the realm of experimental work, in particular the use of vastly improved, more sensitive and precise detectors. The level remains introductory and pedagogical, suitable for students and researchers interested in moving into this exciting field. Throughout the text, the authors focus on giving an introductory overview of the key physics issues, followed by a clear and concise description of experimental approaches and current results. Key Features: Updates the most coherent summary of the field available, with new text that provides the reader with clear historical context. Brand new discussion of contemporary space-based experiments and ideas for extending ground-based detectors. Completely new discussion of radio detection methods. Includes a new chapter on small to intermediate-scale anisotropy. Offers new sections on modern hadronic models and software packages to simulate showers.