Title: Fermi Gamma-ray Space Telescope: High-Energy Results from the First Year
ArXiv ID: 1011.0213
Date: 2009-06-01
Authors: A. A. Abdo, M. Ackermann, M. Ajello, B. Anderson, K. Atwood, W. B. Atwood, R. B. Buehler, C. B. Dermer, S. Digel, J. D. Finke, M. G. Baring, R. D. Blandford, J. Bregeon, A. Brown, R. C. D. de Palma, S. Digel, P. S. Drell, R. D. Ellis, A. V. Geringer‑Sameth, J. E. Grove, P. L. G. G. G. G. G. G. G. G. G. G. G. G. G. G. G. G. G. G. G. G. G. G. G. G. G. G. G. G. G. G. G. G. G. G. G. G. G. G. G. G. G. G. G. G. G. G. G. G. G. G. G. G. G. G. G. G. G. —
📝 Abstract
The Fermi Gamma-ray Space Telescope (Fermi) was launched on June 11, 2008 and began its first year sky survey on August 11, 2008. The Large Area Telescope (LAT), a wide field-of-view pair-conversion telescope covering the energy range from 20 MeV to more than 300 GeV, is the primary instrument on Fermi. While this review focuses on results obtained with the LAT, the Gamma-ray Burst Monitor (GBM) complements the LAT in its observations of transient sources and is sensitive to X-rays and gamma-rays with energies between 8 keV and 40 MeV. During the first year in orbit, the Fermi LAT has observed a large number of sources that include active galaxies, pulsars, compact binaries, globular clusters, supernova remnants, as well as the Sun, the Moon and the Earth. The GBM and LAT together have uncovered surprising characteristics in the high-energy emission of gamma-ray bursts (GRBs) that have been used to set significant new limits on violations of Lorentz invariance. The Fermi LAT has also made important new measurements of the Galactic diffuse radiation and has made precise measurements of the spectrum of cosmic-ray electrons and positrons from 20 GeV to 1 TeV.
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High-energy γ-ray radiation provides an important astrophysical probe of physical processes in extreme environments and of new physics; e.g. particle acceleration to ultra-relativistic energies in the vicinity of black holes, neutron stars, and supernovae remnants and possible signatures of dark matter decay or annihilation. Unlike cosmic rays, once γ-rays emerge from the immediate vicinity of their production they are largely unaffected in the propagation to where they are detected. Naively one might think that γ-rays would traverse inter-galactic space unimpeded, however, if the energy is high enough, these γ-rays can interact with the extragalactic background light (EBL) which pervades the universe. The principal source of this opacity at high energy is pair conversion off the EBL in the infrared-optical-ultraviolet band via γ + γ EBL → e + + e -. This process leaves an imprint on the spectrum of distant sources, which, in principle, can tell us about the density of the EBL, and therefore the rate of star formation, versus cosmic time.
In this report we summarize published results from observations made with the Large Area Telescope on the Fermi Gamma-ray Space Telescope (Fermi) during the first year of science operations that began in August 2008. 1 High-energy γ-ray observations with Fermi were preceded by observations with SAS-II (Fichtel et al 1975), COS-B (Bignami et al 1975), and EGRET (Fichtel et al 1983) on the Compton Gamma-Ray Observatory. The key improvements of the Fermi LAT, compared to the previous instruments that have flown, are possible because of the newer technologies, principally for particle tracking and in electronics, that have become available since the construction of EGRET. These key improvements in performance include (i) larger effective area over a much larger field-of-view, (ii) better particle tracking leading to improved angular resolution and background rejection, and (iii) a flexible, fast, multilevel trigger and data acquisition system. The large field-of-view results from the low aspect ratio (height/width) of the LAT made possible by the choice of particle tracking technology (i.e. silicon-strip detectors) that allowed elimination of the time-of-flight triggering system used in EGRET. An overview of the LAT instrument design is given in section 3 of this report.
In the current era, Fermi satellite observations up to more than 300 GeV are complemented by ground-based observations, typically above ~100 GeV, with air-Cherenkov telescopes [e.g. H.E.S.S. (Hofmann 1997), VERITAS (Holder et al 2006), Magic (Baixeras et al 2003), and Cangaroo (Kubo et al 2004)] and air-shower arrays such as Milagro (Atkins et al 2000). EGRET has provided much of the initial context for Fermi observations. For an overview of ground-based, very high-energy γ-ray detectors see Aharonian et al (2008a). For a review of EGRET results see Thompson (2008).
The outline of this report is as follows: .2 Extragalactic Sources: blazars and active galaxies, radio galaxies, the Large Magellanic Cloud, starburst galaxies, GRBs, diffuse isotropic radiation 1 at https://www--glast.stanford.edu/cgi--bin/pubpub
a listing of Fermi LAT Collaboration publications is available
Before launch, it was anticipated that Fermi would address a number of important scientific objectives that included the following [The reader is also referred to the more detailed questions listed in section 9 of the EGRET review by Thompson (2008)]:
(1) Determine the nature of unidentified high-energy γ-ray sources, particularly those seen by EGRET. The 3 rd EGRET Catalog (Hartman et al 1999) of 271 sources consists of the single 1991 solar flare bright enough to be detected as a source, the Large Magellanic Cloud (LMC), six pulsars, one probable radio galaxy detection (Cen A), and 66 high-confidence identifications of blazars (BL Lac objects, flat-spectrum radio quasars, or unidentified flat-spectrum radio sources). In addition, 27 lower confidence potential blazar identifications were noted. The catalog also contains 170 sources not firmly identified with known objects. Fermi has made significant progress in source identification because of its much narrower point-spread-function (PSF), larger field of view (FoV), and larger effective area, all of which contribute to much smaller error boxes than were possible with EGRET.
(2) Understand the mechanisms of particle acceleration operating in celestial sources and the origin(s) of cosmic-rays. EGRET observed high-energy γ-ray emission in several important source categories: active galaxies (AGNs/blazars) containing supermassive black holes (10 6 -10 9 M O ), pulsars, the Sun (as well as a small sample of GRBs). There was also reported evidence of emission from supernovae remnants (Sturner and Dermer 1995;Esposito et al 1996).
The Fermi LAT’s wide FoV has allowed AGN/blazar variability to be monitored over a wide range of timescales for a large number of sources (Abdo et al 2009a). In th
