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Register a new userFermi Gamma-ray Space Telescope: High-Energy Results from the First Year
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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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We have measured the gamma-ray emission spectrum of the Moon using the data collected by the Large Area Telescope onboard the Fermi satellite during its first 7 years of operation, in the energy range from 30 MeV up to a few GeV. We have also studied the time evolution of the flux, finding a correlation with the solar activity. We have developed a full Monte Carlo simulation describing the interactions of cosmic rays with the lunar surface. The results of the present analysis can be explained in the framework of this model, where the production of gamma rays is due to the interactions of cosmic-ray proton and helium nuclei with the surface of the Moon. Finally, we have used our simulation to derive the cosmic-ray proton and helium spectra near Earth from the Moon gamma-ray data.
We report on the first Fermi Large Area Telescope (LAT) measurements of the so-called extra-galactic diffuse gamma-ray emission (EGB). This component of the diffuse gamma-ray emission is generally considered to have an isotropic or nearly isotropic distribution on the sky with diverse contributions discussed in the literature. The derivation of the EGB is based on detailed modelling of the bright foreground diffuse Galactic gamma-ray emission (DGE), the detected LAT sources and the solar gamma-ray emission. We find the spectrum of the EGB is consistent with a power law with differential spectral index g = 2.41+/-0.05 and intensity, I(> 100 MeV) = (1.03+/-0.17) 10^-5 cm^-2 s^-1 sr^-1, where the error is systematics dominated. Our EGB spectrum is featureless, less intense, and softer than that derived from EGRET data.
We examine 288 GRBs detected by the Fermi Gamma-ray Space Telescopes Gamma-ray Burst Monitor (GBM) that fell within the field-of-view of Fermis Large Area Telescope (LAT) during the first 2.5 years of observations, which showed no evidence for emission above 100 MeV. We report the photon flux upper limits in the 0.1-10 GeV range during the prompt emission phase as well as for fixed 30 s and 100 s integrations starting from the trigger time for each burst. We compare these limits with the fluxes that would be expected from extrapolations of spectral fits presented in the first GBM spectral catalog and infer that roughly half of the GBM-detected bursts either require spectral breaks between the GBM and LAT energy bands or have intrinsically steeper spectra above the peak of the { u}F{ u} spectra (Epk). In order to distinguish between these two scenarios, we perform joint GBM and LAT spectral fits to the 30 brightest GBM-detected bursts and find that a majority of these bursts are indeed softer above Epk than would be inferred from fitting the GBM data alone. Approximately 20% of this spectroscopic subsample show statistically significant evidence for a cut-off in their high-energy spectra, which if assumed to be due to {gamma}{gamma} attenuation, places limits on the maximum Lorentz factor associated with the relativistic outflow producing this emission. All of these latter bursts have maximum Lorentz factor estimates that are well below the minimum Lorentz factors calculated for LAT- detected GRBs, revealing a wide distribution in the bulk Lorentz factor of GRB outflows and indicating that LAT-detected bursts may represent the high end of this distribution.
Black holes with masses below approximately $10^{15}$ g are expected to emit gamma rays with energies above a few tens of MeV, which can be detected by the Fermi Large Area Telescope (LAT). Although black holes with these masses cannot be formed as a result of stellar evolution, they may have formed in the early Universe and are therefore called Primordial Black Holes (PBHs). Previous searches for PBHs have focused on either short timescale bursts or the contribution of PBHs to the isotropic gamma-ray emission. We show that, in case of individual PBHs, the Fermi LAT is most sensitive to PBHs with temperatures above approximately 16 GeV and masses $6times 10^{11}$ g, which it can detect out to a distance of about 0.03 pc. These PBHs have a remaining lifetime of months to years at the start of the Fermi mission. They would appear as potentially moving point sources with gamma-ray emission that becomes spectrally harder and brighter with time until the PBH completely evaporates. In this paper, we develop a new algorithm to detect the proper motion of a gamma-ray point sources, and apply it to 318 unassociated point sources at high galactic latitude in the third Fermi-LAT source catalog (3FGL). None of unassociated point sources with spectra consistent with PBH evaporation show significant proper motion. Using the non-detection of PBH candidates, we derive a 99% confidence limit on PBH evaporation rate in the vicinity of the Earth $dot{rho}_{rm PBH} < 7.2 times 10^3: {rm {pc}^{-3} {yr}^{-1}}$. This limit is similar to the limits obtained with ground-based gamma-ray observatories.
The inner region of the Milky Way is one of the most interesting and complex regions of the gamma-ray sky. The intense interstellar emission and resolved point sources, as well as potential contributions by other sources such as unresolved source populations and dark matter, complicate the interpretation of the data. In this paper the Fermi LAT team analysis of a 15x15 degree region about the Galactic centre is described. The methodology for point-source detection and treatment of the interstellar emission is given. In general, the bulk of the gamma-ray emission from this region is attributable to a combination of these two contributions. However, low-intensity residual emission remains and its characterisation is discussed.
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