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Register a new userThe Spectrum of the Isotropic Diffuse Gamma-Ray Emission Derived From First-Year Fermi Large Area Telescope Data
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The Fermi-LAT collaboration
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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.
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The isotropic gamma-ray background arises from the contribution of unresolved sources, including members of confirmed source classes and proposed gamma-ray emitters such as the radiation induced by dark matter annihilation and decay. Clues about the properties of the contributing sources are imprinted in the anisotropy characteristics of the gamma-ray background. We use 81 months of Pass 7 Reprocessed data from the Fermi Large Area Telescope to perform a measurement of the anisotropy angular power spectrum of the gamma-ray background. We analyze energies between 0.5 and 500 GeV, extending the range considered in the previous measurement based on 22 months of data. We also compute, for the first time, the cross-correlation angular power spectrum between different energy bins. We find that the derived angular spectra are compatible with being Poissonian, i.e. constant in multipole. Moreover, the energy dependence of the anisotropy suggests that the signal is due to two populations of sources, contributing, respectively, below and above 2 GeV. Finally, using data from state-of-the-art numerical simulations to model the dark matter distribution, we constrain the contribution from dark matter annihilation and decay in Galactic and extragalactic structures to the measured anisotropy. These constraints are competitive with those that can be derived from the average intensity of the isotropic gamma-ray background. Data are available at https://www-glast.stanford.edu/pub_data/552.
At a distance of 50 kpc and with a dark matter mass of $sim10^{10}$ M$_{odot}$, the Large Magellanic Cloud (LMC) is a natural target for indirect dark matter searches. We use five years of data from the Fermi Large Area Telescope (LAT) and updated models of the gamma-ray emission from standard astrophysical components to search for a dark matter annihilation signal from the LMC. We perform a rotation curve analysis to determine the dark matter distribution, setting a robust minimum on the amount of dark matter in the LMC, which we use to set conservative bounds on the annihilation cross section. The LMC emission is generally very well described by the standard astrophysical sources, with at most a $1-2sigma$ excess identified near the kinematic center of the LMC once systematic uncertainties are taken into account. We place competitive bounds on the dark matter annihilation cross section as a function of dark matter particle mass and annihilation channel.
The remnant of supernova explosion is widely believed to be the acceleration site of high-energy cosmic ray particles. The acceleration timescale is, however, typically very long. Here we report the detection of a variable $gamma$-ray source with the Fermi Large Area Telescope, which is positionally and temporally consistent with a peculiar supernova, iPTF14hls. A quasi-stellar object SDSS J092054.04+504251.5, which is probably a blazar according to the infrared data, is found in the error circle of the $gamma$-ray source. More data about the $gamma$-ray source and SDSS J092054.04+504251.5 are needed to confirm their association. On the other hand, if the association between the $gamma$-ray source and the supernova is confirmed, this would be the first time to detect high-energy $gamma$-ray emission from a supernova, suggesting very fast particle acceleration by supernova explosions.
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.
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.
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