86 research outputs found
Anisotropies in the diffuse gamma-ray background measured by the Fermi LAT
The contribution of unresolved sources to the diffuse gamma-ray background could induce anisotropies in this emission on small angular scales. We analyze the angular power spectrum of the diffuse emission measured by the Fermi Large Area Telescope at Galactic latitudes |b|>30° in four energy bins spanning 1–50 GeV. At multipoles ℓ≥155, corresponding to angular scales ≲2°, angular power above the photon noise level is detected at >99.99% confidence level in the 1–2 GeV, 2–5 GeV, and 5–10 GeV energy bins, and at >99% confidence level at 10–50 GeV. Within each energy bin the measured angular power takes approximately the same value at all multipoles ℓ≥155, suggesting that it originates from the contribution of one or more unclustered source populations. The amplitude of the angular power normalized to the mean intensity in each energy bin is consistent with a constant value at all energies, C_P/⟨I⟩^2=9.05±0.84×10^(-6) sr, while the energy dependence of C_P is consistent with the anisotropy arising from one or more source populations with power-law photon spectra with spectral index Γ_s=2.40±0.07. We discuss the implications of the measured angular power for gamma-ray source populations that may provide a contribution to the diffuse gamma-ray background
The Fermi Large Area Telescope on Orbit: Event Classification, Instrument Response Functions, and Calibration
The Fermi Large Area Telescope (Fermi-LAT, hereafter LAT), the primary instrument on the Fermi Gamma-ray Space Telescope (Fermi) mission, is an imaging, wide field-of-view, high-energy γ-ray telescope, covering the energy range from 20 MeV to more than 300 GeV. During the first years of the mission, the LAT team has gained considerable insight into the in-flight performance of the instrument. Accordingly, we have updated the analysis used to reduce LAT data for public release as well as the instrument response functions (IRFs), the description of the instrument performance provided for data analysis. In this paper, we describe the effects that motivated these updates. Furthermore, we discuss how we originally derived IRFs from Monte Carlo simulations and later corrected those IRFs for discrepancies observed between flight and simulated data. We also give details of the validations performed using flight data and quantify the residual uncertainties in the IRFs. Finally, we describe techniques the LAT team has developed to propagate those uncertainties into estimates of the systematic errors on common measurements such as fluxes and spectra of astrophysical sources
Search for gamma-ray spectral lines with the Fermi Large Area Telescope and dark matter implications
Weakly interacting massive particles (WIMPs) are a theoretical class of particles that are excellent dark matter candidates. WIMP annihilation or decay may produce essentially monochromatic γ rays detectable by the Fermi Large Area Telescope (LAT) against the astrophysical γ-ray emission of the Galaxy. We have searched for spectral lines in the energy range 5–300 GeV using 3.7 years of data, reprocessed with updated instrument calibrations and an improved energy dispersion model compared to the previous Fermi-LAT Collaboration line searches. We searched in five regions selected to optimize sensitivity to different theoretically motivated dark matter density distributions. We did not find any globally significant lines in our a priori search regions and present 95% confidence limits for annihilation cross sections of self-conjugate WIMPs and decay lifetimes. Our most significant fit occurred at 133 GeV in our smallest search region and had a local significance of 3.3 standard deviations, which translates to a global significance of 1.5 standard deviations. We discuss potential systematic effects in this search, and examine the feature at 133 GeV in detail. We find that the use both of reprocessed data and of additional information in the energy dispersion model contributes to the reduction in significance of the linelike feature near 130 GeV relative to significances reported in other works. We also find that the feature is narrower than the LAT energy resolution at the level of 2 to 3 standard deviations, which somewhat disfavors the interpretation of the 133 GeV feature as a real WIMP signal
Unresolved Unidentified Source Contribution to the Gamma-ray Background
The large majority of EGRET point sources remain without an identified
low-energy counterpart, and a large fraction of these sources are most likely
extragalactic. Whatever the nature of the extragalactic EGRET unidentified
sources, faint unresolved objects of the same class must have a contribution to
the diffuse extragalactic gamma-ray background (EGRB). Understanding this
component of the EGRB, along with other guaranteed contributions from known
sources, is essential if we are to use this emission to constrain exotic
high-energy physics. Here, we follow an empirical approach to estimate whether
a potential contribution of unidentified sources to the EGRB is likely to be
important, and we find that it is. Additionally, we show how upcoming GLAST
observations of EGRET unidentified sources, as well as of their fainter
counterparts, can be combined with GLAST observations of the Galactic and
extragalactic diffuse backgrounds to shed light on the nature of the EGRET
unidentified sources even without any positional association of such sources
with low-energy counterparts.Comment: 11 pages, 5 figures, submitted to Ap
Anisotropies in the diffuse gamma-ray background measured by Fermi LAT
The small angular scale fluctuations of the (on large scale) isotropic
gamma-ray background (IGRB) carry information about the presence of unresolved
source classes. A guaranteed contribution to the IGRB is expected from the
unresolved gamma-ray AGN while other extragalactic sources, Galactic gamma-ray
source populations and dark matter Galactic and extragalactic structures (and
sub-structures) are candidate contributors. The IGRB was measured with
unprecedented precision by the Large Area Telescope (LAT) on-board of the Fermi
gamma-ray observatory, and these data were used for measuring the IGRB angular
power spectrum (APS). Detailed Monte Carlo simulations of Fermi-LAT all-sky
observations were performed to provide a reference against which to compare the
results obtained for the real data set. The Monte Carlo simulations are also a
method for performing those detailed studies of the APS contributions of single
source populations, which are required in order to identify the actual IGRB
contributors. We present preliminary results of an anisotropy search in the
IGRB. At angular scales <2deg (e.g. above multipole 155), angular power above
the photon noise level is detected, at energies between 1 and 10 GeV in each
energy bin, with statistical significance between 7.2 and 4.1 sigmas. The
energy not dependence of the fluctuation anisotropy is pointing to the presence
of one or more unclustered source populations, while the energy dependence of
the intensity anisotropy is consistent with source populations having average
photon index 2.40\pm0.07.Comment: 6 pages, Proceedings of the RICAP 2011 Conference, submitted to NIM
Anisotropies in the diffuse gamma-ray background measured by the Fermi-LAT
The small angular scale fluctuations of the (on large scale) isotropic gamma-ray background (IGRB) carry information about the presence of unresolved source classes. A guaranteed contribution to the IGRB is expected from the unresolved gamma-ray AGN while other extragalactic sources, Galactic gamma-ray source populations and dark matter Galactic and extragalactic structures (and sub-structures) are candidate contributors.
The IGRB was measured with unprecedented precision by the Large Area Telescope (LAT) on-board of the Fermi gamma-ray observatory, and these data were used for measuring the IGRB angular power spectrum (APS). Detailed Monte Carlo simulations of Fermi-LAT all-sky observations were performed to provide a reference against which to compare the results obtained for the real data set. The Monte Carlo simulations are also a method for performing those detailed studies of the APS contributions of single source populations, which are required in order to identify the actual IGRB contributors.
We present preliminary results of an anisotropy search in the IGRB. At angular scales <2° (e.g., above multipole 155), angular power above the photon noise level is detected, at energies between 1 and 10 GeV in each energy bin, with statistical significance between 7.2 and 4.1σ. The obtained energy dependences point to the presence of one or more unclustered source populations with the components having an average photon index Γ=2.40±0.07
Gamma-ray signatures of annihilation to charged leptons in dark matter substructure
Due to their higher concentrations and small internal velocities, Milky Way
subhalos can be at least as important as the smooth halo in accounting for the
GeV positron excess via dark matter annihilation. After showing how this can be
achieved in various scenarios, including in Sommerfeld models, we demonstrate
that, in this case, the diffuse inverse-Compton emission resulting from
electrons and positrons produced in substructure leads to a nearly-isotropic
signal close to the level of the isotropic GeV gamma-ray background seen by
Fermi. Moreover, we show that HESS cosmic-ray electron measurements can be used
to constrain multi-TeV internal bremsstrahlung gamma rays arising from
annihilation to charged leptons.Comment: 8 pages, 4 figures; minor updates to match published versio
Anisotropies in the Diffuse Gamma-Ray Background from Dark Matter with Fermi LAT: a closer look
We perform a detailed study of the sensitivity to the anisotropies related to
Dark Matter (DM) annihilation in the Isotropic Gamma-Ray Background (IGRB) as
measured by the Fermi Large Area Telescope (Fermi-LAT). For the first time, we
take into account the effects of the Galactic foregrounds and use a realistic
representation of the Fermi-LAT. We consider DM anisotropies of extra-galactic
origin and of Galactic origin (which can be generated through annihilation in
the Milky Way sub-structures) as opposed to a background of anisotropies
generated by sources of astrophysical origin, blazars for example. We find that
with statistics from 5 years of observation Fermi is sensitive to a DM
contribution at the level of the thermal-relic cross section depending on the
DM mass and annihilation mode. The anisotropy method for DM searches has a
sensitivity comparable to the usual methods based only on the energy spectrum
and thus constitutes an independent and complementary piece of information in
the DM puzzle. (abridged)Comment: 17 pages, 9 figures, v2: added discussion on unresolved point
sources, matches published version on MNRA
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