| 1. |
- Ackermann, M., et al.
(författare)
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Anisotropies in the diffuse gamma-ray background measured by the Fermi LAT
- 2012
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Ingår i: Physical Review D. - 1550-7998 .- 1550-2368. ; 85:8
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Tidskriftsartikel (refereegranskat)abstract
- 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 vertical bar b vertical bar > 30 degrees in four energy bins spanning 1-50 GeV. At multipoles l >= 155, corresponding to angular scales less than or similar to 2 degrees, 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 x 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 Gamma(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.
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| 2. |
- Ackermann, M., et al.
(författare)
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Constraints on the galactic halo dark matter from fermi-lat diffuse measurements
- 2012
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Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 761:2, s. 91-
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Tidskriftsartikel (refereegranskat)abstract
- We have performed an analysis of the diffuse gamma-ray emission with the Fermi Large Area Telescope (LAT) in the Milky Way halo region, searching for a signal from dark matter annihilation or decay. In the absence of a robust dark matter signal, constraints are presented. We consider both gamma rays produced directly in the dark matter annihilation/decay and produced by inverse Compton scattering of the e(+)/e(-) produced in the annihilation/decay. Conservative limits are derived requiring that the dark matter signal does not exceed the observed diffuse gamma-ray emission. A second set of more stringent limits is derived based on modeling the foreground astrophysical diffuse emission using the GALPROP code. Uncertainties in the height of the diffusive cosmic-ray halo, the distribution of the cosmic-ray sources in the Galaxy, the index of the injection cosmic-ray electron spectrum, and the column density of the interstellar gas are taken into account using a profile likelihood formalism, while the parameters governing the cosmic-ray propagation have been derived from fits to local cosmic-ray data. The resulting limits impact the range of particle masses over which dark matter thermal production in the early universe is possible, and challenge the interpretation of the PAMELA/Fermi-LAT cosmic ray anomalies as the annihilation of dark matter.
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| 3. |
- Ackermann, M., et al.
(författare)
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Limits on dark matter annihilation signals from the Fermi LAT 4-year measurement of the isotropic gamma-ray background
- 2015
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Ingår i: Journal of Cosmology and Astroparticle Physics. - : IOP Publishing. - 1475-7516. ; :9
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Tidskriftsartikel (refereegranskat)abstract
- We search for evidence of dark matter (DM) annihilation in the isotropic gamma-ray background (IGRB) measured with 50 months of Fermi Large Area Telescope (LAT) observations. An improved theoretical description of the cosmological DM annihilation signal, based on two complementary techniques and assuming generic weakly interacting massive particle (WIMP) properties, renders more precise predictions compared to previous work. More specifically, we estimate the cosmologically-induced gamma-ray intensity to have an uncertainty of a factor similar to 20 in canonical setups. We consistently include both the Galactic and extragalactic signals under the same theoretical framework, and study the impact of the former on the IGRB spectrum derivation. We find no evidence for a DM signal and we set limits on the DM-induced isotropic gamma-ray signal. Our limits are competitive for DM particle masses up to tens of TeV and, indeed, are the strongest limits derived from Fermi LAT data at TeV energies. This is possible thanks to the new Fermi LAT IGRB measurement, which now extends up to an energy of 820 GeV. We quantify uncertainties in detail and show the potential this type of search offers for testing the WIMP paradigm with a complementary and truly cosmological probe of DM particle signals.
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| 4. |
- Borriello, Enrico, et al.
(författare)
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Dark matter electron anisotropy : A universal upper limit
- 2012
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Ingår i: Astroparticle physics. - : Elsevier BV. - 0927-6505 .- 1873-2852. ; 35:8, s. 537-546
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Tidskriftsartikel (refereegranskat)abstract
- We study the dipole anisotropy in the arrival directions of high energy CR electrons and positrons (CRE) of dark matter (DM) origin. We show that this quantity is very weakly model dependent and offers a viable criterion to discriminate among CRE from DM or from local discrete sources, like e.g. pulsars. In particular, we find that the maximum anisotropy which DM can provide is to a very good approximation a universal quantity and, as a consequence, if a larger anisotropy is detected, this would constitute a strong evidence for the presence of astrophysical local discrete CRE sources, whose anisotropy, instead, can be naturally larger than the DM upper limit. We further find that the main source of anisotropy from DM is given by the fluctuation in the number density of DM sub-structures in the vicinity of the observer and we thus devote special attention to the study of the variance in the sub-structures realization implementing a dedicated Montecarlo simulation. Such scenarios will be probed in the next years by Fermi-LAT, providing new hints, or constraints, about the nature of DM.
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| 5. |
- Cuoco, Alessandro, et al.
(författare)
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Anisotropies in the diffuse gamma-ray background from dark matter with Fermi LAT : a closer look
- 2011
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Ingår i: Monthly notices of the Royal Astronomical Society. - : Oxford University Press (OUP). - 0035-8711 .- 1365-2966. ; 414:3, s. 2040-2054
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Tidskriftsartikel (refereegranskat)abstract
- 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 implement an analysis pipeline which simulates Fermi LAT data sets starting from model maps of the Galactic foregrounds, the Fermi-resolved point sources, the extragalactic diffuse emission and the signal from DM annihilation. The effects of the detector are taken into account by convolving the model maps with the Fermi LAT instrumental response. We then use the angular power spectrum to characterize the anisotropy properties of the simulated data and to study the sensitivity to DM. We consider DM anisotropies of extragalactic origin and of Galactic origin (which can be generated through annihilation in the MilkyWay substructures) as opposed to a background of anisotropies generated by sources of astrophysical origin, blazars for example. We find that with statistics from 5 yr of observation, Fermi is sensitive to a DM contribution at the level of 1-10 per cent of the measured IGRB depending on the DM mass m. and annihilation mode. In terms of the thermally averaged cross-section , this corresponds to similar to 10(-25) cm(3) s(-1), i. e. slightly above the typical expectations for a thermal relic, for low values of the DM mass m(x) less than or similar to 100 GeV. 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.
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| 6. |
- Cuoco, Alessandro, et al.
(författare)
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Joint anisotropy and source count constraints on the contribution of blazars to the diffuse gamma-ray background
- 2012
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Ingår i: Physical Review D. - 1550-7998 .- 1550-2368. ; 86:6
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Tidskriftsartikel (refereegranskat)abstract
- We place new constraints on the contribution of blazars to the large-scale isotropic gamma-ray background (IGRB) by jointly analyzing the measured source count distribution (logN-logS) of blazars and the measured intensity and anisotropy of the IGRB. We find that these measurements point to a consistent scenario in which unresolved blazars make less than or similar to 20% of the IGRB intensity at 1-10 GeV while accounting for the majority of the measured anisotropy in that energy band. These results indicate that the remaining fraction of the IGRB intensity is made by a component with a low level of intrinsic anisotropy. We determine upper limits on the anisotropy from nonblazar sources, adopting the best-fit parameters of the measured source count distribution to calculate the unresolved blazar anisotropy. In addition, we show that the anisotropy measurement excludes some recently proposed models of the unresolved blazar population.
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| 7. |
- Fornasa, Mattia, et al.
(författare)
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Angular power spectrum of the diffuse gamma-ray emission as measured by the Fermi Large Area Telescope and constraints on its dark matter interpretation
- 2016
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Ingår i: Physical Review D. - 2470-0010. ; 94:12
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Tidskriftsartikel (refereegranskat)abstract
- 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 similar to 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 extra-Galactic 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.
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| 8. |
- Gomez-Vargas, G. A., et al.
(författare)
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Dark matter implications of Fermi-LAT measurement of anisotropies in the diffuse gamma-ray background
- 2014
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Ingår i: Nuclear Instruments and Methods in Physics Research Section A. - : Elsevier BV. - 0168-9002 .- 1872-9576. ; 742, s. 149-153
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Tidskriftsartikel (refereegranskat)abstract
- The detailed origin of the diffuse gamma-ray background is still unknown. However, the contribution of unresolved sources is expected to induce small-scale anisotropies in this emission, which may provide a way to identify and constrain the properties of its contributors. Recent studies have predicted the contributions to the angular power spectrum (APS) from extragalactic and galactic dark matter (DM) annihilation or decay. The Fermi-LAT collaboration reported detection of angular power with a significance larger than 3 sigma in the energy range from 1 GeV to 10 GeV on 22 months of data (Ackermann et al., 2012 [2]). For these preliminary results the already published Fermi-LAT APS measurements (Ackermann et al., 2012 [2]) are compared to the accurate predictions for DM anisotropies from state-of-the-art cosmological simulations as presented in Fornasa et at (2013) [1] to derive constraints on different DM candidates.
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| 9. |
- Korsmeier, Michael, et al.
(författare)
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Implications of lithium to oxygen AMS-02 spectra on our understanding of cosmic-ray diffusion
- 2021
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Ingår i: Physical Review D. - 2470-0010 .- 2470-0029. ; 103:10
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Tidskriftsartikel (refereegranskat)abstract
- We analyze recent AMS-02 comic-ray measurements of lithium, beryllium, boron, carbon, nitrogen, and oxygen. The emphasis of the analysis is on systematic uncertainties related to propagation and nuclear cross sections. To investigate the uncertainties in the propagation scenario, we consider five different frameworks, differing with respect to the inclusion of a diffusion break at a few GV, the presence of reacceleration, and the presence of a break in the injection spectra of primaries. For each framework we fit the diffusion equations of cosmic rays and explore the parameter space with Monte Carlo methods. At the same time, the impact of the uncertainties on the nuclear production cross sections of secondaries is explicitly considered and included in the fit through the use of nuisance parameters. We find that all of the considered frameworks are able to provide a good fit. In particular, two competing scenarios—one including a break in diffusion but no reacceleration and the other with reacceleration but no break in diffusion—are both allowed. The inclusion of cross-section uncertainties is, however, crucial to this result. Thus, at the moment these uncertainties represent a fundamental systematic preventing a deeper understanding of the properties of cosmic-ray propagation. Nonetheless, we find that the slope of diffusion at intermediate rigidities is robustly constrained in the range δ≃0.45–0.5 in models without convection, or δ≃0.4–0.5 if convection is included in the fit. Furthermore, we find that the use of the AMS-02 beryllium data provides a lower limit on the vertical size of the Galactic propagation halo of zh≳3 kpc.
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| 10. |
- Korsmeier, Michael, et al.
(författare)
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Testing the universality of cosmic-ray nuclei from protons to oxygen with AMS-02
- 2022
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Ingår i: Physical Review D. - 2470-0010 .- 2470-0029. ; 105:10
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Tidskriftsartikel (refereegranskat)abstract
- The AMS-02 experiment has provided high-precision measurements of several cosmic-ray (CR) species. The achieved percent-level accuracy gives access to small spectral differences among the different species and, in turn, this allows scrutinizing the universality of CR acceleration, which is expected in the standard scenario of CR shock acceleration. While pre-AMS-02 data already indicated a violation of the universality between protons and helium, it is still an open question if at least helium and heavier nuclei can be reconciled. To address this issue, we performed a joint analysis using the AMS-02 CR measurements of antiprotons, protons, helium, helium 3, boron, carbon, nitrogen, and oxygen. We explore two competing propagation scenarios, one with a break in the diffusion coefficient at a few GVs and no reacceleration, and another one with reacceleration and with a break in the injection spectra of primaries. Furthermore, we explicitly consider the impact of the uncertainties in the nuclear production cross sections of secondaries by including nuisance parameters in the fit. The resulting parameter space is explored with the help of Monte Carlo methods. We find that, contrary to the naive expectation, in the standard propagation scenarios CR universality is violated also for He, on the one hand, and C, N, and O, on the other hand, i.e., different injection slopes (at the level of Δ∼0.05) are required to explain the observed spectra. As an alternative, we explore further propagation scenarios, inspired by nonhomogeneous diffusion, which might save universality. Finally, we also investigate the universality of CR propagation, i.e., we compare the propagation properties inferred using only light nuclei (¯p, p, He, 3He) with the ones inferred using only heavier nuclei (B, C, N, O).
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