for the Fermi LAT Collaboration

Paris Diderot University, Lutetia Parisorum, Île-de-France, France

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Publications (31)32.08 Total impact

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    J. W. Hewitt, F. Acero, T. J. Brandt, J. Cohen, F. de Palma, F. Giordano, for the Fermi LAT Collaboration
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    ABSTRACT: While supernova remnants (SNRs) are widely thought to be powerful accelerators, evidence comes largely from a small number of well-studied cases. Here we systematically determine the gamma-ray emission from all known Galactic SNRs, disentangling them from the sea of Galactic cosmic rays. Using Fermi LAT data we have characterized the GeV emission in all regions containing SNRs, accounting for systematic uncertainties caused by source confusion, diffuse emission modeling, and instrumental response. More than a dozen remnants are identified through spatial extension or detection at TeV energies, with potential associations for >40 more. From this population study, two clear classes of gamma-ray-emitting SNRs emerge: young remnants and those interacting with a dense medium. This large statistical sample also reveals a possible correlation between GeV and radio flux. The growing number of identified SNRs will help to disentangle the effects of age and environment on the aggregate properties of SNRs at high energies.
    07/2013;
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    T. J. Brandt, F. Acero, F. de Palma, J. W. Hewitt, M. Renaud, for the Fermi LAT Collaboration
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    ABSTRACT: Despite tantalizing evidence that supernova remnants (SNRs) are the source of Galactic cosmic rays (CRs), including the recent detection of a spectral signature of hadronic gamma-ray emission from two SNRs, their origin in aggregate remains elusive. We address the long-standing question of Galactic CR nuclei origins using our statistically significant GeV SNR sample to estimate the contribution of SNRs to directly observed CRs. Interactions between CRs and ambient gas near the SNRs emit photons via pion decay at GeV energies, providing an in situ tracer for CRs otherwise measured directly with balloon-borne and satellite experiments near the Earth. To date, the Fermi LAT SNR Catalog has detected more than 50 SNRs and potential associations in classes with a variety of properties, yet all remain possible accelerators. We investigate the GeV and multiwavelength (MW) emission from SNRs to constrain their maximal contribution to observed Galactic CRs. Our work demonstrates the need for improvements to previously sufficient simple models describing the GeV and MW emission from these objects.
    07/2013;
  • C. D. Dermer, A. W. Strong, E. Orlando, L. Tibaldo, for the Fermi Collaboration
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    ABSTRACT: More than 90% of the Galactic gas-related gamma-ray emissivity above 1 GeV is attributed to the decay of neutral pions formed in collisions between cosmic rays and interstellar matter, with lepton-induced processes becoming increasingly important below 1 GeV. Given the high-quality measurements of the gamma-ray emissivity of local interstellar gas between ~50 MeV and ~4 GeV obtained with the Large Area Telescope on board the Fermi space observatory, it is timely to re-investigate this topic in detail, including the hadronic production mechanisms. The emissivity spectrum will allow the interstellar cosmic-ray spectrum to be determined reliably, providing a reference for origin and propagation studies as well as input to solar modulation models. A method for such an analysis and illustrative results are presented.
    07/2013;
  • S Ciprini, D ~J Thompson, Fermi LAT collaboration
    ArXiv e-prints. 03/2013;
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    L. Tibaldo, I. A. Grenier, for the Fermi LAT collaboration
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    ABSTRACT: Cygnus X is a conspicuous massive-star forming region in the Local Arm of the Galaxy at ~1.4 kpc from the solar system. Gamma-ray observations can be used to trace cosmic rays (CRs) interacting with the ambient interstellar gas and low-energy radiation fields. Using the Fermi Large Area Telescope (LAT) we have discovered the presence of a 50-pc wide cocoon of freshly-accelerated CRs in the region bounded by the ionization fronts from the young stellar clusters. On the other hand, the LAT data show that the CR population averaged over the whole Cygnus complex on a scale of 400 pc is similar to that found in the interstellar space near the Sun. These results confirm the long-standing hypothesis that massive star-forming regions host CR factories and shed a new light on the early phases of CR life in such a turbulent environment.
    Nuclear Physics B - Proceedings Supplements 11/2012; s 239–240. · 0.88 Impact Factor
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    Alex Drlica-Wagner, for the Fermi LAT Collaboration
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    ABSTRACT: Due to a large mass-to-light ratio and low astrophysical backgrounds, dwarf spheroidal galaxies (dSphs) are considered to be one of the most promising targets for dark matter searches via gamma rays. The Fermi LAT Collaboration has recently reported robust constraints on the dark matter annihilation cross section from a combined analysis of 10 dSphs. These constraints have been applied to experimentally valid, super-symmetric particle models derived from a phenomenological scan of the Minimal Supersymmetric Standard Model (the pMSSM). Additionally, the LAT Collaboration has searched for spatially extended, hard-spectrum gamma-ray sources lacking counterparts in other wavelengths, since they may be associated with dark matter substructures predicted from simulations.
    10/2012;
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    ABSTRACT: Recent detections of the starburst galaxies M82 and NGC 253 by gamma-ray telescopes suggest that galaxies rapidly forming massive stars are more luminous at gamma-ray energies compared to their quiescent relatives. Building upon those results, we examine a sample of 69 dwarf, spiral, and luminous and ultraluminous infrared galaxies at photon energies 0.1-100 GeV using 3 years of data collected by the Large Area Telescope (LAT) on the \textit{Fermi Gamma-ray Space Telescope} (\textit{Fermi}). Measured fluxes from significantly detected sources and flux upper limits for the remaining galaxies are used to explore the physics of cosmic rays in galaxies. We find further evidence for quasi-linear scaling relations between gamma-ray luminosity and both radio continuum luminosity and total infrared luminosity which apply both to quiescent galaxies of the Local Group and low-redshift starburst galaxies (conservative $P$-values $\lesssim0.05$ accounting for statistical and systematic uncertainties). The normalizations of these scaling relations correspond to luminosity ratios of $\log(L_{0.1-100 \rm{GeV}}/L_{1.4 \rm{GHz}}) = 1.7 \pm 0.1_{\rm (statistical)} \pm 0.2_{\rm (dispersion)}$ and $\log(L_{0.1-100 \rm{GeV}}/L_{8-1000 \mu\rm{m}}) = -4.3 \pm 0.1_{\rm (statistical)} \pm 0.2_{\rm (dispersion)}$ for a galaxy with a star formation rate of 1 $M_{\odot}$ yr$^{-1}$, assuming a Chabrier initial mass function. Using the relationship between infrared luminosity and gamma-ray luminosity, the collective intensity of unresolved star-forming galaxies at redshifts $0<z<2.5$ above 0.1 GeV is estimated to be 0.4-2.4 $\times 10^{-6}$ ph cm$^{-2}$ s$^{-1}$ sr$^{-1}$ (4-23% of the intensity of the isotropic diffuse component measured with the LAT). We anticipate that $\sim10$ galaxies could be detected by their cosmic-ray induced gamma-ray emission during a 10-year \textit{Fermi} mission.
    06/2012;
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    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 LAT at Galactic latitudes |b| > 30 deg in four energy bins spanning 1 to 50 GeV. At multipoles \ell \ge 155, corresponding to angular scales \lesssim 2 deg, angular power above the photon noise level is detected at >99.99% CL in the 1-2 GeV, 2-5 GeV, and 5-10 GeV energy bins, and at >99% CL at 10-50 GeV. Within each energy bin the measured angular power takes approximately the same value at all multipoles \ell \ge 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/ ^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.
    02/2012;
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    ABSTRACT: Gamma-ray binaries are stellar systems containing a neutron star or black hole, with gamma-ray emission produced by an interaction between the components. These systems are rare, even though binary evolution models predict dozens in our Galaxy. A search for gamma-ray binaries with the Fermi Large Area Telescope (LAT) shows that 1FGL J1018.6-5856 exhibits intensity and spectral modulation with a 16.6-day period. We identified a variable x-ray counterpart, which shows a sharp maximum coinciding with maximum gamma-ray emission, as well as an O6V((f)) star optical counterpart and a radio counterpart that is also apparently modulated on the orbital period. 1FGL J1018.6-5856 is thus a gamma-ray binary, and its detection suggests the presence of other fainter binaries in the Galaxy.
    Science 01/2012; 335(6065):189-93. · 31.20 Impact Factor
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    Luigi Tibaldo, Isabelle A. Grenier, for the Fermi LAT collaboration
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    ABSTRACT: The Cygnus region hosts the most conspicuous star-forming region close to the Sun, embedded in a giant complex of molecular clouds in the Local Arm. We present an analysis of the Fermi-LAT observations of Cygnus intended to probe the cosmic-ray and interstellar-matter content of the region. From gamma-ray data we estimate a total of (8 +5 -1) x 10^6 solar masses of interstellar gas in the complex at a distance of 1.4 kpc. The gamma-ray emission from the atomic gas supports the average HI spin temperature derived from radio absorption/emission pairs to estimate its column densities. The X(CO)=N(H2)/W(CO) ratio derived in the massive Cygnus complex is consistent with other LAT estimates for clouds in the Local and Perseus arms. The mass of dark gas, escaping HI and CO observations but traced by dust and gamma rays, amounts to ~10% of the total. We find an average gamma-ray emissivity per interstellar H atom in the 0.1--100 GeV energy band in good agreement with measurements in other segments of the Local Arm. We infer that the cosmic-ray population averaged over a few hundred parsecs is fairly uniform in density and spectrum along the Local Arm. Despite the presence of potential accelerators and much larger interstellar densities in Cygnus compared to the solar neighborhood, their cosmic-ray populations are similar on such a scale.
    10/2011;
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    Stephan Zimmer, Jan Conrad, for the Fermi LAT Collaboration, Anders Pinzke
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    ABSTRACT: Multiwavelength observations suggest that clusters are reservoirs of vast amounts relativistic electrons and positrons that are either injected into and accelerated directly in the intra-cluster medium, or produced as secondary pairs by cosmic ray ions scattering on ambient protons. In these possible scenarios gamma rays are produced either through electrons upscattering low-energy photons or by decay of neutral pions produced by hadronic interactions. In addition, the high mass-to-light ratios in clusters in combination with considerable Dark Matter (DM) overdensities makes them interesting targets for indirect DM searches with gamma rays. The resulting signals are different from known point sources or from diffuse emission and could possibly be detected with the Fermi-LAT. Both WIMP annihilation/decay spectra and cosmic ray induced emission are determined by universal parameters, which make a combined statistical likelihood analysis feasible. We present initial results of this analysis leading to limits on the DM annihilation cross section or decay time and on the hadron injection efficiency.
    10/2011;
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    ABSTRACT: We present the discovery of a new gamma-ray binary system from the search for periodic modulation in the 100 MeV to 200 GeV Fermi LAT light curves of all sources in the first Fermi-LAT catalog. 1FGL J1018.6-5856 was found to have a 16.6 day modulation in its gamma-ray light curve that is accompanied by spectral variability. We also identify counterparts in the X-ray, radio and optical wavebands using data from the Swift XRT, ATCA, and telescopes at SAAO and LCO. The X-ray and radio counterparts are highly variable - the X-ray flux is modulated on the orbital period with maximum X-ray flux coinciding with the phase of maximum gamma-ray flux. The optical counterpart has a spectral type of approximately O6V((f)) and shows little variability in a series of Swift UVOT observations. The overall properties of 1FGL J1018.6-5856 indicate that it is a member of the rare gamma-ray binary class of objects, and that it shares several properties with LS 5039. However, there are also several differences from LS 5039, including the relative phasing of the gamma-ray flux and spectral modulation and the shape of the X-ray light curve. The similarities and differences are expected to allow us to more fully develop our understanding of the astrophysics involved in these enigmatic objects.
    08/2011; 12.
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    Luigi Tibaldo, Isabelle A. Grenier, Tsunefumi Mizuno, for the Fermi LAT collaboration
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    ABSTRACT: Gamma-ray emission produced by interactions between cosmic rays (CRs) and interstellar gas traces the product of their densities throughout the Milky Way. The outer Galaxy is a privileged target of investigation to separate interstellar structures seen along the line of sight. Recent observations by the Fermi Large Area Telescope (LAT) shed light on open questions of the EGRET era about the distribution of CR densities and the census of the interstellar medium. The gradient of gamma-ray emissivities measured in the outer Galaxy is significantly flatter than predictions from widely used CR propagation models given the rapid decline of putative CR sources beyond the solar circle. Large propagation volumes, with halo heights up to 20 kpc, or a flat CR source distribution are required to match the data. Other viable possibilities include non-uniform CR diffusion properties or more gas than accounted for by the radio/mm-wave data. Gamma-ray data constrain the evolution of the Xco=N(H2)/W(CO) ratio within a few kpc from the Sun. There is a significant increase by a factor 2 from nearby clouds in the Gould Belt to the local spur. No further significant variations are measured from the local spur to the Perseus spiral arm. At the level of statistical accuracy provided by the LAT data, the most important source of uncertainty, often overlooked so far, is due to the optical depth correction applied to derive the column densities of H I. Reliable determinations of the amount of atomic gas in the plane are key to better probe the properties of CRs in the Galaxy.
    12/2010;
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    Luigi Tibaldo, for the Fermi LAT collaboration
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    ABSTRACT: For more than one year the Fermi Large Area Telescope has been surveying the gamma-ray sky from 20 MeV to more than 300 GeV with unprecedented statistics and angular resolution. One of the key science targets of the Fermi mission is diffuse gamma-ray emission. Galactic interstellar gamma-ray emission is produced by interactions of high-energy cosmic rays with the interstellar gas and radiation field. We review the most important results on the subject obtained so far: the non-confirmation of the excess of diffuse GeV emission seen by EGRET, the measurement of the gamma-ray emissivity spectrum of local interstellar gas, the study of the gradient of cosmic-ray densities and of the X(CO)=N(H2)/W(CO) ratio in the outer Galaxy. We also catch a glimpse at diffuse gamma-ray emission in the Large Magellanic Cloud. These results allow the improvement of large-scale models of Galactic diffuse gamma-ray emission and new measurements of the extragalactic gamma-ray background. Comment: Contribution to the Workshop SciNeGHe 2009/Gamma-ray Physics in the LHC era (Assisi - Italy, Oct. 7-9 2009); 10 pages, 6 figures
    02/2010;
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    Gudlaugur Johannesson, Igor Moskalenko, Seth Digel, for the Fermi LAT Collaboration
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    ABSTRACT: The diffuse high-energy gamma-ray emission of the Milky Way arises from interactions of cosmic-rays (CRs) with interstellar gas and radiation field in the Galaxy. The neutral hydrogen (H I) gas component is by far the most massive and broadly distributed component of the interstellar medium. Using the 21-cm emission line from the hyperfine structure transition of atomic hydrogen it is possible to determine the column density of H I if the spin temperature (Ts) of the emitting gas is known. Studies of diffuse gamma-ray emission have generally relied on the assumption of a fixed, constant spin temperature for all H I in the Milky Way. Unfortunately, observations of H I in absorption against bright background sources has shown it to vary greatly with location in the Milky Way. We will discuss methods for better handling of spin temperatures for Galactic diffuse emission modeling using the Fermi-LAT data and direct observation of the spin temperature using H I absorption. Comment: "2009 Fermi Symposium", "eConf Proceedings C091122"
    02/2010;
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    ABSTRACT: PKS 2005-489 is a very extreme high-frequency peaked BL Lac object which shows at its detection in VHE gamma-rays the softest TeV spectrum ever measured. Multi-year studies show large flux and spectral variation in the X-ray regime while only weak variations have been detected in the VHE band. In 2009, PKS 2005-489 exhibited a very strong flare in the synchrotron regime which was covered by our broadband multi-wavelength campaign. With the simultaneous observations by HESS (TeV), Fermi/LAT (GeV), RXTE (keV), Swift (keV, UV, optical) and ATOM (optical) the synchrotron and inverse Compton emission peaks were well covered. With all these observations the upper end of both spectral components in the X-ray and TeV regime was measured in several epochs while a variation by a factor of 50 was detected in the X-rays. We will present the results of the new multi-wavelength campaign during the synchrotron flare and discuss the broadband characteristics in respect to earlier observations of lower states.
    01/2010; 42:709.
  • S Ciprini, E Antolini, G Tosti, Fermi LAT Collaboration
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    ABSTRACT: Highlights on the first Fermi LAT multi-frequency campaign dedicated on the source BL Lacer-tae are presented in this poster. The coordinated campaign held from the end of August to the end of October 2008 saw the participation of radio (Metsahovi) telescopes, near-IR and optical telescopes, and the participation of RXTE and Swift telescopes. For the first time the broad band radio-to-gamma-ray behavior of the eponymous blazar is revealed during a quiescent state with unprecedented energy coverage thanks to this campaign. This allows to put new insights on the location of the emitting regions with respect to the Broad Line Region, on the variability behavior during the long quiescence periods between major outbursts, and on the sites were the high-energy radiation (X-ray and gamma-ray photons) is produced.
    38th COSPAR Scientific Assembly; 01/2010
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    ABSTRACT: The Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope discovered a high-energy gamma-ray outburst (characterized by a rapid rise and about five days duration) from a source identified with the blazar PKS 1502+106 (OR 103, S3 1502+10, z=1.839) starting on August 05, 2008 (MJD 54683.9). The outburst was followed by bright and variable gamma-ray flux over the next few months. A few selected highlights on the Fermi LAT multi-frequency campaign on PKS 1502+106 are introduced as an example of the optimal capabilities of Fermi LAT in monitoring cosmic high-energy flares, variability and being an essential companion of multi-waveband studies.
    Memorie della Societa Astronomica Italiana Supplementi. 01/2010; 14:258.
  • S Ciprini, D Gasparrini, Fermi LAT Collaboration
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    ABSTRACT: The Flare Advocate/Gamma-ray Sky Watcher (FA-GSW) activity is part of the Fermi LAT Science Operations aiming to supply a prompt outlook service to the quick-look Automatic Science Processing (ASP) products, and in general to the LAT sky, day by day. The FA-GSW points out potentially interesting seeds for LAT science and the different LAT science groups, while communicating basic and relevant news to the external astrophysical community, in order to increase the rate of multi-frequency observations and follow-ups that could maximize science return. During the first two years of the Fermi mission, FA-GSWs discovered, for example, many gamma-ray flares and longer-term brightening from variable blazars, unidentified transients near the Galactic plane, confirmed the quiet sun gamma-ray emission, compiled many Astronomical Telegrams, pointed out possible new gamma-ray sources, and provided starting seeds for more than a dozen LAT papers on single sources and follow-up multifrequency campaigns. Some highlights of these two year of crucial service-activity are summarized in this poster.
    38th COSPAR Scientific Assembly; 01/2010
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    Akira Okumura, Tune Kamae, for the Fermi LAT Collaboration
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    ABSTRACT: We report on a preliminary analysis of the diffuse gamma-ray observations of local giant molecular clouds Orion A and B with the Large Area Telescope onboard the Fermi Gamma-ray Space Telescope. The gamma-ray emission of the clouds is well explained by hadronic and electromagnetic interactions between cosmic rays and nuclei in the clouds. In consequence, we obtain the total masses of the Orion A and B clouds to be (80.6 +/- 7.5 +/- 4.8) x 10^3 Msun and (39.5 +/- 5.2 +/- 2.6) x 10^3 Msun, respectively, for the distance to the clouds of 400 pc and the Galactic CR spectrum predicted by GALPROP on the local observations of CRs. The structure of molecular clouds have been extensively studied by radio telescopes, especially using the line intensity of CO molecules (WCO) and a constant conversion factor from Wco to N (H_2) (= Xco). However, this factor is found to be significantly different for Orion A and B: 1.76 +/- 0.04 +/- 0.02 and 1.27 +/- 0.06 +/- 0.01, respectively. Comment: 2009 Fermi Symposium, eConf Proceedings C091122
    12/2009;

Publication Stats

25 Citations
32.08 Total Impact Points

Institutions

  • 2012
    • Paris Diderot University
      Lutetia Parisorum, Île-de-France, France
    • Stanford University
      • Department of Physics
      Stanford, CA, United States
  • 2010
    • Heidelberg University
      Tiffin, Ohio, United States