D. Bastieri

Dark Matter Constraints from Observations of 25 Milky Way Satellite Galaxies with the Fermi Large Area Telescope

The dwarf spheroidal satellite galaxies of the Milky Way are some of the most dark-matter-dominated objects known. Due to their proximity, high dark matter content, and lack of astrophysical backgrounds, dwarf spheroidal galaxies are widely considered to be among the most promising targets for the indirect detection of dark matter via γ rays. Here we report on γ-ray observations of 25 Milky Way dwarf spheroidal satellite galaxies based on 4 years of Fermi Large Area Telescope (LAT) data. None of the dwarf galaxies are significantly detected in γ rays, and we present γ-ray flux upper limits between 500 MeV and 500 GeV. We determine the dark matter content of 18 dwarf spheroidal galaxies from stellar kinematic data and combine LAT observations of 15 dwarf galaxies to constrain the dark matter annihilation cross section. We set some of the tightest constraints to date on the annihilation of dark matter particles with masses between 2 GeV and 10 TeV into prototypical standard model channels. We find these results to be robust against systematic uncertainties in the LAT instrument performance, diffuse γ-ray background modeling, and assumed dark matter density profile.

Fermi Establishes Classical Novae as a Distinct Class of Gamma-ray Sources

Gamma-ray novas may be garden variety When astronomers detected gamma rays from the nova V407 Cyg, an explosive mass transfer from a red giant onto a white dwarf, they found it surprising enough. They blamed the rays on strong stellar winds enabling particle acceleration. Now, the Fermi-LAT Collaboration has observed gamma rays from three more novas, all lacking the strong winds. Although the three sources vary slightly in nature, none is particularly unusual. If all novas emit gamma rays, then astronomers would expect to see the same number of novas that they did in fact see within a 5-kpc distance over 5 years. Science, this issue p. 554 Three classical novae exhibit unexpected high-energy particle acceleration and may represent the norm for that object class. A classical nova results from runaway thermonuclear explosions on the surface of a white dwarf that accretes matter from a low-mass main-sequence stellar companion. In 2012 and 2013, three novae were detected in γ rays and stood in contrast to the first γ-ray–detected nova V407 Cygni 2010, which belongs to a rare class of symbiotic binary systems. Despite likely differences in the compositions and masses of their white dwarf progenitors, the three classical novae are similarly characterized as soft-spectrum transient γ-ray sources detected over 2- to 3-week durations. The γ-ray detections point to unexpected high-energy particle acceleration processes linked to the mass ejection from thermonuclear explosions in an unanticipated class of Galactic γ-ray sources.

FERMI LARGE AREA TELESCOPE STUDY OF COSMIC RAYS AND THE INTERSTELLAR MEDIUM IN NEARBY MOLECULAR CLOUDS

We report an analysis of the interstellar γ -ray emission from the Chamaeleon, R Coronae Australis (R CrA), and Cepheus and Polaris flare regions with the Fermi Large Area Telescope. They are among the nearest molecular cloud complexes, within ∼300 pc from the solar system. The γ -ray emission produced by interactions of cosmic rays (CRs) and interstellar gas in those molecular clouds is useful to study the CR densities and distributions of molecular gas close to the solar system. The obtained γ -ray emissivities above 250 MeV are (5.9 ± 0.1stat +0.9 −1.0sys) × 10−27 photons s−1 sr−1 H-atom−1, (10.2 ± 0.4stat +1.2 −1.7sys) × 10−27 photons s−1 sr−1 H-atom−1, and (9.1 ± 0.3stat +1.5 −0.6sys) × 10−27 photons s−1 sr−1 H-atom−1 for the Chamaeleon, R CrA, and Cepheus and Polaris flare regions, respectively. Whereas the energy dependences of the emissivities agree well with that predicted from direct CR observations at the Earth, the measured emissivities from 250 MeV to 10 GeV indicate a variation of the CR density by ∼20% in the neighborhood of the solar system, even if we consider systematic uncertainties. The molecular mass calibrating ratio, XCO = N(H2)/WCO, is found to be (0.96 ± 0.06stat +0.15 −0.12sys) × 1020 H2-molecule cm−2 (K km s−1)−1, (0.99 ± 0.08stat +0.18 −0.10sys) × 1020 H2-molecule cm−2 (K km s−1)−1, and (0.63 ± 0.02stat +0.09 −0.07sys) × 1020 H2-molecule cm−2 (K km s−1)−1 for the Chamaeleon, R CrA, and Cepheus and Polaris flare regions, respectively, suggesting a variation of XCO in the vicinity of the solar system. From the obtained values of XCO, the masses of molecular gas traced by WCO in the Chamaeleon, R CrA, and Cepheus and Polaris flare regions are estimated to be ∼5 × 103M , ∼103M , and ∼3.3 × 104M , respectively. A comparable amount of gas not traced well by standard Hi and CO surveys is found in the regions investigated.We report an analysis of the interstellar γ-ray emission from the Chamaeleon, R Coronae Australis (R CrA), and Cepheus and Polaris flare regions with the Fermi Large Area Telescope. They are among the nearest molecular cloud complexes, within ∼ 300 pc from the solar system. The γ-ray emission produced by interactions of cosmicrays (CRs) and interstellar gas in those molecular clouds is useful to study the CR densities and distributions of molecular gas close to the solar system. The obtained γ-ray emissivities above 250 MeV are (5.9 ± 0.1stat +0.9 −1.0sys) × 10 −27 photons s−1 sr−1 H-atom−1, (10.2 ± 0.4stat +1.2 −1.7sys) × 10 −27 photons s−1 sr−1 H-atom−1, and (9.1 ± 0.3stat +1.5 −0.6sys) × 10 −27 photons s−1 sr−1 H-atom−1 for the Chamaeleon, R CrA, and Cepheus and Polaris flare regions, respectively. Whereas the energy dependences of the emissivities agree well with that predicted from direct CR observations at the Earth, the measured emissivities from 250 MeV to 10 GeV indicate a variation of the CR density by ∼ 20 % in the neighborhood of the solar system, even if we consider systematic uncertainties. The molecular mass calibrating ratio, XCO = N(H2)/WCO, is found to be (0.96 ± 0.06stat +0.15 −0.12sys) ×10 20 H2-molecule cm −2 (K km s−1)−1, (0.99 ± 0.08stat +0.18 −0.10sys) ×10 20 H2-molecule cm −2 (K km s−1)−1, and (0.63 ± 0.02stat +0.09 −0.07sys) ×10 20 H2-molecule cm −2 (K km s−1)−1 for the Chamaeleon, R CrA, and Cepheus and Polaris flare regions, respectively, suggesting a variation of XCO in the vicinity of the solar system. From the obtained values of XCO, the masses of molecular gas traced by WCO in the Chamaeleon, R CrA, and Cepheus and Polaris flare regions are estimated to be ∼ 5×103 M⊙, ∼ 10 3 M⊙, and ∼ 3.3×10 4 M⊙, respectively. A comparable amount of gas not traced well by standard H I and CO surveys is found in the regions investigated. University of California at Santa Cruz, Santa Cruz, CA 95064, USA Institut für Astround Teilchenphysik and Institut für Theoretische Physik, Leopold-Franzens-Universität Innsbruck, A-6020 Innsbruck, Austria Department of Physics, University of Washington, Seattle, WA 98195-1560, USA NYCB Real-Time Computing Inc., Lattingtown, NY 11560-1025, USA Department of Chemistry and Physics, Purdue University Calumet, Hammond, IN 46323-2094, USA Institut für Theoretische Physik and Astrophysik, Universität Würzburg, D-97074 Würzburg, Germany Institució Catalana de Recerca i Estudis Avançats (ICREA), Barcelona, Spain Consorzio Interuniversitario per la Fisica Spaziale (CIFS), I-10133 Torino, Italy INTEGRAL Science Data Centre, CH-1290 Versoix, Switzerland NASA Postdoctoral Program Fellow, USA Dipartimento di Fisica, Università di Roma “Tor Vergata”, I-00133 Roma, Italy Institut Universitaire de France, France