Ackermann et al.

We report on the first Fermi Large Area Telescope (LAT) measurements of the so-called "extragalactic" diffuse γ-ray emission (EGB). This component of the diffuse γ-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 γ-ray emission (DGE), the detected LAT sources and the solar γ-ray emission. We find the spectrum of the EGB is consistent with a power law with differential spectral index γ = 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. PACS numbers: 95.30.Cq,95.55.Ka,95.85.Pw,96.50.sb,98.70.Sa Introduction: The high-energy diffuse γ-ray emission is dominated by γ-rays produced by cosmic rays (CR) interacting with the Galactic interstellar gas and radiation fields, the so-called diffuse Galactic emission (DGE). A much fainter component, commonly designated as "extragalactic γ-ray background" (EGB), was first detected against the bright DGE foreground by the SAS-2 satellite [1] and later confirmed by analysis of the EGRET data [2]. The EGB by definition has an isotropic sky distribution and is considered by many to be the superposition of contributions from unresolved extragalactic sources including active galactic nuclei, starburst galaxies and γ-ray bursts ([3] and references therein) and truly-diffuse emission processes. These diffuse processes include the possible signatures of large-scale structure formation [4], emission produced by the interactions of 3 ultra-high-energy CRs with relic photons [5], the annihilation or decay of dark matter, and many other processes (e.g., [3] and references therein). However, the diffuse γ-ray emission from inverse Compton (IC) scattering by an extended Galactic halo of CR electrons could also be attributed to such a component if the size of the halo is large enough (i.e., ∼ 25 kpc) [6]. In addition, γ-ray emission from CRs interacting in populations of small solar system bodies [7] and the all-sky contribution of IC scattering of solar photons with local CRs can provide contributions [8][9][10]. Hence, an extragalactic origin for such a component is not clear, even though we will use the abbreviation 'EGB' throughout this paper.In this paper, we present analysis and first results for the EGB derived from the Fermi Large Area Telescope (LAT) [11] data. Our analysis uses data from the initial 10 months of the science phase of the mission. Essential to this study is an event-level data selection with a higher level of background rejection than the standard LAT data selections, and improvements to the instrument simulation. These have been made following extensive on-orbit studies of the LAT performance and of charged particle backgrounds. Together, these improvements over the pre-launch modelling and bac...

The grand challenges of contemporary fundamental physics—dark matter, dark energy, vacuum energy, inflation and early universe cosmology, singularities and the hierarchy problem—all involve gravity as a key component. And of all gravitational phenomena, black holes stand out in their elegant simplicity, while harbouring some of the most remarkable predictions of General Relativity: event horizons, singularities and ergoregions. The hitherto invisible landscape of the gravitational Universe is being unveiled before our eyes: the historical direct detection of gravitational waves by the LIGO-Virgo collaboration marks the dawn of a new era of scientific exploration. Gravitational-wave astronomy will allow us to test models of black hole formation, growth and evolution, as well as models of gravitational-wave generation and propagation. It will provide evidence for event horizons and ergoregions, test the theory of General Relativity itself, and may reveal the existence of new fundamental fields. The synthesis of these results has the potential to radically reshape our understanding of the cosmos and of the laws of Nature. The purpose of this work is to present a concise, yet comprehensive overview of the state of the art in the relevant fields of research, summarize important open problems, and lay out a roadmap for future progress. This write-up is an initiative taken within the framework of the European Action on ‘Black holes, Gravitational waves and Fundamental Physics’.

Designed as a high-sensitivity gamma-ray observatory, the Fermi Large Area Telescope is also an electron detector with a large acceptance exceeding 2 m;{2} sr at 300 GeV. Building on the gamma-ray analysis, we have developed an efficient electron detection strategy which provides sufficient background rejection for measurement of the steeply falling electron spectrum up to 1 TeV. Our high precision data show that the electron spectrum falls with energy as E-3.0 and does not exhibit prominent spectral features. Interpretations in terms of a conventional diffusive model as well as a potential local extra component are briefly discussed.

We model the transport of cosmic ray nuclei in the Galaxy by means of a new numerical code. Differently from previous numerical models we account for a generic spatial distribution of the diffusion coefficient. We found that in the case of radially uniform diffusion, the main secondary/primary ratios (B/C, N/O and sub-Fe/Fe) and the modulated antiproton spectrum match consistently the available observations. Convection and re-acceleration do not seem to be required in the energy range we consider: 1 ≤ E ≤ 10 3 GeV/nucleon. We generalize these results accounting for radial dependence of the diffusion coefficient, which is assumed to trace that of the cosmic ray sources. While this does not affect the prediction of secondary/primary ratios, the simulated longitude profile of the diffuse γ-ray emission is significantly different from the uniform case and may agree with EGRET measurements without invoking ad hoc assumptions on the galactic gas density distribution. ‡ This unit of measure corresponds to the energy E competing on average to each nucleon within a nucleus with A nucleons and energy A × E.