Keywords

The attenuation produced by extragalactic background light (EBL) in γ-ray spectra of blazars has been used to constrain the Hubble constant (H₀) and matter density (Ω_m) of the universe. We propose to estimate H₀ and Ω_m using the well-measured >10 GeV extragalactic γ-ray background (EGB). This suggestion is based on the fact that the >10 GeV EGB is totally explained by the emissions from blazars, and an EBL-absorption cutoff occurs at ∼50 GeV in the EGB spectrum. We fit the >10 GeV EGB data with modeled EGB spectrum. This results in ${H}_{0}={64.9}_{-4.3}^{+4.6}\,\mathrm{km}\,{{\rm{s}}}^{-1}\,{\mathrm{Mpc}}^{-1}$ and ${{\rm{\Omega }}}_{{\rm{m}}}={0.31}_{-0.14}^{+0.13}$. Note that the uncertainties may be underestimated due to the limit of our realization for EBL model. H₀ and Ω_m are degenerate in our method. Independent determination of Ω_m by other methods would improve the constraint on H₀.

The Andromeda galaxy is the closest spiral galaxy to us and has been the subject of numerous studies. It harbors a massive dark matter halo, which may span up to ∼600 kpc across and comprises ∼90% of the galaxy's total mass. This halo size translates into a large diameter of 42° on the sky, for an M31–Milky Way (MW) distance of 785 kpc, but its presumably low surface brightness makes it challenging to detect with γ-ray telescopes. Using 7.6 yr of Fermi Large Area Telescope (Fermi–LAT) observations, we make a detailed study of the γ-ray emission between 1–100 GeV toward M31's outer halo, with a total field radius of 60° centered at M31, and perform an in-depth analysis of the systematic uncertainties related to the observations. We use the cosmic-ray propagation code GALPROP to construct specialized interstellar emission models to characterize the foreground γ-ray emission from the MW, including a self-consistent determination of the isotropic component. We find evidence for an extended excess that appears to be distinct from the conventional MW foreground, having a total radial extension upward of ∼120–200 kpc from the center of M31. We discuss plausible interpretations of the excess emission, but emphasize that uncertainties in the MW foreground—and in particular, modeling of the H i-related components—have not been fully explored and may impact the results.

The discovery of a binary neutron star merger (NSM) through both its gravitational wave and electromagnetic emission has revealed these events to be key sites of r-process nucleosynthesis. Here, we evaluate the prospects of finding the remnants of Galactic NSMs by detecting the gamma-ray decay lines from their radioactive r-process ejecta. We find that ¹²⁶Sn, which has several lines in the energy range 415–695 keV and resides close to the second r-process peak, is the most promising isotope, because of its half-life t_1/2 = 2.30(14) × 10⁵ yr being comparable to the ages of recent NSMs. Using a Monte Carlo procedure, we predict that multiple remnants are detectable as individual sources by next-generation γ-ray telescopes which achieve sub-MeV line sensitivities of ∼10⁻⁸–10⁻⁶ γ cm⁻² s⁻¹. However, given the unknown locations of the remnants, the most promising search strategy is a systematic survey of the Galactic plane and bulge extending to high Galactic latitudes. Individual known supernova remnants which may be misclassified NSM remnants could also be targeted, especially those located outside the Galactic plane. Detection of a moderate sample of Galactic NSM remnants would provide important clues to unresolved issues such as the production of actinides in NSMs, properties of merging NS binaries, and even help distinguish them from rare supernovae as current Galactic r-process sources. We also investigate the diffuse flux from longer-lived nuclei (e.g., ¹⁸²Hf) that could in principle trace the Galactic spatial distribution of NSMs over longer timescales, but find that the detection of the diffuse flux appears challenging even with next-generation telescopes.

The High Altitude Water Cherenkov (HAWC) telescope recently observed extended emission around the Geminga and PSR B0656+14 pulsar wind nebulae (PWNe). These observations have been used to estimate cosmic-ray (CR) diffusion coefficients near the PWNe that appear to be more than two orders of magnitude smaller than the diffusion coefficients typically derived for the interstellar medium from measured abundances of secondary species in CRs. Two-zone diffusion models have been proposed as a solution to this discrepancy, where the slower diffusion zone (SDZ) is confined to a small region around the PWN. Such models are shown to successfully reproduce the HAWC observations of the Geminga PWN while retaining consistency with other CR data. It is found that the size of the SDZ influences the predicted positron flux and the spectral shape of the extended γ-ray emission at lower energies that can be observed with the Fermi Large Area Telescope. If the two observed PWNe are not unique, then it is likely that there are similar pockets of slow diffusion around many CR sources elsewhere in the Milky Way. The consequences of such a picture for Galactic CR propagation is explored.

Recent radio surveys have discovered a large number of low-luminosity core-dominated radio galaxies that are much more abundant than those at higher luminosities. These objects will be too faint in γ-rays to be detected individually by Fermi. Nevertheless, they may contribute significantly to the unresolved extragalactic γ-ray background. We consider here the possible contribution of these core-dominated radio galaxies to the diffuse extragalactic γ-ray background. Using published data available for all 45 of the radio galaxies listed as detected counterparts in the Fermi FL8Y source list update to the 3FGL catalog, we have searched for radio maps that can resolve the core flux from the total source flux. Using high-resolution radio maps we were able to obtain core fluxes for virtually every source. We then derived a relation between core radio flux and γ-ray flux that we extrapolated to sources with low radio luminosities that are known to be highly core-dominated. We then employed a very recent determination of the luminosity function for core-dominated radio galaxies in order to obtain the contribution of all possible γ-ray-emitting radio galaxies to the unresolved extragalactic γ-ray background. We find this contribution to be possibly non-negligible, 4%–18% of the unresolved γ-ray background observed using the Fermi-LAT telescope.

The Fermi Gamma-Ray Space Telescope has provided evidence for diffuse gamma-ray emission in the central parts of the Milky Way and the Andromeda galaxy. This excess has been interpreted either as dark-matter annihilation emission or as emission from thousands of millisecond pulsars (MSPs). We have recently shown that old massive globular clusters (GCs) may move toward the center of the Galaxy by dynamical friction and carry within them enough MSPs to account for the observed gamma-ray excess. In this Letter we revisit the MSP scenario for the Andromeda galaxy by modeling the formation and disruption of its GC system. We find that our model predicts gamma-ray emission ∼2–3 times larger than for the Milky Way, but still nearly an order of magnitude smaller than the observed Fermi excess in the Andromeda. Our MSP model can reproduce the observed excess only by assuming ∼8 times a larger number of old clusters than inferred from galaxy scaling relations. To explain the observations we require either that Andromeda deviates significantly from the scaling relations, or that a large part of its high-energy emission comes from additional sources.

Various studies have implied the existence of a gaseous halo around the Galaxy extending out to ∼100 kpc. Galactic cosmic rays (CRs) that propagate to the halo, either by diffusion or by convection with the possibly existing large-scale Galactic wind, can interact with the gas therein and produce gamma-rays via proton–proton collision. We calculate the CR distribution in the halo and the gamma-ray flux, and explore the dependence of the result on model parameters such as diffusion coefficient, CR luminosity, and CR spectral index. We find that the current measurement of isotropic gamma-ray background (IGRB) at ≲TeV with the Fermi Large Area Telescope already approaches a level that can provide interesting constraints on the properties of Galactic CR (e.g., with CR luminosity L_CR ≤ 10⁴¹ erg s⁻¹). We also discuss the possibilities of the Fermi bubble and IceCube neutrinos originating from the proton–proton collision between CRs and gas in the halo, as well as the implication of our results for the baryon budget of the hot circumgalactic medium of our Galaxy. Given that the isotropic gamma-ray background is likely to be dominated by unresolved extragalactic sources, future telescopes may extract more individual sources from the IGRB, and hence put even more stringent restrictions on the relevant quantities (such as Galactic CR luminosity and baryon budget in the halo) in the presence of a turbulent halo that we consider.

An intergalactic magnetic field (IGMF) stronger than 3 × 10⁻¹³ G would explain the lack of a bright, extended degree-scale, GeV-energy inverse Compton component in the gamma-ray spectra of TeV blazars. A robustly predicted consequence of the presence of such a field is the existence of degree-scale GeV-energy gamma-ray halos (gamma-ray bow ties) about TeV-bright active galactic nuclei, corresponding to more than half of all radio galaxies. However, the emitting regions of these halos are confined to and aligned with the direction of the relativistic jets associated with gamma-ray sources. Based on the orientation of radio jets, we align and stack corresponding degree-scale gamma-ray images of isolated Fanaroff–Riley class I and II objects and exclude the existence of these halos at overwhelming confidence, limiting the intergalactic field strength to <10⁻¹⁵ G for large-scale fields and progressively larger in the diffusive regime when the correlation length of the field becomes small in comparison to 1 Mpc. When combined with prior limits on the strength of the IGMF, this excludes a purely magnetic explanation for the absence of halos. Thus, it requires the existence of novel physical processes that preempt the creation of halos, e.g., the presence of beam-plasma instabilities in the intergalactic medium or a drastic cutoff of the very high-energy spectrum of these sources.

The existence of diffuse Galactic neutrino production is expected from cosmic-ray interactions with Galactic gas and radiation fields. Thus, neutrinos are a unique messenger offering the opportunity to test the products of Galactic cosmic-ray interactions up to energies of hundreds of TeV. Here we present a search for this production using ten years of Astronomy with a Neutrino Telescope and Abyss environmental RESearch (ANTARES) track and shower data, as well as seven years of IceCube track data. The data are combined into a joint likelihood test for neutrino emission according to the KRA${}_{\gamma }$ model assuming a 5 PeV per nucleon Galactic cosmic-ray cutoff. No significant excess is found. As a consequence, the limits presented in this Letter start constraining the model parameter space for Galactic cosmic-ray production and transport.

Various observations are revealing the widespread occurrence of fast and powerful winds in active galactic nuclei (AGNs) that are distinct from relativistic jets, likely launched from accretion disks and interacting strongly with the gas of their host galaxies. During the interaction, strong shocks are expected to form that can accelerate nonthermal particles to high energies. Such winds have been suggested to be responsible for a large fraction of the observed extragalactic gamma-ray background (EGB) and the diffuse neutrino background, via the decay of neutral and charged pions generated in inelastic pp collisions between protons accelerated by the forward shock and the ambient gas. However, previous studies did not properly account for processes such as adiabatic losses that may reduce the gamma-ray and neutrino fluxes significantly. We evaluate the production of gamma rays and neutrinos by AGN-driven winds in detail by modeling their hydrodynamic and thermal evolution, including the effects of their two-temperature structure. We find that they can only account for less than ∼30% of the EGB flux, as otherwise the model would violate the independent upper limit derived from the diffuse isotropic gamma-ray background. If the neutrino spectral index is steep with Γ ≳ 2.2, a severe tension with the isotropic gamma-ray background would arise as long as the winds contribute more than 20% of the IceCube neutrino flux in the 10–100 TeV range. At energies ≳ 100 TeV, we find that the IceCube neutrino flux may still be accountable by AGN-driven winds if the spectral index is as small as Γ ∼ 2.0–2.1.