Diffuse Galactic gamma-ray flux at very high energy

Abstract

The observation of the diffuse Galactic gamma ray flux is the most powerful tool to study cosmic rays in different regions of the Galaxy, because the energy and angular distributions of the photons encode information about the density and spectral shape of relativistic particles in the entire MilkyWay. An open problem of fundamental importance is whether cosmic rays in distant regions of the Milky Way have the same spectral shape observed at the Earth or not. If the spectral shape of protons and nuclei is equal in all the Galaxy, the dominant, hadronic component of the diffuse gamma ray flux must have an angular distribution that, after correcting for absorption effects, is energy independent. To study experimentally the validity of this factorization of the energy and angular dependence of the diffuse flux it is necessary to compare observations in a very broad energy range. The extension of the observations to energies Eγ 0.1-10 PeV is of great interest, because it allows the study of the cosmic ray spectra around the feature known as the "knee". The absorption probability for photons in this energy range is not negligible, and distorts the energy and angular distributions of the diffuse flux, therefore a precise calculation of the absorption effects is necessary for the interpretation of the data. In this work we present predictions of the diffuse gamma ray flux at very high energy, constructed under different hypothesis for the space dependence of the cosmic ray energy spectra, and discuss the potential of the observations for present and future detectors.PACS numbers: 98.35Gi,95.85Pw,95.85Ry * Electronic address: paolo.lipari@roma1.infn.it † Electronic address: vernetto@to.infn.it

Search for PeV Gamma-Ray Emission from the Southern Hemisphere with 5 Yr of Data from the IceCube Observatory

“…Dotted lines show the E −3 spectrum, used for obtaining IceCube upper limits, over the energy range containing 5%-95% events in the final sample. Also shown for two models are the absorbed flux predictions for the IC-86 FOV calculated by Lipari & Vernetto (2018) by special request. The two models assume space-independent and space-dependent cosmic-ray spectra throughout the Galaxy, respectively.…”

“…Figure 12 compares the scaled flux limits from this analysis with the existing IC-40 (Aartsen et al 2013a) and CASA-MIA (Borione et al 1998) limits, as well as the measured flux by ARGO-YBJ (Bartoli et al 2015). In addition, the gammaray flux from the diffuse Galactic plane in the analysis FOV is shown for two model predictions from Lipari & Vernetto (2018). The first assumes that cosmic-ray spectra have a spaceindependent spectral shape throughout the Galaxy.…”

“…The projected flux at 2 PeV of these sources, assuming there is no cutoff in their energy spectra, is shown in Figure 10. Attenuation effects were calculated for the Galactic coordinates and distance of each source using model results provided by Lipari & Vernetto (2018). Source distances were estimated from associated X-ray or radio observations when possible (Wakely & Horan 2008).…”

“…In purple are the extrapolated fluxes up to 2 PeV of H.E.S.S. sources in the analysis FOV, under the scenario of no break in the best-fit energy spectrum, with an approximate correction for absorption using model predictions from Lipari & Vernetto (2018). Error bars indicate the statistical uncertainty, while the systematic uncertainty is represented by the shaded areas.…”

A combined astrophysical and dark matter interpretation of the IceCube HESE and throughgoing muon events

“…In each panel, we show the current 90% CL upper limits on the integrated photon flux over the whole energy range from 10 GeV to 10 8 GeV, obtained from a combination of various experiments, such as CASA-MIA [129], MILARGO [130], Fermi-LAT [131], GRAPES [132], KASCADE [133,134], ARGO [135] and HAWC [136]. We also adopted a combined best fit [137] in the 1-100 TeV region for the upper bound on photon flux based on the diffuse electron flux limit from AMS-02 [138], DAMPE [139], Fermi-LAT [140], MAGIC [141], HESS [142], and VERITAS [143], since air showers from electrons, positrons and photons behave the same. The light brown region corresponds to the 2σ region for the IceCube HESE-only bestfit for the one-component flux.…”