A surprisingly large flux of extraterrestrial high-energy neutrinos was discovered by the IceCube experiment. While the flux of muon neutrinos with energies E > 100 TeV is consistent with the ...extragalactic gamma-ray background determined by Fermi-LAT, the softer component of the cascade neutrino flux at E < 100 TeV is larger than expected. Moreover, a gamma-ray excess at high galactic latitudes at energies E > 300 GeV was found in the data of Fermi-LAT. The gamma-ray excess at TeV energies and the neutrino excess at E < 100 TeV may have a common galactic origin. In this work, we study the possibility that both excesses are caused by interactions of cosmic rays (CRs) with energies up to PeV in the wall of the local bubble. Source of these CRs may be a recent nearby source like Vela. We show that such a scenario can explain the observed CR flux around the knee, while CR interactions in the bubble wall can generate a substantial fraction of the observed astrophysical high-energy neutrino flux below ∼ few × 100 TeV.
A signal of high-energy extraterrestrial neutrinos from unknown source(s) was recently discovered by the IceCube experiment. Neutrinos are always produced together with γ-rays, but the γ-ray flux ...from extragalactic sources is suppressed due to attenuation in the intergalactic medium. We report the discovery of a γ-ray excess at high galactic latitudes starting at energies 300 GeV in the data of the Fermi telescope. We show that the multi-TeV γ-ray diffuse emission has spectral characteristics at both low and high galactic latitudes compatible with those of the IceCube high neutrino signal in the same sky regions. This suggests that these γ-rays are the counterpart of the IceCube neutrino signal, implying that a sizable part of the IceCube neutrino flux originates from the Milky Way. We argue that the diffuse neutrino and γ-ray signal at high galactic latitudes originates either from previously unknown nearby cosmic ray “PeVatron” source(s), an extended galactic cosmic ray halo or from decays of heavy dark matter particles.
We aim to explain in a unified way the experimental data on ultrahigh-energy cosmic rays (UHECRs) and neutrinos, using a single source class and obeying limits on the extragalactic diffuse gamma-ray ...background. If UHECRs only interact hadronically with gas around their sources, the resulting diffuse cosmic-ray (CR) flux can be matched well to the observed one, providing at the same time large neutrino fluxes. Since the required fraction of heavy nuclei is, however, rather large, the maxima of air showers in the Earth’s atmosphere induced by UHECRs with energies E≳3×1018 eV would be too high. Therefore, additional photohadronic interactions of UHECRs close to the accelerator have to be present, in order to modify the nuclear composition of CRs in a relatively narrow energy interval. We thus include both photon and gas backgrounds and combine the resulting CR spectra with the high-energy part of the Galactic CR fluxes predicted by the escape model. As result, we find a good description of experimental data on the total CR flux, the mean shower maximum depth Xmax and its width rms(Xmax) in the whole energy range above E≃1017 eV. The predicted high-energy neutrino flux matches IceCube measurements, while the contribution to the extragalactic diffuse gamma ray background is of order 30%.
Context.
Measuring the diffuse Galactic
γ
-ray flux in the TeV range is difficult for ground-based
γ
-ray telescopes because of the residual cosmic-ray background, which is higher than the
γ
-ray ...flux by several orders of magnitude. Its detection is also challenging for space-based telescopes because of low signal statistics.
Aims.
We characterise the diffuse TeV flux from the Galaxy using decade-long exposures of the
Fermi
Large Area Telescope.
Methods.
Considering that the level of diffuse Galactic emission in the TeV band approaches the level of residual cosmic-ray background, we estimated the level of residual cosmic-ray background in the SOURCEVETO event selection and verified that the TeV diffuse Galactic emission flux is well above the residual cosmic-ray background up to high Galactic latitude regions.
Results.
We study spectral and imaging properties of the diffuse TeV signal from the Galactic plane. We find much stronger emission from the inner Galactic plane than in previous HESS telescope estimates (lower bound). We also find a significant difference in the measurement of the Galactic longitude and latitude profiles of the signal measured by
Fermi
and HESS. These discrepancies are presumably explained by the fact that regions of background estimate in HESS have non-negligible
γ
-ray flux. Comparing
Fermi
measurements with those of ARGO-YBJ, we find better agreement, with the notable exception of the Cygnus region, where we find much higher flux (by a factor 1.5). We also measure the TeV diffuse emission spectrum up to high Galactic latitude and show that the spectra of different regions of the sky have spectral slopes consistent with Γ = 2.34 ± 0.04, which is harder than the slope of the locally observed spectrum of cosmic rays with energies 10–100 TeV, which produce TeV diffuse emission on their way through the interstellar medium. We discuss the possible origin of the hard slope of the TeV diffuse emission.
Conclusions.
Fermi
/LAT provides reliable measurements of the diffuse Galactic emission spectrum in the TeV range, which are almost background free at low Galactic latitudes. The diffuse flux becomes comparable to the residual cosmic-ray background at Galactic latitudes |
b
| > 50°. Its measurement in these regions might suffer from systematic uncertainty stemming from the uncertainty of our phenomenological model of the residual cosmic-ray background in the Pass 8
Fermi
/LAT data.
The supernova explosion which deposited Fe60 isotopes on Earth 2–3 million years ago should have also produced cosmic rays which contribute to the locally observed cosmic ray flux. We show that the ...contribution of this “local source” causes the “anomalies” observed in the positron and antiproton fluxes and explains why their spectral shapes agree with that of the proton flux. At the same time, this local source component accounts for the difference in the slopes of the spectra of cosmic ray nuclei as the result of the slightly varying relative importance of the “local” and the average component for distinct CR nuclei. Such a “local supernova” model for the spectra of nuclei can be tested via a combined measurement of the energy dependence of the boron-to-carbon (primary-to-secondary cosmic rays) ratio and of the antiproton spectrum: while the antiproton spectrum is predicted to extend approximately as a power law into the TeV range without any softening break, the B/C ratio is expected to show a “plateau” at a level fixed by the observed positron excess in the 30–300 GeV range. We discuss the observability of such a plateau with dedicated experiments for the measurement of the cosmic ray composition in the 10 TeV energy range (NUCLEON, ISS-CREAM).
The locally observed cosmic ray spectrum has several puzzling features, such as the excess of positrons and antiprotons above ~20 GeV and the discrepancy in the slopes of the spectra of cosmic ray ...protons and heavier nuclei in the TeV-PeV energy range. We show that these features are consistently explained by a nearby source which was active approximately two million years ago and has injected (2-3)×10^{50} erg in cosmic rays. The transient nature of the source and its overall energy budget point to the supernova origin of this local cosmic ray source. The age of the supernova suggests that the local cosmic ray injection was produced by the same supernova that has deposited ^{60}Fe isotopes in the deep ocean crust.
In lectures presented at the ISAPP-Baikal summer school I summarised the status and expectations of the multi-messenger astroparticle physics observations using astrophysical neutrinos, cosmic rays ...and gamma-rays. In this lectures I presented a summary of existing observations as well as an interpretation of the results.
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