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The IceCube experiment has recently reported the first observation of high-energy cosmic neutrinos. Their origin is still unknown. In this paper, we investigate the possibility that they originate in active galaxies. We show that hadronic interactions (pp) in the generally less powerful, more frequent, FR-I radio galaxies are one of the candidate source classes being able to accommodate the observation while the more powerful, less frequent, class of FR-II radio galaxies has too low of a column depths to explain the signal.
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With infrared luminosities $L_{mathrm{IR}} geq 10^{12} L_{odot}$, Ultra-Luminous Infrared Galaxies (ULIRGs) are the most luminous objects in the infrared sky. They are predominantly powered by starburst regions with star-formation rates $gtrsim 100~ M_{odot}~ mathrm{yr^{-1}}$. ULIRGs can also host an active galactic nucleus (AGN). Both the starburst and AGN environments contain plausible hadronic accelerators, making ULIRGs candidate neutrino sources. We present the results of an IceCube stacking analysis searching for high-energy neutrinos from a representative sample of 75 ULIRGs with redshift $z leq 0.13$. While no significant excess of ULIRG neutrinos is found in 7.5 years of IceCube data, upper limits are reported on the neutrino flux from these 75 ULIRGs as well as an extrapolation for the full ULIRG source population. In addition, constraints are provided on models predicting neutrino emission from ULIRGs.
We study the propagation of cosmic rays generated by sources residing inside superbubbles. We show that the enhanced magnetic field in the bubble wall leads to an increase of the interior cosmic ray density. Because of the large matter density in the wall, the probability for cosmic ray interactions on gas peaks there. As a result, the walls of superbubbles located near young cosmic ray sources emit efficiently neutrinos. We apply this scenario to the Loop~I and Local Superbubble: These bubbles are sufficiently near such that cosmic rays from a young source as Vela interacting in the bubble wall can generate a substantial fraction of the observed astrophysical high-energy neutrino flux below $sim$ few $times 100$ TeV.
IceCube has observed an excess of neutrino events over expectations from the isotropic background from the direction of NGC 1068. The excess is inconsistent with background expectations at the level of $2.9sigma$ after accounting for statistical trials. Even though the excess is not statistical significant yet, it is interesting to entertain the possibility that it corresponds to a real signal. Assuming a single power-law spectrum, the IceCube Collaboration has reported a best-fit flux $phi_ usim 3 times 10^{-8} (E_ u/{rm TeV})^{-3.2}~({rm GeV , cm^2 , s})^{-1}$, where $E_ u$ is the neutrino energy. Taking account of new physics and astronomy developments we give a revised high-energy neutrino flux for the Stecker-Done-Salamon-Sommers AGN core model and show that it can accommodate IceCube observations.
Cosmic-ray (CR) protons can accumulate for cosmological times in clusters of galaxies. Their hadronic interactions with protons of the intra-cluster medium (ICM) generate secondary electrons, gamma-rays and high-energy neutrinos. In light of the high-energy neutrino events recently discovered by the IceCube observatory, we estimate the contribution from galaxy clusters to the diffuse gamma-ray and neutrino backgrounds. For the first time, we consistently take into account the synchrotron emission generated by secondary electrons and require the clusters radio counts to be respected. For a choice of parameters respecting current constraints from radio to gamma-rays, and assuming a proton spectral index of -2, we find that hadronic interactions in clusters contribute by less than 10% to the IceCube flux, and much less to the total extragalactic gamma-ray background observed by Fermi. They account for less than 1% for spectral indexes <-2. The high-energy neutrino flux observed by IceCube can be reproduced without violating radio constraints only if a very hard (and speculative) spectral index >-2 is adopted. However, this scenario is in tension with the high-energy IceCube data, which seem to suggest a spectral energy distribution of the neutrino flux that decreases with the particle energy. We stress that our results are valid for all kind of sources injecting CR protons into the ICM, and that, while IceCube can test the most optimistic scenarios for spectral indexes >=-2.2 by stacking few nearby massive objects, clusters of galaxies cannot give any relevant contribution to the extragalactic gamma-ray and neutrino backgrounds in any realistic scenario.
Ultra-luminous infrared galaxies (ULIRGs) have infrared luminosities $L_{mathrm{IR}} geq 10^{12} L_{odot}$, making them the most luminous objects in the infrared sky. These dusty objects are generally powered by starbursts with star-formation rates that exceed $100~ M_{odot}~ mathrm{yr}^{-1}$, possibly combined with a contribution from an active galactic nucleus. Such environments make ULIRGs plausible sources of astrophysical high-energy neutrinos, which can be observed by the IceCube Neutrino Observatory at the South Pole. We present a stacking search for high-energy neutrinos from a representative sample of 75 ULIRGs with redshift $z leq 0.13$ using 7.5 years of IceCube data. The results are consistent with a background-only observation, yielding upper limits on the neutrino flux from these 75 ULIRGs. For an unbroken $E^{-2.5}$ power-law spectrum, we report an upper limit on the stacked flux $Phi_{ u_mu + bar{ u}_mu}^{90%} = 3.24 times 10^{-14}~ mathrm{TeV^{-1}~ cm^{-2}~ s^{-1}}~ (E/10~ mathrm{TeV})^{-2.5}$ at 90% confidence level. In addition, we constrain the contribution of the ULIRG source population to the observed diffuse astrophysical neutrino flux as well as model predictions.
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