Tomographic Constraints on High-Energy Neutrinos of Hadronuclear Origin

Abstract

Mounting evidence suggests that the TeV-PeV neutrino flux detected by the IceCube telescope has mainly an extragalactic origin. If such neutrinos are primarily produced by a single class of astrophysical sources via hadronuclear (pp) interactions, a similar flux of gamma-ray photons is expected. For the first time, we employ tomographic constraints to pinpoint the origin of the IceCube neutrino events by analyzing recent measurements of the cross correlation between the distribution of GeV gamma rays, detected by the Fermi satellite, and several galaxy catalogs in different redshift ranges. We find that the corresponding bounds on the neutrino luminosity density are up to 1 order of magnitude tighter than those obtained by using only the spectrum of the gamma-ray background, especially for sources with mild redshift evolution. In particular, our method excludes any hadronuclear source with a spectrum softer than E −2.1 as a main component of the neutrino background, if its evolution is slower than ð1 þ zÞ 3 . Starburst galaxies, if able to accelerate and confine cosmic rays efficiently, satisfy both spectral and tomographic constraints. Introduction.-The discovery of the PeV neutrinos by IceCube [1,2] has launched the era of high-energy neutrino astronomy. The current data set is compatible with a flux in excess with respect to the atmospheric background, with an isotropic allocation of events on the celestial sphere and flavor equipartition [1][2][3][4][5][6]. Because of the current low statistics, the origin of the high-energy IceCube events is not yet known, but an extragalactic and mostly diffuse origin appears to be favored [7,8].High-energy neutrino production from cosmic accelerators has been the subject of a cascade of theoretical studies, especially after the IceCube results were announced [7,8]. Many papers discuss the neutrino emission from one specific source class by adopting a modeldependent approach, for active galactic nuclei (AGNs) [9][10][11][12][13][14][15][16][17][18], star-forming galaxies [19][20][21][22][23][24][25][26][27][28], gamma-ray bursts [29][30][31][32][33][34][35][36], galaxy clusters [37][38][39][40], and dark matter decays [41][42][43][44][45].Alternatively, a more generic approach focuses on the phenomenological aspects of the potential sources. For example, assuming photomeson production (pγ) of neutrinos, Ref.[46] obtained constraints on the source size and magnetic field strength needed to match the IceCube flux. Reference [47] hypothesized that the TeV-PeV neutrinos were generated via hadronuclear interactions (pp) and concluded that the cosmic ray spectrum of the dominant neutrino sources should be harder than E −2.2 . This is because the associated gamma-ray spectrum will extend down to GeV energies, where the flux of the isotropic gamma-ray background (IGRB) measured with the Fermi Large Area Telescope (LAT) [48] cannot be overshot. The connection with sources of ultrahigh-energy cosmic rays has also been considered [49][50][51].

Constraining high-energy cosmic neutrino sources: Implications and prospects

“…yses [57,58,110,111] can be used for additional constraints, and the cross correlation gives stringent limits on contributions from star-forming galaxies including SBGs [112].…”

Evidence against Star-forming Galaxies as the Dominant Source of Icecube Neutrinos

“…This process is expected to be the dominant high-energy emission mechanism in star-forming galaxies (SFGs) and may be relevant for other hadronuclear sources, such as galaxy clusters. Several authors have jointly considered the cumulative neutrino and γ-ray emissions of extragalactic source populations in light of recent results from IceCube and the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope (Fermi ) (e.g., Chang & Wang 2014;Chang et al 2015;Tamborra et al 2014;Ando et al 2015).…”

Multi-messenger aspects of cosmic neutrinos*

“…This places even stronger bounds on pp scenarios that are expected not to coincide with AGN [31]. Recently, it was argued that limits derived from the cross-correlation of Fermi gamma-rays with galaxy catalogues exclude the pp scenarios with soft emission and weak redshift evolution [32]. Other electromagnetic emission bands can also become important in constraining pp emission scenarios, for instance radio emission of galaxy clusters [18].…”