Hidden Cosmic-Ray Accelerators as an Origin of TeV-PeV Cosmic Neutrinos

The latest IceCube data suggest that the all-flavor cosmic neutrino flux may be as large as ${10}^{-7}\mathrm{GeV}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}{\mathrm{sr}}^{-1}$ around 30 TeV. We show that, if sources of the TeV-PeV neutrinos are transparent to $\gamma$ rays with respect to two-photon annihilation, strong tensions with the isotropic diffuse $\gamma$-ray background measured by Fermi are unavoidable, independently of the production mechanism. We further show that, if the IceCube neutrinos have a photohadronic ($p\gamma$) origin, the sources are expected to be opaque to 1–100 GeV $\gamma$ rays. With these general multimessenger arguments, we find that the latest data suggest a population of cosmic-ray accelerators hidden in GeV-TeV $\gamma$ rays as a neutrino origin. Searches for x-ray and MeV $\gamma$-ray counterparts are encouraged, and TeV-PeV neutrinos themselves will serve as special probes of dense source environments.

Figure 1

Left panel: All-flavor neutrino (thick blue lines) and isotropic diffuse $\gamma$-ray (thin red lines) fluxes for $pp$ and minimal $p\gamma$ scenarios of Eqs. (4) and (5) that account for the latest IceCube data from $\sim 10\mathrm{TeV}$ to $\sim 2\mathrm{PeV}$ energies [5], where $s^{\prime }=s_{\text{ob}}=2.5$ is used. While $pp$ scenarios require ${ϵ}_{\nu }^b=25\mathrm{TeV}$ with a strong tension with the Fermi IGRB [13], minimal $p\gamma$ scenarios allow the range ${ϵ}_{\nu }^b$ of 6–25 TeV (shaded regions) as long as the sources are transparent to $\gamma$ rays (see the main text for details). Right panel: Same as the left panel, but now showing neutrino fluxes of AGN core and choked jet models from Refs. [21, 24]. To illustrate the strength of diffuse $\gamma$-ray constraints, we pretend that the sources were transparent to $\gamma$ rays.

Figure 2

Neutrino and CR bounds on the optical depth to $\gamma \gamma \to e^{+}{e}^{-}$ in the sources of diffuse TeV-PeV neutrinos. We calculate ${\tau }_{\gamma \gamma }$ and $f_{p\gamma }$ as functions of ${ϵ}_{\gamma }$ and ${ϵ}_p$, respectively, imposing $f_{p\gamma }\ge 0.01$. We consider simple power laws with $\alpha =2.5$ and $\alpha =2/3$ for ${ϵ}_{\nu }^b=6–25\mathrm{TeV}$ (shaded bands), and the gray-body case with the temperature $kT/{\mathrm{\Gamma }}^2=112\mathrm{eV}$.