2017ApJ...847...95S

We analyze the IceCube four-year neutrino data in search of a signal from the Fermi bubbles. No signal is found from the bubbles or from their dense shell, even when taking into account the softer background. This imposes a conservative ξ_i < 8% upper limit on the cosmic-ray ion (CRI) acceleration efficiency, and an η ≡ ξ_e/ξ_i >= 0.006 lower limit on the electron-to-ion ratio of acceleration efficiencies (at the 2σ confidence level). For typical ξ_i, a signal should surface once the number of IceCube neutrinos increases by ∼an order of magnitude, unless there is a <PeV cutoff on the CRI spectrum.