IceCube and HAWC constraints on very-high-energy emission from the Fermi bubbles

Abstract

The nature of the γ-ray emission from the Fermi bubbles is unknown. Both hadronic and leptonic models have been formulated to explain the peculiar γ-ray signal observed by the Fermi-LAT between 0.1-500 GeV. If this emission continues above ∼30 TeV, hadronic models of the Fermi bubbles would provide a significant contribution to the high-energy neutrino flux detected by the IceCube observatory. Even in models where leptonic γ-rays produce the Fermi bubbles flux at GeV energies, a hadronic component may be observable at very high energies. The combination of IceCube and HAWC measurements have the ability to distinguish these scenarios through a comparison of the neutrino and γ-ray fluxes at a similar energy scale. We examine the most recent four-year dataset produced by the IceCube collaboration and find no evidence for neutrino emission originating from the Fermi bubbles. In particular, we find that previously suggested excesses are consistent with the diffuse astrophysical background with a p-value of 0.22 (0.05 in an extreme scenario that all the IceCube events that overlap with the bubbles come from them). Moreover, we show that existing and upcoming HAWC observations provide independent constraints on any neutrino emission from the Fermi bubbles, due to the close correlation between the γ-ray and neutrino fluxes in hadronic interactions. The combination of these results disfavors a significant contribution from the Fermi bubbles to the IceCube neutrino flux.

Hadronic Models of the Fermi Bubbles: Future Perspectives

“…Detection of these high energy neutrinos can serve as one of the major discriminator between the hadronic and leptonic models. It was pointed out in the past that a fraction of the high-energy astrophysical neutrinos detected by IceCube [15] could originate from the FB in hadronic models [16][17][18][19]21]. The corresponded neutrino flux from the bubbles is consistent with the hadronic flux model that reproduce FB gamma ray data.…”

Can Winds Driven by Active Galactic Nuclei Account for the Extragalactic Gamma-Ray and Neutrino Backgrounds?

“…Such diffuse gamma-ray limits can constrain pp scenarios, in which a significant fraction of IceCube neutrinos are explained by Galactic sources (Ahlers & Murase 2014). For example, if the Fermi bubbles (Fang et al 2017;Sherf et al 2017) or Loop I (Andersen et al 2017) dominantly contribute to the 10-100 TeV neutrino flux, the diffuse gamma-ray flux from the sky region ΔΩis expected to be E 4 2 F~ǵ g 10 3 sr GeV cm s sr…”

“…However, we note that in the case of a hard index, Γ CR = 2.1, although the neutrino flux is about 5 times lower than the best-fit value at 10 TeV, the flux above 100 TeV is consistent with that inferred from throughgoing muon neutrino detection (assuming the neutrino flavor ratio to be 1:1:1). Indeed, the two-component scenario is possible, in which a hard component above 100 TeV (Aartsen et al 2015(Aartsen et al , 2016 can be explained by cosmic-ray reservoir models, which may be even related to the sources of ultra-highenergy cosmic rays (Liu et al 2014;Murase & Waxman 2016;Fang et al 2017).…”

Unveiling the Origin of the Fermi Bubbles

“…The presence of PeV neutrinos could also be used to infer the existence of high-energy CR protons (CRp) from the bubbles. Although IceCube has detected neutrinos with incoming directions coincident with the bubbles, estimating the neutrino background has been a difficult task due to the low number counts and so far it is still unclear whether there are neutrinos associated with the bubbles themselves after background subtraction [23][24][25][26].…”