Neutrino events at IceCube and the Fermi bubbles

We discuss the possibility that the IceCube neutrino telescope might be observing the Fermi bubbles. If the bubbles discovered in gamma rays originate from accelerated protons, they should be strong emitters of high energy ($\gtrsim \mathrm{GeV}$) neutrinos. These neutrinos are detectable as showerlike or tracklike events at a ${\mathrm{Km}}^3$ neutrino observatory. For a primary cosmic ray flux with spectrum $\propto E^{-2.1}$ and cutoff energy at or above 10 PeV, the Fermi bubble flux substantially exceeds the atmospheric background, and could account for up to $\sim 4–5$ of the 28 events detected above $\sim 30\mathrm{TeV}$ at IceCube. Running the detector for $\sim 5–7$ more years should be sufficient to discover this flux at high significance. For a primary cosmic ray flux with steeper spectrum, and/or lower cutoff energy, longer running times will be required to overcome the background.

Figure 1

(a) The IceCube events in equatorial coordinates, with their median angular errors, from [2]. The contours of the Fermi bubbles are shown as well. (b) The time and (deposited) energy distribution of the events that are spatially correlated with the bubbles.

Figure 2

(a) The expected ${\nu }_{\mu }+{\overline{\nu }}_{\mu }$ flux (solid lines) from the FB (before oscillations), normalized to the gamma-ray flux, as a function of the energy, for different proton spectral indices, $k$, and different cutoffs of the primary proton spectrum, $E_0$. Solid, red: $k=2.1$; dotted, black: $k=2.3$. For each we show, from thin to thick: $E_0=1,3,10,30\mathrm{PeV}$. For comparison, we also show: (i) the atmospheric neutrino flux [34] averaged over 25°–95° zenith angle, (ii) the ANTARES upper limit [32] and (iii) the diffuse flux that best fits the IceCube data [2]. The other two panels show the distribution (normalized to 1) of the flux in $\sin \theta$ (with $\theta$ the declination angle) (b), and in the right ascension, $\varphi$, (c), for each bubble (solid) and the total for both (dotted).

Figure 3

Events expected at IceCube per decade, as a function of the neutrino energy threshold $E_{\text{th}}$, for the primary proton spectrum index $k=2.1$ (upper panel) and $k=2.3$ (lower panel). Solid: FB signal, for the total of showerlike and tracklike events (thick) and for tracklike events only (thin). The arrows indicate the effect of varying the primary spectrum cutoff in the interval $E_0=1–30\mathrm{PeV}$. Dashed: the same but from atmospheric fluxes. Dot-dot-dashed: showerlike and tracklike events from the IceCube best-fit flux in Fig. 2.

Figure 4

Expected neutrino energy distribution of the total showerlike and tracklike events per decade, for signal (with $k=2.1$) and for atmospheric background. (a) $E_0=1,3\mathrm{PeV}$; (b) $E_0=10,30\mathrm{PeV}$. For $E_0=1\mathrm{PeV}$, signal and background are very close in all bins.

Figure 5

The same as Fig. 4 for the IceCube running time of 662 days. The IceCube events that correlate with the FB (from Fig. 1) are shown for comparison. Their coordinates on the vertical axis give a visual representation of the number of events for which the central value of the observed energy falls in a given bin. The solid (dashed) error bars represent, respectively, the error on the observed energy and the factor of $\sim 3–4$ difference between neutrino energy and observed energy for neutral current events.