Probing the Galactic origin of the IceCube excess with gamma rays

The IceCube Collaboration has recently reported evidence for a high-energy extraterrestrial neutrino flux. During two years of operation 28 events with energies between 30 TeV and 1.2 PeV were observed while only 10.6 events were expected from conventional atmospheric backgrounds. The hadronic interactions responsible for this IceCube excess will also produce a flux of high-energy $\gamma$-rays that can serve as a probe of source direction and distance. We show that existing TeV to PeV diffuse $\gamma$-ray limits support the interpretation that the IceCube excess is mostly of extragalactic origin. However, we point out that $\gamma$-ray surveys are biased in the Northern Hemisphere whereas the recent IceCube data tentatively show a weak preference for the Southern Sky. Possible sub-dominant contributions from Galactic neutrino sources like remnants of supernovae and hypernovae are marginally consistent with present $\gamma$-ray limits. This emphasizes the importance of future diffuse TeV to PeV $\gamma$-ray surveys in the Southern Hemisphere, particularly in the extended region around the Galactic center including the Fermi Bubbles.

Figure 1

Mollweide projection of the 21 cascades (circles) and 7 muons (diamonds) with time-ordered event numbers according to Ref. [2]. The green square and red star indicate the hot spot in the cascade clustering search and the hottest spot (with low significance) in the PeV $\gamma$-ray search [4]. The light-gray shaded area is the region in the sky which is presently uncharted in PeV $\gamma$-ray emission. The northern and southern edge of this unaccessible region is given by the reach of CASA-MIA [5] and IceCube’s $\gamma$-ray search with the IC-40 configuration [4], respectively. The dark-gray shaded area shows this region assuming future observations with the full IC-86 configuration and HAWC [6].

Figure 2

Measurements of the isotropic diffuse $\gamma$-ray flux in the TeV-PeV range by various experiments (see Table 1). Left panel: The black lines shows the $\gamma$-ray flux corresponding to IceCube’s best fit assuming $pp$-interactions ($K=2$) and an exponential cutoff at 6 PeV (i.e., 3 PeV for neutrinos). We show the unattenuated flux and the flux from 8.5 kpc, 20 kpc, and 30 kpc, respectively, taking into account pair production via scattering off CMB photons. For the conversion of photon fractions into photon flux we use the CR flux of Ref. [8]. For comparison we also show the total neutrino flux as a thin gray line. Right panel: Comparison to the Galactic $\gamma$-ray emission of a generic DM decay scenario assuming a scalar $X$ with mass $m_X=5\mathrm{PeV}$ and lifetime ${\tau }_X=7\times {10}^{27}\mathrm{s}$. The solid, dashed, and dotted black lines show the diffuse emission from the three sky regions divided by the red dashed circles in Fig. 3. The solid gray line shows the total average neutrino flux, which also accounts for the extragalactic contribution shown separately as a dashed gray line.

Figure 3

The fluctuation of the nonisotropic diffuse flux described by Eq. (7) assuming extended emission regions with radius 15° around each direction in the sky. The events are weighted according to the approximation described in the text. The bilobal region (blue dashed lines) indicate the position of the FBs. The concentric circles (red dashed lines) show the GC region containing 25% and 50% of the emission from DM decay in the Galactic halo.

Figure 4

Diffuse measurements of the $\gamma$-ray flux in the GP in comparison to the expected diffuse flux from the propagation of Galactic CRs (light-gray lines) and from Galactic SNRs (black lines) and HNRs (dark-gray lines) with power index $\mathrm{\Gamma }=2.2$. The solid lines indicate the estimate in Eqs. (8), (10) and (11) using relation (2) without attenuation and the dashed lines indicate the contribution from a source at the GC. We adopt the calculation of Ref. [35] for the interstellar radiation field on top of the CMB. We also show estimated sensitivities w.r.t. the diffuse TeV-PeV $\gamma$-ray emission in the GP ($|b|<2°$) for the observatories (in ascending energy of maximum sensitivity) CTA (green dotted), HAWC (blue dotted), LHAASO (red dotted) and HiSCORE (brown dotted). Note that the model-dependent theoretical fluxes are averaged over Galactic longitude and latitude $|b|<2°$, whereas the measurements only apply to the intersection of the GP with the FoV and in some case extend to larger absolute latitudes as indicated in the plot ($cf$. Figure 5). Extending the GP to $|b|<5°$ or $|b|<10°$ reduces the theoretical fluxes (only ${\pi }^0$-decay and ignoring absorption) by about a factor 2 or 3, respectively. The relative intensity of the diffuse flux between Galactic Center and anti-Center is less than $\pm 25\%$ (see text).

Figure 5

The on-source regions of GP diffuse emission used for the experimental results shown in Fig. 4 using the same color coding. We also show the distribution of IceCube events in the vicinity of the GP (cf. Fig. 1). The circled areas indicate the uncertainty of the cascade reconstruction as in Fig. 4. Note that the limits on diffuse $\gamma$-ray emission along the GP from HEGRA [49] assume a larger zenith angle range than for the isotropic diffuse emission listed in Table 1.

Figure 6

Left: Histogram of the event distribution in declination (top) and deposited energy (bottom). The hatched area shows the contribution of the seven events in the extended GC region with a possible association with the FBs. The lines shows the expected background from atmospheric muons (dotted), conventional atmospheric neutrinos (dashed) and the sum of these backgrounds and the best-fit diffuse flux (solid) from Ref. [2]. Right: The diffuse flux from the FB in comparison with diffuse $\gamma$-ray limits in the 0.1–1 PeV range corrected for the overlap of the FoV with the FB region. The horizontal dashed line is a preliminary upper limit from ANTARES on the per flavor neutrino flux of the FB [100]. The green point indicates the equivalent diffuse flux from the FB of $1.4J_{{\nu }_{\alpha }}^{\mathrm{IC}}$ (see main text). The dotted (solid) line shows a possible intrinsic (absorbed) $\gamma$-ray emission from the FB with a spectral index $\mathrm{\Gamma }=2.2$ and exponential cutoff at 6 PeV according to Eq. (12). The corresponding neutrino flux (per flavor) is shown as a dashed line. We also show estimated sensitivities of the observatories (in ascending energy of maximum sensitivity) CTA (green dotted), HAWC (blue dotted), LHAASO (red dotted) and HiSCORE (brown dotted) w.r.t. the diffuse TeV-PeV $\gamma$-ray emission in the FBs.