Evidence for a Spectral Break or Curvature in the Spectrum of Astrophysical Neutrinos from 5 TeV--10 PeV

Abstract

We report improved measurements of the all flavor astrophysical neutrino spectrum with IceCube by combining complementary neutrino samples in two independent analyses. Both analyses show evidence of a harder spectrum at energies below $\sim$30~TeV compared to higher energies where the spectrum is well characterized by a power law. The spectrum is better described by a log parabola or a broken power law, the latter being the preferred model. Both, however, reject a single power law over an energy range 5~TeV-10~PeV with a significance $>4σ$, providing new constraints on properties of cosmic neutrino sources.

Figures (3)

• Figure 1: Energy distribution of cascade- and track-identified events used in the CF and MESE analyses: contributions from astrophysical neutrinos, assuming the best-fit broken power-law spectrum, total background (atmospheric neutrinos and muons) along with the total contributions compared to data for CF on the left (8.5 years of tracks and 10 years of cascades) and MESE on the right (11.4 years). Note the different scales for tracks (left axis) and cascades (right axis). Here, energy proxy represents the reconstructed energy for the given event type which is different for cascades, MESE tracks, and CF tracks. Hence these energy proxies are not directly comparable.
• Figure 2: Two-dimensional profile likelihood scan on the spectral indices of the broken power law: the star markers indicate the best fit with $\gamma_1=1.72$ and $\gamma_2=2.84$ for MESE, and $\gamma_1=1.31$ and $\gamma_2=2.74$ for CF. Contours represent the 68% and 95% confidence regions based on Wilks’ theorem. The dotted line signifies the expectation of $\gamma_1=\gamma_2$, indicating the transition to a single power-law spectrum.
• Figure 3: Segmented flux: the fit to the astrophysical flux normalization assuming an $E^{-2}$ spectrum is shown in each segment. Orange markers and shaded regions represent CF while MESE is shown in blue. The shaded regions show the 68% uncertainties and sensitive energy ranges for each analysis obtained from profile likelihood scans of the four parameters of the BPL model. The flux from NGC1068 NGC1068 and the galactic plane GalacticPlaneScience measured with IceCube lie at much lower scales.