$\gamma$-ray observations of extraterrestrial neutrino track event positions

TL;DR

This study analyzed 70 months of Fermi-LAT gamma-ray data to search for emission associated with IceCube's extraterrestrial neutrino events, finding no direct correlation but discovering two new gamma-ray sources, likely AGNs.

Contribution

First comprehensive gamma-ray analysis of IceCube neutrino candidates using long-term Fermi-LAT data, establishing upper limits and identifying new gamma-ray sources.

Findings

No significant gamma-ray emission associated with IceCube neutrino events.

Discovery of two new gamma-ray point sources, likely active galactic nuclei.

Upper limits set on gamma-ray flux from neutrino source locations.

Abstract

In this paper we report the results of a $\gamma$-ray study of IceCube's extraterrestrial neutrino candidates detected as track-like events. Using 70 months of Fermi-LAT observations, a likelihood analysis of all $1-300$ GeV photons within 5 degrees of the track-like neutrino candidates' origin was undertaken, to search for spatially coincident $\gamma$-ray emission. One of IceCube's HESE track events was found to be spatially coincident with a $\gamma$-ray bright active galactic nucleus (AGN), PKS 0723-008. We find however, that the chance probability for Fermi-LAT detected AGN to be spatially coincident with a single HESE track-like event is high ($\sim37$\%). We therefore find no evidence of $\gamma$-ray emission associated with the detection of IceCube's HESE track-like neutrino candidates. Upper limits were calculated in the energy range of $1-300$ GeV, assuming a point source origin for the neutrino events considered. The implications for the non-detection of $\gamma$-ray emission from the source of the HESE track-like events are briefly discussed. The large time period analysed in our study did however, reveal two new $\gamma$-ray point sources. With a flux of ($1.28 \pm 0.08$) $\times 10^{-9}$ photons cm$^{-2}$ s$^{-1}$, in the $1-300$ GeV energy range, and an associated TS value of 220.6, one of these new point sources is positionally coincident with the AGN PKS 1346-112. The other point source has a $1-300$ GeV flux of ($7.95 \pm 1.23$) $\times 10^{-10}$ photons cm$^{-2}$ s$^{-1}$ and an associated TS value of 92.4. This new point source is spatially coincident with the radio source NVSS J072534+021645 suggesting that it too is an AGN.