${\hat{I}}^3$-ray observations of extraterrestrial neutrino track event positions

Abstract

In this paper we report the results of a ${\hat{I}}^3$-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 ${\hat{I}}^3$-ray emission. One of IceCube's HESE track events was found to be spatially coincident with a ${\hat{I}}^3$-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 ($\sim 37$\%). We therefore find no evidence of ${\hat{I}}^3$-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 ${\hat{I}}^3$-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 ${\hat{I}}^3$-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.

Accepted for publication in MNRAS, 10 pages, 8 figures, 3 tables