Constrains on the extragalactic origin of IceCube's neutrinos using HAWC

TL;DR

This paper uses HAWC gamma-ray observations to constrain the extragalactic origins of IceCube's neutrinos, finding that the lack of detected sources limits certain source population models.

Contribution

It introduces FIRESONG, a simulation of neutrino source properties, and uses HAWC data to set new constraints on the density and evolution of extragalactic neutrino sources.

Findings

HAWC detected only 2 extragalactic sources in 760 days.

The lack of sources constrains models with no source evolution.

At least 8 sources should be detected if sources are uniformly distributed.

Abstract

IceCube has discovered an unresolved and isotropic flux of neutrinos between 10 TeV and 8 PeV. Extragalactic origin for this flux is usually assumed, as well as a correlation with the sources of cosmic rays. To date, no clear association with a class of objects has been made. HAWC is a very-high-energy (VHE, or $\gtrsim$100 GeV) gamma ray observatory in operation in central Mexico. HAWC has studied 2/3 of the entire sky ($\sim8~sr)$. We use this survey to search for optically thin sources of cosmic rays and neutrinos, responsible for IceCube's observations. We have written a simulation of the cosmological properties of neutrino sources, FIRESONG, to show that under certain conditions, HAWC should be observing multiple sources. However, HAWC has only detected 2 extragalactic objects in 760 days of livetime: Mrk 421 and Mrk 501. This deficiency of detected sources in the extragalactic sky can be used to constrain the properties of neutrino sources, such as their density in the local universe. The case HAWC restricts the most is for no evolution in the density of sources, in which at least 8 objects should have been detected for the cases not excluded already by IceCube.