# How bright can the brightest neutrino source be?

## TL;DR

This paper constrains the maximum brightness of the brightest astrophysical neutrino sources using flux distribution models and observational limits, providing insights into source populations and guiding future neutrino telescope searches.

## Contribution

It introduces a method to estimate the maximum neutrino source flux by combining flux distribution assumptions with observational constraints, improving understanding of source brightness limits.

## Key findings

- The maximum flux for abundant sources like starburst galaxies is an order of magnitude below current limits.
- Current IceCube limits are already competitive for rare, bright sources such as blazars.
- Future neutrino telescopes will significantly tighten constraints on source brightness with increased exposure.

## Abstract

After the discovery of extraterrestrial high-energy neutrinos, the next major goal of neutrino telescopes will be identifying astrophysical objects that produce them. The flux of the brightest source $F_{\rm max}$, however, cannot be probed by studying the diffuse neutrino intensity. We aim at constraining $F_{\rm max}$ by adopting a broken power-law flux distribution, a hypothesis supported by observed properties of any generic astrophysical sources. The first estimate of $F_{\rm max}$ comes from the fact that we can only observe one universe, and hence, the expected number of sources above $F_{\rm max}$ cannot be too small compared with one. For abundant source classes such as starburst galaxies, this one-source constraint yields a value of $F_{\rm max}$ that is an order of magnitude lower than the current upper limits from point-source searches. Then we derive upper limits on $F_{\rm max}$ assuming that the angular power spectrum is consistent with neutrino shot noise yet. We find that the limits obtained with upgoing muon neutrinos in IceCube can already be quite competitive, especially for rare but bright source populations such as blazars. The limits will improve nearly quadratically with exposure, and therefore be even more powerful for the next generation of neutrino telescopes.