How bright can the brightest neutrino source be?

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.Comment: 13 pages, 10 figures, accepted by Phys.Rev.

Astrophysical interpretation of the anisotropies in the unresolved gamma-ray background

“…The index above the break F 0 is fixed to the Euclidean value, i.e. 2.5, typical of sources that are homogenously distributed in a local volume [71,72]. This is particularly appropriate for rare emitters, as for the large-flux end of the distribution.…”

Angular power spectrum analysis on current and future high-energy neutrino data

“…This turns out to yield a significant effect on projections, which weakens sensitivity with the angular power spectrum approach compared with the earlier estimates of Ref. [30] based on analytic calculations. We use the characteristic number of sources (N ) to assess the sensitivities, which is roughly the number of neutrino sources over which the measured intensity is divided, and bright sources like BL Lacs have therefore smaller values (N = 6 × 10 2 ) than numerous, weak sources like starburst galaxies (N = 10 7 ).…”

“…The work extends fully analytic treatment of Ref. [30] by adopting more realistic simulations of the expected sky map of neutrino events of given exposure. By repeating these Monte Carlo simulations, we make error estimates more robust by taking cross correlations between different multipole bins fully into account.…”

“…where N s is the total number of neutrino sources, F is the flux of each source, the slopes (α, β) are fixed to (2.5, 1.5), F is the characteristic flux at the break, and N is the characteristic number of sources at F (see Ref. [30] for further details). The slope at high-flux regime (α = 2.5) is generically true for any sources distributed homogeneously in the local volume where expansion of the Universe can be neglected.…”

“…Another method in order to reveal sources that give dominant contribution to the measured diffuse flux, is by studying angular clustering of registered events [28][29][30] and is shown to be more efficient than the conventional point-source searches [20] for individually bright, rare source populations such as blazars [30]. References [29,31] set, for instance, stringent limits on the number density of neutrino sources, which has the advantage that a wide range of neutrino source populations is discussed without using prior theoretical models.…”