Starburst galaxies strike back: a multi-messenger analysis with Fermi-LAT and IceCube data

Starburst galaxies, which are known as 'reservoirs' of high-energy cosmic-rays, can represent an important high-energy neutrino 'factory' contributing to the diffuse neutrino flux observed by IceCube. In this paper, we revisit the constraints affecting the neutrino and gamma-ray hadronuclear emissions from this class of astrophysical objects. In particular, we go beyond the standard prototype-based approach leading to a simple power-law neutrino flux, and investigate a more realistic model based on a data-driven blending of spectral indexes, thereby capturing the observed changes in the properties of individual emitters. We then perform a multi-messenger analysis considering the extragalactic gamma-ray background (EGB) measured by Fermi-LAT and different IceCube data samples: the 7.5-yr high-energy starting events (HESE) and the 6-yr high-energy cascade data. Along with starburst galaxies, we take into account the contributions from blazars and radio galaxies as well as the secondary gamma-rays from electromagnetic cascades. Remarkably, we find that, differently from the highly-constrained prototype scenario, the spectral index blending allows starburst galaxies to account for up to 40 per cent of the HESE events at 95.4 per cent CL, while satisfying the limit on the non-blazar EGB component. Moreover, values of O(100 PeV) for the maximal energy of accelerated cosmic-rays by supernovae remnants inside the starburst are disfavoured in our scenario. In broad terms, our analysis points out that a better modelling of astrophysical sources could alleviate the tension between neutrino and gamma-ray data interpretation.

Automated summary

The study reexamines the role of starburst galaxies as reservoirs of high-energy cosmic-rays and high-energy neutrinos, proposing a more realistic model with spectral index blending to capture variations in individual emitter properties. Multi-messenger analysis reveals that this approach allows starburst galaxies to contribute significantly to the high-energy neutrino events while still satisfying constraints from gamma-ray data, potentially easing the tension between neutrino and gamma-ray interpretations.