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

Abstract

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-year High-Energy Starting Events (HESE) and the 6-year 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% of the HESE events at 95.4% CL, while satisfying the limit on the non-blazar EGB component. Moreover, values of O (100 PeV) for the maximal energy of accelerated cosmicrays by supernovae remnants inside the starburst are disfavoured in our scenario. In broad terms, our analysis points out that a better modeling of astrophysical sources could alleviate the tension between neutrino and gamma-ray data interpretation.

Gamma-Rays from Star Forming Activity Appear to Outshine Misaligned Active Galactic Nuclei

“…where d 2 V /dzdΩ is the comoving volume element, and dN γ /dE γ is the gamma-ray spectrum, which is taken to be a power law with a cutoff at at 100 TeV. For the SF activity contribution, we use a power law spectral index of Γ = 2.2 ± 0.04, while for the mAGN activity contribution we use Γ = 2.25 ± 0.28 [9,51]. We note, however, that throughout this paper we use Γ = 2.2 for the source gammaray fits which feed into Eq.…”

“…Depending on the dispersion in spectral indices, the neutrino contribution from SF activity can vary significantly. Models with mixed spectral indices can contribute significantly below 100 TeV and less so above that energy [51]. However, in models where the SF spectrum is relatively soft, it may be difficult to explain the intensity of the neutrino flux within any SFG model [82], particularly at energies between 10-100 TeV [84].…”

“…Finally, we point out that the intrinsic spectral shape above 100 GeV would also make a significant difference in the relative contribution of different source classes to the diffuse high-energy neutrino flux (see also ref. [51]).…”

Particle acceleration and multimessenger emission from starburst-driven galactic winds

“…A peculiar aspect of SBGs is represented by the amount of target material for nuclear interactions, potentially leading to copious production of neutrinos and gamma rays. The contribution of SBGs to the neutrino flux measured by the IceCube Observatory (IceCube Collaboration: Aartsen et al 2013;Abbasi et al 2020) has been extensively discussed by many authors (Loeb & Waxman 2006;Tamborra et al 2014;Bechtol et al 2017;Sudoh et al 2018;Palladino et al 2018;Peretti et al 2020;Ajello et al 2020;Ambrosone et al 2020), together with the compatibility of the predictions with existing constraints imposed by gamma-ray observations (Ackermann et al 2012;Lisanti et al 2016). The seriousness of these constraints stimulated the search for powerful hidden CR accelerators in environments highly opaque to gamma rays and yet transparent to neutrinos (Capanema et al 2021) like the inner core of Active Galactic Nuclei (AGNi; see e.g.…”