Modeling of Cosmic-Ray Production and Transport and Estimation of Gamma-Ray and Neutrino Emissions in Starburst Galaxies

TL;DR

This paper models cosmic-ray production and transport in starburst galaxies to estimate gamma-ray and neutrino emissions, highlighting the challenges in constraining models with current observations and suggesting future detection prospects.

Contribution

It introduces new models for cosmic-ray production and transport in starburst galaxies, comparing their predictions with gamma-ray and neutrino observations to better understand cosmic-ray behavior.

Findings

Current gamma-ray observations cannot tightly constrain cosmic-ray models.

Certain diffusion models better reproduce observed gamma-ray spectra.

Arp220's high star formation rate suggests a significant contribution from stellar winds.

Abstract

Starburst galaxies (SBGs) with copious massive stars and supernova (SN) explosions are the sites of active cosmic-ray production. Based on the predictions of nonlinear diffusive shock acceleration theory, we model the cosmic-ray proton (CRP) production by both pre-SN stellar winds (SWs) and supernova remnants (SNRs) from core-collapse SNe inside the starburst nucleus. Adopting different models for the transport of CRPs, we estimate the $\gamma$-ray and neutrino emissions due to $pp$ collisions from nearby SBGs such as M82, NGC253, and Arp220. We find that with the current $\gamma$-rays observations by Fermi-LAT, Veritas, and H.E.S.S., it would be difficult to constrain CRP production and transport models. Yet, the observations are better reproduced with (1) the combination of the single power-law (PL) momentum distribution for SNR-produced CRPs and the diffusion model in which the CRP diffusion is mediated by the strong Kolmogorov-type turbulence of $\delta B/B\sim1$, and (2) the combination of the double PL model for SNR-produced CRPs and the diffusion model in which the scattering of CRPs is controlled mostly by self-excited waves rather than the pre-existing turbulence. The contribution of SW-produced CRPs could be substantial in Arp220, where the star formation rate is higher and the slope of the initial mass function would be flatter. We suggest that M82 and NGC253 might be detectable as point sources of high-energy neutrinos in the upcoming KM3NET and IceCube-Gen2, when optimistic models are applied. Future observations of neutrinos as well as $\gamma$-rays would provide constraints for the production and diffusion of CRPs in SBGs.