Stochastic galactic supernova flux of semi-relativistic particles

TL;DR

This paper critically examines the assumption that galactic supernovae produce a smooth, stationary flux of semi-relativistic particles, revealing instead a stochastic, energy-dependent flux with implications for terrestrial detection and constraints.

Contribution

The authors develop a numerical tool to simulate stochastic galactic supernova fluxes of semi-relativistic particles, challenging the smooth flux approximation used in previous bounds.

Findings

Flux is highly stochastic and energy-dependent.

Previous smooth flux models overestimate constraints.

New bounds on axion-like particles and dark matter are weaker.

Abstract

New exotic particles with MeV masses, such as axion-like particles or light dark matter, can be emitted from core-collapse supernovae (SNe) with semi-relativistic velocities. Due to their speed dispersion, they would arrive at Earth as an extended packet with a time spread that can be as large as tens of millennia for typical detectors. It has been argued in the literature that the superposition of packets from all galactic SNe would give rise to a smooth and stationary diffuse flux that could be observable on terrestrial experiments. In this article, we critically examine this hypothesis by carrying out a numerical simulation of the galactic history of SN explosions. We show that, although the particle packets do overlap, due to the short observational time window, each of them only contributes with a very narrow range of energies and with an intensity that depends on the SN distance. As a consequence, the energy dependence of the resulting flux is extremely sensitive to the stochastic nature of the SN population and far from smooth. This has profound implications for the expected signature in terrestrial experiments, which displays a spectral shape that is not properly described by the smooth approximation. We develop a numerical tool to compute this stochastic galactic flux for generic semi-relativistic particles, which also allows us to explore sub-MeV particles, where the smooth diffuse flux approach does not hold. To test this framework, we revisit existing bounds on axion-like particles and fermionic dark matter, finding weaker constraints than previously reported.