Diffuse Axion Background

TL;DR

This paper develops a framework for the diffuse axion background from transient astrophysical sources, constrains its flux using existing data, and discusses future detection prospects across a wide axion mass range.

Contribution

It introduces a general framework for the diffuse axion background from transient sources and derives constraints from existing observations, highlighting future experimental sensitivities.

Findings

Strong constraints on light axion flux from existing gamma-ray and X-ray data.

Future experiments will significantly improve sensitivity to the diffuse axion background.

D$a$B can serve as a probe for axion emission sources and mechanisms.

Abstract

Relativistic axions can be readily produced in a broad variety of transient sources, such as axion star bosenova explosions, supernovae or even evaporating primordial black holes. We develop a general framework describing the resulting persistent diffuse axion background (D$a$B) due to accumulated axions from historic transient events. We derive strong constraints on the D$a$B flux from light axions $m\lesssim 10^{-3}\,{\rm eV}$ emitted from sources with energies $\omega \gtrsim{\rm MeV}$ considering the non-observation of excess photons associated with axion-photon coupling from experiments, including COMPTEL, NuSTAR, XMM-Newton, INTEGRAL, EGRET and Fermi. Future searches in experiments such as SKA, JWST, XRISM, Vera C. Rubin Observatory, AMEGO/e-ASTROGAM will allow probing D$a$B and associated axion-photon couplings with unprecedented sensitivity covering a wide range of possible source energies as low as $0.1\,\mu$eV and multiple decades in axion masses. We highlight the differences between astrophysical and dark sector sources of D$a$B. Further, we discuss complementarity with direct detection as well as prospects for other D$a$B searches. Our analysis demonstrates that D$a$B can act as a promising probe of populations of axion emission sources as well as emission mechanisms.