First indirect detection constraints on axions in the Solar basin

TL;DR

This paper establishes the first observational constraints on a solar basin of keV-mass axions by analyzing NuSTAR X-ray data, significantly improving existing bounds on axion-photon coupling in this mass range.

Contribution

It introduces the concept of a solar basin of trapped axions and provides the first limits on their properties using X-ray observations.

Findings

Set world-leading limits on axion-photon coupling for 5-30 keV axions.

Demonstrated that solar basin axions can produce detectable X-ray signals.

Improved constraints over stellar cooling bounds by more than an order of magnitude.

Abstract

Axions with masses of order keV can be produced in great abundance within the Solar core. The majority of Sun-produced axions escape to infinity, but a small fraction of the flux is produced with speeds below the escape velocity. Over time, this process populates a basin of slow-moving axions trapped on bound orbits. These axions can decay to two photons, yielding an observable signature. We place the first limits on this solar basin of axions using recent quiescent solar observations made by the NuSTAR X-ray telescope. We compare three different methodologies for setting constraints, and obtain world-leading limits for axions with masses between 5 and 30 keV, in some cases improving on stellar cooling bounds by more than an order of magnitude in coupling.