NuSTAR bounds on radiatively decaying particles from M82

TL;DR

This paper uses NuSTAR X-ray observations of M82 to set new constraints on the properties of radiatively decaying axions and similar particles in the 30-500 keV mass range, expanding the parameter space of viable models.

Contribution

It provides the first constraints on heavy axion-like particles in the 30-500 keV mass range using astrophysical observations of M82.

Findings

No excess X-ray flux detected from axion decay in M82.

Constraints on axion-photon coupling strength of $10^{-10}$ to $10^{-12}$ GeV$^{-1}$.

Limits extend into previously unexplored parameter space for axion masses.

Abstract

Axions and other putative feebly interacting particles (FIPs) with a mass of tens to several hundreds of keVs can be produced in stellar cores with a Lorentz boost factor $E_a/m_a\lesssim 10$. Thus, starburst galaxies such as M82 are efficient factories of slow axions. Their decay $a\rightarrow\gamma\gamma$ would produce a large flux of X-ray photons, peaking around $100$ keV and spread around the galaxy by an angle that can be relatively large. We use observations of the Nuclear Spectroscopic Telescope Array (NuSTAR) mission to show that the absence of these features can constrain $30-500$ keV axion masses into uncharted regions for axion-photon coupling of $g_{a\gamma}\sim 10^{-10}-10^{-12}\,\rm GeV^{-1}$. Our argument can be applied to other heavy FIPs and astrophysical sources that are hot enough to produce them, yet cold enough to avoid large boost factors which slow down the decay.