NuSTAR as an Axion Helioscope

TL;DR

This work addresses the search for axions/ALPs by exploiting solar Primakoff production and subsequent axion–photon conversion in the Sun’s atmosphere to yield a distinctive X-ray signal. The authors combine a robust solar-core axion flux model with realistic 3D solar magnetic-field models and NuSTAR X-ray spectroscopy, using a Bayesian likelihood to set limits on the coupling $g_{a\gamma}$. They report a 95% CL bound of $g_{a\gamma}\lesssim 7.3\times 10^{-12}$ GeV$^{-1}$ for $m_a\lesssim 4\times 10^{-7}$ eV, improving on CAST and probing new regions of the $(m_a,g_{a\gamma})$ plane. The results demonstrate NuSTAR’s potential as an axion helioscope and constrain parameter space independently of dark-matter assumptions, with implications for future helioscopes such as IAXO and BabyIAXO.

Abstract

We present a novel approach to investigating axions and axion-like particles (ALPs) by studying their potential conversion into X-rays within the Sun's atmospheric magnetic field. Utilizing high-sensitivity data from the Nuclear Spectroscopic Telescope Array (NuSTAR) collected during the 2020 solar minimum, along with advanced solar atmospheric magnetic field models, we establish a new limit on the axion-photon coupling strength $g_{aγ}\lesssim 7.3\times 10^{-12}$~GeV$^{-1}$ at 95\% CL for axion masses $m_a\lesssim 4\times 10^{-7}$\,eV. This constraint surpasses current ground-based experimental limits, studying previously unexplored regions of the axion-photon coupling parameter space up to masses of $m_a\lesssim 3.4\times 10^{-4}$\,eV. These findings mark a significant advancement in our ability to probe axion properties and strengthen indirect searches for dark matter candidates.

Figures (11)

• Figure 1: NuSTAR's 95% CL exclusion on axion-photon coupling strength $g_{a\gamma }$ (blue line). Regions excluded by this work are shown in shaded blue. We present our results in comparison to current PhysRevLett.133.221005Asztalos_2010Du_2018Braine_2020Bartram_2021Boutan_2018Bartram_2023PhysRevD.42.1297Zhong_2018Backes_2021haystaccollaboration2023newLee_2020Jeong_2020Kwon_2021Lee_2022Kim_2023Yi_2023Yang_2023kim2023experimentalPhysRevX.14.031023Quiskamp_2022Abeln:2021McAllister_2017Alesini_2019Alesini_2021Di_Vora_2023Devlin:2021fpqOuellet_2019Salemi_2021Crisosto:2019fcj and future RBahre_2013PhysPotIAXOAbeln:2021Armengaud:2014gea laboratory experiments.
• Figure 2: Modeling of the transverse component of the solar atmospheric magnetic field (top) Rempel_2014ps2017, and contributions to axion plasma frequency (bottom) for the photosphere (orange), chromosphere (yellow), and corona (green) regions as function of height in units of the solar radius.
• Figure 3: NuSTAR's observed spectra for module A (left) and B(right) in the signal (red) and area normalized background (cyan) regions. Background-subtracted total spectrum (bottom row) and 95% CL spectral shape of the axion component folded through the instrument response for an exemplary axion mass of $m_a = 10^{-7}$ eV is shown.
• Figure A: Top: Solar axion surface luminosity depending on energy and the radius $r/R_{\odot}$ on the solar disk. The flux ($\frac{dN_{a}/g^{2}_{10}}{dE\, dA\,dt\,dA_\odot}$) is given in units of axions $\rm{keV^{-1}cm^{-2}s^{-1}}$ per unit surface area on the solar disk. Bottom: Differential solar axion spectrum, derived by integrating the model shown on the top up to different values of $r/R_{\odot}$ in units of the solar radius $R_{\odot}$.
• Figure B: Soft X-ray images of the full solar disk from the Hinode X-Ray Telescope2008ApJ...672.1237L2011AA...526A..78K. The 2019 image (top) shows an active region near the disk center. The image at the bottom, during the NuSTAR observations, shows only a few X-ray bright points.