[Paper Review] X-Ray Searches for Solar Axions

H. S. Hudson, L. W. Acton+4 more|arXiv (Cornell University)|Jan 22, 2012

Solar and Space Plasma Dynamics3 citations

TL;DR

This paper investigates X-ray searches for solar axions via their conversion to photons in the Sun's magnetic field, using data from Yohkoh, RHESSI, and Hinode. Despite no detection, the study sets competitive upper limits on the axion-photon coupling constant, reaching $ g_{10} \lesssim 0.2 \, \mu\text{eV} $ for axion masses below $ 10^{-4} \, \mu\text{eV} $, demonstrating the potential of solar X-ray telescopes for axion detection.

ABSTRACT

Axions generated thermally in the solar core can convert nearly directly to X-rays as they pass through the solar atmosphere via interaction with the magnetic field. The result of this conversion process would be a diffuse centrally-concentrated source of few-keV X-rays at disk center; it would have a known dimension, of order 10% of the solar diameter, and a spectral distribution resembling the blackbody spectrum of the solar core. Its spatial structure in detail would depend on the distribution of mass and field in the solar atmosphere. The brightness of the source depends upon these factors as well as the unknown coupling constant and the unknown mass of the axion; this particle is hypothetical and no firm evidence for its existence has been found yet. We describe the solar magnetic environment as an axion/photon converter and discuss the upper limits obtained by existing and dedicated observations from three solar X-ray observatories: Yohkoh, RHESSI, and Hinode

Motivation & Objective

• To search for solar axions by detecting their X-ray conversion signatures in the solar magnetic field using existing X-ray observatories.
• To assess the sensitivity of solar X-ray telescopes to axion-photon conversion, particularly in the quiet Sun and active regions.
• To establish upper limits on the axion-photon coupling constant $ g_{a\gamma} $ based on non-detection in deep X-ray observations.
• To explore the feasibility of detecting axion signals using temporal and spectral signatures, especially during sunspot transits.
• To highlight the role of solar magnetic field structure and atmospheric density in determining axion conversion efficiency and detection sensitivity.

Proposed method

• Utilizes the Primakoff effect, where solar axions convert to X-ray photons in the presence of magnetic fields in the solar atmosphere.
• Analyzes archival X-ray data from three observatories: Yohkoh/SXT, RHESSI, and Hinode/XRT, focusing on diffuse X-ray emission near solar disk center.
• Applies spectral and histogram-based analysis techniques to identify potential axion-induced X-ray signals above background noise.
• Estimates conversion efficiency based on assumed magnetic field configurations, including dipole and horizontal fields, with field strength and scale height as key parameters.
• Uses Monte Carlo simulations and background modeling to distinguish axion signals from instrumental noise and astrophysical backgrounds (e.g., Compton scattering, neutron-induced events).
• Compares observed X-ray fluxes to theoretical predictions for axion conversion, using the solar core blackbody spectrum as a reference.

Experimental results

Research questions

• RQ1 Can X-ray telescopes detect axion-to-photon conversion in the solar atmosphere via the Primakoff effect?
• RQ2 What are the upper limits on the axion-photon coupling constant $ g_{a\gamma} $ derived from existing solar X-ray observations?
• RQ3 How do uncertainties in solar magnetic field structure and atmospheric density affect the detectability of axion signals?
• RQ4 Can temporal signatures from sunspot transits enhance sensitivity to axion signals beyond diffuse emission searches?
• RQ5 How do the limits from solar X-ray observations compare with those from laboratory-based helioscopes like CAST?

Key findings

• No definitive signal of solar axion emission was detected in data from Yohkoh, RHESSI, or Hinode.
• The most stringent upper limit on the axion-photon coupling constant $ g_{10} $ (in units of $ \mu\text{eV} $) is $ 0.2 \, \mu\text{eV} $, derived from Hinode/XRT histogram analysis at 1.53 keV.
• RHESSI's off-point observation yields a limit of $ g_{10} \leq 1.8 \, \mu\text{eV} $, with a flux limit of 340 ph cm⁻² s⁻¹ keV⁻¹ at 5 keV.
• Hinode/XRT data yield a flux limit of 0.01 ph cm⁻² s⁻¹ keV⁻¹ at 1.53 keV, corresponding to a $ g_{10} $ limit of 0.2 $ \mu\text{eV} $.
• The Yohkoh/AlMg observation yields a flux limit of 1.2 ph cm⁻² s⁻¹ keV⁻¹ at 2.04 keV, corresponding to a $ g_{10} $ limit of 0.4 $ \mu\text{eV} $.
• The study demonstrates that solar X-ray telescopes can achieve axion coupling limits competitive with laboratory-based helioscopes for axion masses below $ 10^{-4} \, \mu\text{eV} $.