NuSTAR as an Axion Helioscope: probing axion-nucleon and axion-electron couplings

This paper utilizes NuSTAR solar X-ray observations from the 2020 solar minimum to derive significantly improved 95% confidence level limits on axion-nucleon and axion-electron couplings for axion masses below $10^{-6}$ eV, establishing solar X-ray monitoring as a powerful method for axion searches.

The Gist

Imagine the Sun as a giant, glowing factory. For decades, scientists have been trying to find out if this factory is secretly producing invisible, ghost-like particles called axions. These particles are hypothetical; they were invented to solve a mystery in physics, but nobody has ever seen one.

This paper is like a new detective story where the researchers use a space telescope called NuSTAR to catch these ghosts in the act.

Here is the story of how they did it, explained simply:

1. The Invisible Factory Workers (Axion Production)

Inside the Sun's hot core, energy is constantly being made. Usually, this energy stays as light (photons). But the scientists suspect that sometimes, this light might turn into axions.

In previous studies, scientists only looked for one specific way this could happen (called the "Primakoff effect"). But this paper says, "Wait, there might be other ways!"

• The Electron Route: Imagine axions being born when they bump into electrons (tiny charged particles) inside the Sun.
• The Nucleon Route: Imagine axions being born when they interact with the heavy cores of atoms (protons and neutrons).

The paper calculates how many of these "ghosts" would be created if these interactions exist.

2. The Magnetic Trap (Conversion)

Here is the tricky part: Axions are invisible. You can't see them with a telescope. So, how do you catch them?

The researchers use the Sun's own atmosphere as a trap. The Sun has a giant, invisible magnetic field surrounding it, like a massive, invisible net.

• When the invisible axions fly out of the Sun and hit this magnetic net, there is a small chance they will turn back into X-ray light.
• Think of it like a ghost hitting a magic wall and suddenly turning into a flash of light that you can see.

3. The Detective Work (NuSTAR)

The team used the NuSTAR telescope, which is like a super-sensitive X-ray camera floating in space. They pointed it at the Sun during a time when the Sun was very quiet (in 2020).

They looked for a specific "glow" in the X-ray data.

• The Background Noise: The sky is full of random X-rays from space, and the telescope itself makes some noise. It's like trying to hear a whisper in a crowded, noisy room.
• The Signal: If axions exist, they would create a specific pattern of X-rays—a little extra glow that shouldn't be there.

4. The Results: "No Ghosts Found, But We Got Better at Hunting"

The team looked at the data and said, "We didn't see the ghost."

However, in science, finding nothing is actually a huge victory. Because they didn't see the signal, they can now say: "If axions exist, they must be even weaker or rarer than we thought."

They set new, very strict "speed limits" on how strong the axion's connection to electrons and atomic nuclei can be.

• The Electron Limit: They proved that the connection between axions and electrons is weaker than a very specific, tiny number.
• The Nucleon Limit: They proved the connection between axions and atomic nuclei is also weaker than a specific limit.

5. Why This Matters

The paper claims these new limits are much better than what ground-based experiments (like the CAST experiment in a cave in Europe) have found so far.

• The Analogy: Imagine previous experiments were trying to find a needle in a haystack using a metal detector that was a bit rusty. This new study used a high-tech laser scanner that can see the needle from space. Even though they still didn't find the needle, they proved that if it is there, it's hiding in a much smaller, more specific spot than we thought.

The Special "Fingerprint"

The paper also mentions a special "fingerprint" they are looking for. If axions are created by the heavy iron atoms in the Sun, they would produce a very sharp, single note of X-ray energy (14.4 keV), like a pure musical tone. This is different from the "static noise" of other types of axions. If they ever find this pure tone in the future, it would be a smoking gun proof that axions exist.

Summary

In short, this paper says: "We used a space telescope to look at the Sun's magnetic field, hoping to see invisible particles turn into light. We didn't see them, but we proved that if they are there, they are much more elusive than we previously believed. This makes our search for them much more precise."

Technical Summary

Technical Summary: NuSTAR as an Axion Helioscope

Problem and Motivation
Axions and axion-like particles (ALPs) are hypothetical pseudoscalars proposed to solve the strong CP problem and serve as dark matter candidates. While their coupling to photons ($g_{a\gamma}$) is well-studied via helioscopes like the CERN Axion Solar Telescope (CAST), generic ALP models also predict couplings to electrons ($g_{ae}$) and nucleons ($g_{aN}$). Previous analyses of solar X-ray data, specifically from the Nuclear Spectroscopic Telescope Array (NuSTAR), focused exclusively on the Primakoff production mechanism (photon-to-axion conversion in the solar core). However, these studies did not account for axion production mediated by axion-electron or axion-nucleon interactions, which could dominate the solar axion flux in specific parameter regimes. This work addresses that gap by extending the NuSTAR analysis to include these additional production channels, thereby probing the product of couplings $g_{ae} \cdot g_{a\gamma}$ and $g_{aN} \cdot g_{a\gamma}$.

Methodology
The study utilizes X-ray observations of the quiet Sun taken by NuSTAR on February 21, 2020, during the solar minimum. The analysis focuses on two distinct regions: a source region (a circle of radius $0.1 R_\odot$ centered on the Sun) and a background region (an annulus from $0.15 R_\odot$ to $0.30 R_\odot$).

1. Axion Production Models:
   • Primakoff Effect: Thermal photons in the solar core convert to axions via interaction with electric fields of ions and electrons.
   • Axion-Electron Coupling: Production occurs via bremsstrahlung, Compton scattering, axio-deexcitation, and axio-recombination (BCA processes).
   • Axion-Nucleon Coupling: Production is dominated by nuclear transitions, specifically the M1 magnetic transition of $^{57}\text{Fe}$ at 14.4 keV, as nucleon-nucleon bremsstrahlung is suppressed in the solar interior.
2. Conversion Mechanism: Produced axions travel to the solar atmosphere, where they may convert back into X-ray photons via the Primakoff effect in the solar magnetic field. The conversion probability is calculated using models for the solar magnetic field (combining magneto-convection simulations for the photosphere and MHD simulations for the corona) and plasma density profiles.
3. Data Analysis: The analysis covers an energy range of 4–15 keV, extending the previous 4–11 keV window to capture the 14.4 keV $^{57}\text{Fe}$ line. The expected photon signal is modeled as a function of the couplings. A Bayesian statistical framework is employed, treating the observed counts in the source and background regions as Poisson variables. The likelihood is constructed to constrain the product of couplings ($g_{a\gamma} \cdot g_{aY}$) in regimes where the Primakoff flux is subdominant, and to map the full $(g_{a\gamma}, g_{aY})$ plane.

Key Contributions

• Extension of Production Channels: The paper is the first to incorporate axion production via electron and nucleon couplings into the NuSTAR solar axion search framework.
• Broadened Energy Range: By extending the analysis up to 15 keV, the study captures the harder spectral components relevant for nucleon-coupled production and the specific 14.4 keV line from $^{57}\text{Fe}$.
• Robust Systematics: The authors emphasize that using the Sun as a target offers better control over systematic uncertainties compared to other astrophysical probes (e.g., white dwarfs or supernovae), as solar structure and magnetic fields are comparatively well-constrained.

Results
The analysis derives 95% Confidence Level (CL) upper limits on the relevant coupling products for axion masses $m_a \lesssim 10^{-6}$ eV:

• Axion-Electron Coupling: The product of couplings is constrained to $g_{ae} \cdot g_{a\gamma} \lesssim 1.1 \times 10^{-24} \text{ GeV}^{-1}$.
• Axion-Nucleon Coupling: The product of couplings is constrained to $g_{aN}^{\text{eff}} \cdot g_{a\gamma} \lesssim 2.3 \times 10^{-19} \text{ GeV}^{-1}$.

These constraints are presented in the $(g_{a\gamma}, g_{ae})$ and $(g_{a\gamma}, g_{aN}^{\text{eff}})$ planes, showing exclusion regions that interpolate between the Primakoff-dominated regime and the electron/nucleon-coupling-dominated regimes. The results significantly improve upon current laboratory limits from CAST and CUORE in the specified mass range, approaching the projected sensitivity of the next-generation helioscope IAXO for the nucleon channel.

Significance
The paper establishes solar X-ray observations as a powerful and robust method for axion searches, particularly for constraining couplings to electrons and nucleons. The derived limits probe a substantial region of parameter space currently inaccessible to ground-based experiments. While some astrophysical bounds (e.g., from white dwarfs or supernovae) reach deeper into the parameter space, the authors note that these rely on less certain stellar modeling. In contrast, the NuSTAR limits offer a systematics-controlled complement to these probes. The 2020 dataset is identified as likely remaining the best available X-ray satellite data for solar axion studies until the next solar minimum around 2030.