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arxiv: 2605.26265 · v1 · pith:7GMLOYAMnew · submitted 2026-05-25 · 🌌 astro-ph.CO · astro-ph.HE· hep-ph

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

Tiziano Zanzarella , Francisco R. Cand\'on , Maurizio Giannotti , Marco Regis , Jaime Ruz , Marco Taoso , Elisa Todarello , Julia K. Vogel This is my paper

Pith reviewed 2026-06-29 20:08 UTC · model grok-4.3

classification 🌌 astro-ph.CO astro-ph.HEhep-ph
keywords axionsaxion-like particlessolar X-raysNuSTARhelioscopecoupling boundssolar magnetic field
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The pith

NuSTAR solar X-ray data sets new bounds g_ae g_aγ ≲ 1.1×10^{-24} GeV^{-1} and g_aN^eff g_aγ ≲ 2.3×10^{-19} GeV^{-1} for m_a below 10^{-6} eV.

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper examines whether axions produced inside the Sun via electron or nucleon interactions can be detected as X-rays after converting in the solar magnetic field. NuSTAR observations taken during the 2020 solar minimum show no excess flux, allowing the authors to translate the non-detection into upper limits on the products of the relevant couplings. These limits improve on existing laboratory constraints and position solar X-ray telescopes as a viable search channel. The work focuses on the regime where production is dominated by the electron or nucleon couplings rather than the photon coupling alone.

Core claim

In the regime where ALP production is dominated by couplings to electrons or nucleons, NuSTAR data yield bounds on the product of couplings g_ae·g_aγ ≲ 1.1×10^{-24} GeV^{-1} and g_aN^eff·g_aγ ≲ 2.3×10^{-19} GeV^{-1} at 95% CL, for axion masses m_a ≲10^{-6} eV.

What carries the argument

Production of axions in the solar interior through Primakoff, Compton and bremsstrahlung processes, followed by their reconversion into X-ray photons in the Sun's atmospheric magnetic field, with the predicted flux compared against NuSTAR measurements.

If this is right

  • The new bounds are stronger than current ground-based experimental limits on the same coupling products.
  • Solar X-ray observations constitute a robust and independent method for axion searches in the sub-micro-eV mass window.
  • Repeated or deeper solar exposures with similar telescopes can tighten the same coupling constraints without new hardware.

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • The same dataset could be reanalyzed with updated solar magnetic-field models to test sensitivity of the bounds.
  • Analogous searches around other stars with known surface fields could extend the method beyond the Sun.
  • If the production-rate modeling uncertainty is reduced, the same non-detection would translate into even stronger coupling limits.

Load-bearing premise

The calculated axion production rates inside the Sun and the photon reconversion probability in the solar magnetic field are accurate enough that the observed X-ray limits map directly onto coupling bounds.

What would settle it

Detection of a statistically significant excess X-ray flux above standard solar emission in the energy band expected for the axion mass range would directly contradict the reported coupling limits.

read the original abstract

We investigate solar X-ray observations as a probe of axions and axion-like particles. These particles can be produced in the interior of the Sun via the conversion of thermal photons, as well as through processes involving axion-electron and axion-nucleon interactions. The resulting axions can then reconvert into photons in the Sun's atmospheric magnetic field, generating a signal in the X-ray energy range. In this work, we derive new limits on axions using X-ray observations with the Nuclear Spectroscopic Telescope Array (NuSTAR) during the 2020 solar minimum. In the regime where ALP production is dominated by couplings to electrons or nucleons, we obtain bounds on the product of couplings $g_{ae}\cdot g_{a\gamma}\lesssim 1.1\times10^{-24}\,\rm GeV^{-1}$ and $g_{aN}^{\rm eff}\cdot g_{a\gamma}\lesssim 2.3\times 10^{-19}\,\rm GeV^{-1}$ at 95% CL, for axion masses $m_a\lesssim10^{-6}\,\rm eV$. These constraints strongly improve current ground-based experimental limits, establishing solar X-ray observations as a powerful and robust method for axion searches.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

2 major / 2 minor

Summary. The paper investigates solar X-ray observations with NuSTAR during the 2020 solar minimum as a probe of axions and axion-like particles. These particles are produced in the solar interior via Primakoff conversion of thermal photons as well as Compton and bremsstrahlung processes involving axion-electron or axion-nucleon couplings, then reconvert to photons in the solar atmospheric magnetic field. The central result is new 95% CL upper limits on the coupling products g_ae · g_aγ ≲ 1.1×10^{-24} GeV^{-1} and g_aN^eff · g_aγ ≲ 2.3×10^{-19} GeV^{-1} for m_a ≲ 10^{-6} eV, claimed to improve on existing ground-based limits.

Significance. If the adopted solar-interior production rates and atmospheric reconversion probabilities are accurate, the limits would strengthen constraints on axion couplings and demonstrate the value of repurposing solar X-ray data for particle-physics searches. The use of real observational data during solar minimum is a positive aspect of the approach.

major comments (2)
  1. [Abstract and limits derivation section] Abstract and the section deriving the numerical limits: the quoted bounds on g_ae·g_aγ and g_aN^eff·g_aγ are stated without any description of the background model, the treatment of the solar atmospheric B-field integral, or the statistical procedure that converts non-observation into 95% CL limits, so the support for these specific numbers cannot be assessed from the text.
  2. [Production and reconversion modeling section] Section describing axion production and reconversion: the expected signal is obtained by multiplying the solar production rate (scaling with g_ae² or g_aN²) by the reconversion probability (scaling with g_aγ²); no uncertainty budget or sensitivity study is provided for variations in the adopted solar temperature/density profiles or the line-of-sight B_perp integral, both of which directly rescale the predicted count rate and therefore shift the reported coupling-product bounds.
minor comments (2)
  1. Notation for g_aN^eff is introduced without an explicit definition or reference to the combination of nucleon couplings used.
  2. [Abstract] The energy range and exposure details of the NuSTAR data set used for the analysis are not stated in the abstract.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for the careful review and constructive comments. We agree that the original manuscript lacked sufficient detail in the areas identified and have revised the text accordingly to include the requested descriptions, modeling details, and sensitivity studies. Point-by-point responses follow.

read point-by-point responses

  1. Referee: [Abstract and limits derivation section] Abstract and the section deriving the numerical limits: the quoted bounds on g_ae·g_aγ and g_aN^eff·g_aγ are stated without any description of the background model, the treatment of the solar atmospheric B-field integral, or the statistical procedure that converts non-observation into 95% CL limits, so the support for these specific numbers cannot be assessed from the text.

    Authors: We agree that the original text did not provide adequate detail on these elements. In the revised manuscript we have expanded the limits derivation section (and made corresponding adjustments to the abstract) to explicitly describe: the background model (instrumental plus cosmic X-ray background estimated from off-Sun observations during solar minimum); the line-of-sight integration of the solar atmospheric B_perp field using the adopted magnetic field profile; and the statistical procedure (a binned likelihood analysis with Poisson statistics yielding 95% CL upper limits on the signal normalization given the non-observation). These additions allow the quoted numerical bounds to be assessed from the text. revision: yes

  2. Referee: [Production and reconversion modeling section] Section describing axion production and reconversion: the expected signal is obtained by multiplying the solar production rate (scaling with g_ae² or g_aN²) by the reconversion probability (scaling with g_aγ²); no uncertainty budget or sensitivity study is provided for variations in the adopted solar temperature/density profiles or the line-of-sight B_perp integral, both of which directly rescale the predicted count rate and therefore shift the reported coupling-product bounds.

    Authors: We acknowledge the absence of an uncertainty budget and sensitivity study in the original version. The revised manuscript now includes a new subsection that presents: (i) sensitivity studies varying the solar temperature and density profiles across standard solar models (e.g., GS98 and AGSS09); (ii) variations in the B_perp integral with uncertainties drawn from different solar magnetic field models; and (iii) a quantitative uncertainty budget showing the fractional impact on the predicted count rate and the consequent shifts in the derived coupling-product limits. This demonstrates the robustness of the reported bounds. revision: yes

Circularity Check

0 steps flagged

No circularity; bounds derived from external NuSTAR data via independent solar models

full rationale

The paper computes expected NuSTAR count rates from axion production (Primakoff/Compton/bremsstrahlung scaling with g_ae² or g_aN²) followed by reconversion (scaling with g_aγ²) in the solar atmosphere, then sets 95% CL upper limits on the products g_ae·g_aγ and g_aN^eff·g_aγ from non-observation in 2020 solar-minimum data. All inputs (solar profiles, magnetic field integrals, plasma frequency) are taken from external literature or standard models; the resulting limits are not fitted to any quantity defined in terms of themselves, nor obtained via self-citation chains, ansatz smuggling, or renaming of known results. The derivation chain is therefore self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The central claim rests on standard solar-model inputs for axion production and on a model of the solar atmospheric magnetic field; no new free parameters, axioms beyond domain standards, or invented entities are introduced in the abstract.

axioms (2)
  • domain assumption Standard solar model for axion production rates via electron and nucleon couplings
    Invoked to compute the axion flux from the solar interior.
  • domain assumption Model of solar atmospheric magnetic field for axion-photon reconversion
    Used to calculate the probability that axions convert back to X-ray photons.

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discussion (0)

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Reference graph

Works this paper leans on

43 extracted references · 30 canonical work pages · 7 internal anchors

  1. [1]

    R. D. Peccei and H. R. Quinn,CP Conservation in the Presence of Instantons,Phys. Rev. Lett.38(1977) 1440

    1977

  2. [2]

    R. D. Peccei and H. R. Quinn,Constraints Imposed by CP Conservation in the Presence of Instantons,Phys. Rev. D16(1977) 1791

    1977

  3. [3]

    Weinberg,A New Light Boson?,Phys

    S. Weinberg,A New Light Boson?,Phys. Rev. Lett.40(1978) 223

    1978

  4. [4]

    Wilczek,Problem of StrongPandTInvariance in the Presence of Instantons,Phys

    F. Wilczek,Problem of StrongPandTInvariance in the Presence of Instantons,Phys. Rev. Lett.40(1978) 279

    1978

  5. [5]

    The Low-Energy Frontier of Particle Physics

    J. Jaeckel and A. Ringwald,The Low-Energy Frontier of Particle Physics,Ann. Rev. Nucl. Part. Sci.60(2010) 405 [1002.0329]

    work page internal anchor Pith review Pith/arXiv arXiv 2010

  6. [6]

    P. W. Graham, I. G. Irastorza, S. K. Lamoreaux, A. Lindner and K. A. van Bibber, Experimental Searches for the Axion and Axion-Like Particles,Ann. Rev. Nucl. Part. Sci.65 (2015) 485 [1602.00039]

    work page internal anchor Pith review Pith/arXiv arXiv 2015

  7. [7]

    Giannotti,Status and Perspectives on Axion searches,PoSCOSMICWISPers2024 (2025) 033 [2412.08733]

    M. Giannotti,Status and Perspectives on Axion searches,PoSCOSMICWISPers2024 (2025) 033 [2412.08733]

    work page arXiv 2025

  8. [8]

    Raffelt and L

    G. Raffelt and L. Stodolsky,Mixing of the Photon with Low Mass Particles,Phys. Rev. D37 (1988) 1237

    1988

  9. [9]

    Sikivie,Experimental Tests of the Invisible Axion,Phys

    P. Sikivie,Experimental Tests of the Invisible Axion,Phys. Rev. Lett.51(1983) 1415. [10]CASTCollaboration, K. Altenm¨ uller et al.,New Upper Limit on the Axion-Photon Coupling with an Extended CAST Run with a Xe-Based Micromegas Detector,Phys. Rev. Lett.133 (2024) 221005 [2406.16840]

    work page arXiv 1983

  10. [10]

    E. D. Carlson and L.-S. Tseng,Pseudoscalar conversion and x-rays from the sun,Phys. Lett. B 365(1996) 193 [hep-ph/9507345]

    work page internal anchor Pith review Pith/arXiv arXiv 1996

  11. [11]

    Ruz et al.,NuSTAR as an Axion Helioscope,Phys

    J. Ruz et al.,NuSTAR as an Axion Helioscope,Phys. Rev. Lett.135(2025) 141001 [2407.03828]. [13]IAXOCollaboration, E. Armengaud et al.,Physics potential of the International Axion Observatory (IAXO),JCAP06(2019) 047 [1904.09155]

    work page arXiv 2025

  12. [12]

    S. Hoof, J. Jaeckel and L. J. Thormaehlen,Quantifying uncertainties in the solar axion flux and their impact on determining axion model parameters,JCAP09(2021) 006 [2101.08789]

    work page arXiv 2021

  13. [13]

    Redondo,Solar axion flux from the axion-electron coupling,Journal of Cosmology and Astroparticle Physics2013(2013) 008

    J. Redondo,Solar axion flux from the axion-electron coupling,Journal of Cosmology and Astroparticle Physics2013(2013) 008

    2013

  14. [14]

    Carenza, M

    P. Carenza, M. Giannotti, J. Isern, A. Mirizzi and O. Straniero,Axion astrophysics,Phys. Rept.1117(2025) 1 [2411.02492]

    work page arXiv 2025

  15. [15]

    Di Luzioet al., Eur

    L. Di Luzio et al.,Probing the axion–nucleon coupling with the next generation of axion helioscopes,Eur. Phys. J. C82(2022) 120 [2111.06407]

    work page arXiv 2022

  16. [16]

    De La Luz, A

    V. De La Luz, A. Lara, E. Mendoza and M. Shimojo,3D Simulations of the Quiet Sun Radio Emission at Millimeter and Submillimeter Wavelengths,Geofisica Internacional47(2008) 197

    2008

  17. [17]

    Berger, M. J. and Hubbell, J. H. and Seltzer, S. M. and Chang, J. and Coursey, J. S. and Sukumar, R. and Zucker, D. S and Olsen, K.,NIST Standard Reference Database 8 (XGAM),

  18. [18]

    Rempel,Numerical simulations of quiet sun magnetism: On the contribution from a small-scale dynamo,The Astrophysical Journal789(2014) 132

    M. Rempel,Numerical simulations of quiet sun magnetism: On the contribution from a small-scale dynamo,The Astrophysical Journal789(2014) 132

    2014

  19. [19]

    Miki´ c, C

    Z. Miki´ c, C. Downs, J. A. Linker, R. M. Caplan, D. H. Mackay, L. A. Upton, P. Riley, R. Lionello, T. T¨ or¨ ok, V. S. Titov, J. Wijaya, M. Druckm¨ uller, J. M. Pasachoff and W. Carlos, – 15 – Predicting the corona for the 21 august 2017 total solar eclipse,Nature Astronomy2(2018) 913–921

    2017

  20. [20]

    Riley, J

    P. Riley, J. A. Linker, Z. Miki´ c, R. Lionello, S. A. Ledvina and J. G. Luhmann,A Comparison between Global Solar Magnetohydrodynamic and Potential Field Source Surface Model Results, ApJ653(2006) 1510

    2006

  21. [21]

    Perri, S

    M. Perri, S. Puccetti, N. Spagnuolo, R. Ficcadenti, A. Davis, K. Forster, B. Grefenstette, F. Harrison and K. Madsen,The nustar data analysis software guide.version 1.9.7,

  22. [22]

    D. R. Wik, A. Hornstrup, S. Molendi, G. Madejski, F. A. Harrison, A. Zoglauer, B. W. Grefenstette, F. Gastaldello, K. K. Madsen, N. J. Westergaard, D. D. M. Ferreira, T. Kitaguchi, K. Pedersen, S. E. Boggs, F. E. Christensen, W. W. Craig, C. J. Hailey, D. Stern and W. W. Zhang,Nustar observations of the bullet cluster: Constraints on inverse compton emiss...

    2014

  23. [23]

    Arnaud and et al.,Xspec: An x-ray spectral fitting package,Astrophysics Source Code Library(1999) ascl

    K. Arnaud and et al.,Xspec: An x-ray spectral fitting package,Astrophysics Source Code Library(1999) ascl. [26]CASTCollaboration, K. Altenm¨ uller et al.,Search for solar axions produced through the axion-electron coupling gae using a new GridPix detector at CAST,JHEP12(2025) 009 [2505.05909]. [27]XENONCollaboration, E. Aprile et al.,Search for New Physic...

    work page arXiv 1999

  24. [24]

    D. Li, R. J. Creswick, F. T. Avignone and Y. Wang,Sensitivity of the CUORE detector to14.4 keV solar axions emitted by the M1 nuclear transition of 57Fe,JCAP02(2016) 031 [1512.01298]

    work page internal anchor Pith review Pith/arXiv arXiv 2016

  25. [25]

    Dessert, A

    C. Dessert, A. J. Long and B. R. Safdi,No Evidence for Axions from Chandra Observation of the Magnetic White Dwarf RE J0317-853,Phys. Rev. Lett.128(2022) 071102 [2104.12772]

    work page arXiv 2022

  26. [26]

    M. Xiao, P. Carenza, M. Giannotti, A. Mirizzi, K. M. Perez, O. Straniero and B. W. Grefenstette,Betelgeuse constraints on coupling between axionlike particles and electrons,Phys. Rev. D106(2022) 123019 [2204.03121]

    work page arXiv 2022

  27. [27]

    Ning and B

    O. Ning and B. R. Safdi,Probing the axion-electron coupling with NuSTAR observations of galaxies,Phys. Rev. D113(2026) 015021 [2503.09682]

    work page arXiv 2026

  28. [28]

    Mittal, T

    S. Mittal, T. Siegert, F. Calore, P. Carenza, L. Eisenberger, M. Giannotti, A. Lella, A. Mirizzi, D. Tsatsis and H. Yoneda,Search for Axion-like particles from nearby pre-supernova stars, Astron. Astrophys.707(2026) A108 [2512.19298]

    work page arXiv 2026

  29. [29]

    Fleury, A

    L. Fleury, A. Obertas, H. Richer and J. Heyl,Axion Constraints from White Dwarf Cooling in 47 Tucanae,2511.21676

    work page arXiv

  30. [30]

    M. L. Alberino, M. M. Miller Bertolami, M. E. Camisassa, A. Caputo and S. Torres,New axion bounds derived from the 100-parsec Gaia DR3 white dwarf luminosity function,2603.00901

    work page arXiv

  31. [31]

    Capozzi and G

    F. Capozzi and G. Raffelt,Axion and neutrino bounds improved with new calibrations of the tip of the red-giant branch using geometric distance determinations,Phys. Rev. D102(2020) 083007 [2007.03694]

    work page arXiv 2020

  32. [32]

    Straniero, C

    O. Straniero, C. Pallanca, E. Dalessandro, I. Dominguez, F. R. Ferraro, M. Giannotti, A. Mirizzi and L. Piersanti,The RGB tip of galactic globular clusters and the revision of the axion-electron coupling bound,Astron. Astrophys.644(2020) A166 [2010.03833]

    work page arXiv 2020

  33. [33]

    C. A. Manzari, Y. Park, B. R. Safdi and I. Savoray,Supernova Axions Convert to Gamma Rays in Magnetic Fields of Progenitor Stars,Phys. Rev. Lett.133(2024) 211002 [2405.19393]. – 16 –

    work page arXiv 2024

  34. [34]

    J. A. Grifols, E. Masso and R. Toldra,Gamma-rays from SN1987A due to pseudoscalar conversion,Phys. Rev. Lett.77(1996) 2372 [astro-ph/9606028]

    work page internal anchor Pith review Pith/arXiv arXiv 1996

  35. [35]

    J. W. Brockway, E. D. Carlson and G. G. Raffelt,SN1987A gamma-ray limits on the conversion of pseudoscalars,Phys. Lett. B383(1996) 439 [astro-ph/9605197]

    work page internal anchor Pith review Pith/arXiv arXiv 1996

  36. [36]

    Revisiting the SN1987A gamma-ray limit on ultralight axion-like particles

    A. Payez, C. Evoli, T. Fischer, M. Giannotti, A. Mirizzi and A. Ringwald,Revisiting the SN1987A gamma-ray limit on ultralight axion-like particles,JCAP02(2015) 006 [1410.3747]

    work page internal anchor Pith review Pith/arXiv arXiv 2015

  37. [37]

    Hoof and L

    S. Hoof and L. Schulz,Updated constraints on axion-like particles from temporal information in supernova SN1987A gamma-ray data,JCAP03(2023) 054 [2212.09764]

    work page arXiv 2023

  38. [38]

    Calore, P

    F. Calore, P. Carenza, M. Giannotti, J. Jaeckel and A. Mirizzi,Bounds on axionlike particles from the diffuse supernova flux,Phys. Rev. D102(2020) 123005 [2008.11741]

    work page arXiv 2020

  39. [39]

    F. R. Cand´ on, S. Ganguly, M. Giannotti, T. Kumar, A. Lella and F. Mescia,Fresh look at the diffuse ALP background from supernovae,Phys. Rev. D112(2025) 015006 [2505.05567]

    work page arXiv 2025

  40. [40]

    O. Ning, K. Raman and B. R. Safdi,Cosmological Neutron Stars Produce Diffuse Axion X-Ray Signatures,2512.15849

    work page arXiv

  41. [41]

    F. R. Cand´ on, P. Casaseca, M. Giannotti, M. Kaltschmidt, J. Ruz and J. K. Vogel,Probing the axion-nucleon coupling with supergiant stars,Phys. Rev. D112(2025) 123050 [2504.21107]

    work page arXiv 2025

  42. [42]

    O. Ning, A. Ray and B. R. Safdi,Axion lines from nuclear de-excitations in galactic stellar populations,Phys. Rev. D113(2026) 035010 [2509.03569]

    work page arXiv 2026

  43. [43]

    Carenza, T

    P. Carenza, T. Fischer, M. Giannotti, G. Guo, G. Mart´ ınez-Pinedo and A. Mirizzi,Improved axion emissivity from a supernova via nucleon-nucleon bremsstrahlung,JCAP10(2019) 016 [1906.11844]. [Erratum: JCAP 05, E01 (2020)]. – 17 –

    work page arXiv 2019