Search for a solar-bound axion halo using the Global Network of Optical Magnetometers for Exotic physics searches

TL;DR

This study targets a gravitationally bound solar axion halo around the Sun by analyzing GNOME data for a global, directionally modulated pseudo-magnetic field arising from axion gradient couplings to proton spins. A physically motivated halo model with a ground-state field oscillation at $\omega_a$ and an exponential radial profile informs a daily-modulated signal template, which is cross-correlated across 12 GNOME stations. The analysis finds no evidence for such a halo and places 95% CL upper limits on the axion induced pseudo-magnetic field across $0.05$–$20$ Hz, translating these limits into constraints on axion proton couplings for two halo density benchmarks and showing substantial strengthening for the quadratic coupling relative to astrophysical bounds. The work demonstrates the viability of cross station daily-modulation analyses in a global magnetometer network and points to future gains with Advanced GNOME comagnetometers that could probe proton and neutron spin couplings with orders of magnitude greater sensitivity, expanding searches for solar halos and other ultralight dark matter scenarios.

Abstract

We report on a search for a gravitationally bound solar axion halo using data from the Global Network of Optical Magnetometers for Exotic physics searches (GNOME), a worldwide array of magnetically shielded atomic magnetometers with sensitivity to exotic spin couplings. Motivated by recent theoretical work suggesting that self-interacting ultralight axions can be captured by the Sun's gravitational field and thermalize into the ground state, we develop a signal model for the pseudo-magnetic fields generated by axion-proton gradient couplings in such a halo. The analysis focuses on the fifth GNOME Science Run (69 days, 12 stations), employing a cross-correlation pipeline with time-shifted daily modulation templates to search for the global, direction-dependent, monochromatic signal expected from a solar axion halo. No statistically significant candidate signals are observed. We set 95% confidence-level upper limits on the amplitude of the axion-induced pseudo-magnetic field over the frequency range 0.05 Hz to 20 Hz, translating to constraints on the linear and quadratic axion-proton couplings for halo densities predicted by gravitational capture models and for the maximum overdensities allowed by planetary ephemerides. In the quadratic coupling case, our limits surpass existing astrophysical bounds by over two orders of magnitude across much of the accessible parameter space.

Figures (18)

• Figure 1: The potential enhanced density at the position of the Earth for a solar axion halo as compared to the average local dark matter density $\rho{_{\hbox{\scriptsize dm}}}$ (dashed blue line). The black line represents the maximum solar axion halo mass density, $\rho{_{\hbox{\scriptsize halo}}}({\rm{max}})$, allowed by gravitational constraints as evaluated in Refs. banerjee2020relaxionbanerjee2020searching. The purple line represents the solar axion halo mass density, $\rho{_{\hbox{\scriptsize halo}}}({\rm{quartic}})$, predicted from gravitational capture of axions by the Sun where the dissipation mechanism is provided by a quartic self-interaction of axions as described in detail in Ref. budker2023generic.
• Figure 2: Map and list of locations of presently active GNOME stations (summer 2025).
• Figure 3: Schematic diagram of the Earth moving through a solar axion halo (purple shaded region). The pseudo-magnetic field associated with a solar axion halo coupling to atomic spins has both a radial component in the $-\hat{\boldsymbol{r}}$ direction due to the spatial gradient and a transverse component due to the "axion wind" directed along the axion halo's relative velocity with respect to the lab frame, $\hat{\boldsymbol{k}}$.
• Figure 4: The solid lines show the maximum possible pseudo-magnetic field amplitudes from a solar axion halo as a function of signal frequency based on Eqs. \ref{['eq:Bpl-rad']} -- \ref{['eq:Bpq-tan']}, the dashed lines show the maximum possible pseudo-magnetic field based on the capture model of Ref. budker2023generic. For the solid lines the solar axion halo density is set to its maximum value $\rho{_{\hbox{\scriptsize halo}}}({\rm{max}})$ and for the dashed lines the solar axion halo density is set to the model value $\rho{_{\hbox{\scriptsize halo}}}({\rm{quartic}})$ as shown in Fig. \ref{['Fig:max-solar-ALP-halo-density']}. The upper plot shows the pseudo-magnetic field amplitudes for the radial (blue lines) and transverse (red lines) components of the linear gradient interaction, $\mathcal{B}_{p,l}^{(r)}$ and $\mathcal{B}_{p,l}^{(\perp)}$, respectively. The coupling is set to the strongest value consistent with astrophysical limits, corresponding to $f_l \approx 10^8~{\rm{GeV}}$. The lower plot shows the pseudo-magnetic field amplitudes for the radial (blue lines) and transverse (red lines) components of the quadratic gradient interaction, $\mathcal{B}_{p,q}^{(r)}$ and $\mathcal{B}_{p,q}^{(\perp)}$, respectively. The coupling is set to the strongest value consistent with astrophysical limits, corresponding to $f_q \approx 10^4~{\rm{GeV}}$. Note the different vertical scales on the two plots: the maximum possible pseudo-magnetic field amplitude for the quadratic interaction is orders of magnitude larger than that for the linear interaction.
• Figure 5: Schematic flow chart diagramming key elements of data collection and pre-processing as described in the text.