Limits on the Axion-Photon Coupling from Chandrayaan-2

TL;DR

This work investigates solar axions produced in the Sun and converted into x‑rays in the solar magnetic field, using Chandrayaan‑2 XSM observations of the quiet Sun to constrain the axion–photon coupling $g_{aγγ}$ for low axion masses ($m_a o$ a few $10^{-4}$ eV). The authors model solar axion production via Primakoff, bremsstrahlung, and Compton channels, compute the axion‑photon conversion probability $P_{aγ}$ through the solar atmosphere, and compare the predicted axion‑induced x‑ray signal to the observed quiet‑Sun spectrum with three background subtraction schemes. A Bayesian analysis with a uniform prior on $g_{aγγ}$ yields 95% credible upper limits on $g_{aγγ}$ across $m_a$ up to $5 imes10^{-4}$ eV, giving $(0.47 ext{--}2.2) imes10^{-10}$ GeV$^{-1}$ depending on the background treatment. The results are competitive with CAST and NuSTAR and offer a complementary, disk‑integrated solar constraint, motivating future dedicated solar X‑ray missions to push sensitivity further.

Abstract

Axions and axion-like particles (ALPs) have gained immense attention in searches for beyond Standard Model (BSM) physics. Experiments searching for axions leverage their predicted couplings to Standard Model (SM) particles to look for observable signals. Though weak, these couplings allow axions to be produced abundantly in the interiors of stars such as the Sun. Once created, axions can escape the Sun and while passing through the solar atmosphere, oscillate into photons in the magnetic field producing x-rays. For the first time, we used data from the observation of soft x-rays from the quiet Sun during the 2019-20 solar minimum by the solar x-ray monitor (XSM), onboard India's Chandrayaan-2 lunar exploration mission, to constrain the coupling of axions to photons ($g_{a γγ}$). Using the latest models of the solar atmosphere to calculate the magnetic field and plasma frequency, we constrain $g_{a γγ} \lesssim (0.47\,-\,2.2) \times 10^{-10}$ GeV$^{-1}$ at 95% confidence level for axion masses $m_a \lesssim 5 \times 10^{-4}$eV.

Figures (4)

• Figure 1: The solar axion flux at the solar surface arising from the Primakoff process (red solid) and from the combined Bremsstrahlung and Compton processes (blue solid). The couplings are set to $g_{a \gamma \gamma} = 10^{-10}$ GeV$^{-1}$ and $g_{aee} = 1.3 \times 10^{-13}$.
• Figure 2: Top panel: Variation of the axion to photon conversion probability $P_{a \gamma}$ with distance from the solar surface $h$ for fixed $g_{a \gamma \gamma}$ and $E_a$ and for a range of $m_a$. Bottom panel: Variation of $P_{a \gamma}$ with $m_a$ and $E_a$ for a fixed $g_{a \gamma \gamma}$.
• Figure 3: (a) XSM observed solar x-ray spectrum and background spectrum when the Sun is not in FOV. (b) XSM observed solar x-ray spectrum after background subtraction with three different methods Top panel (Case 1): Only cosmic x-ray background subtraction along with avoiding bright x-ray source and energetic solar events (large SEP events), Center panel (Case 2): Measured XSM background (when the Sun is out of FOV) subtraction along with avoiding multiple bright x-ray sources and large SEP events, Bottom panel (Case 3): Empirically modeled background subtraction.
• Figure 4: 95% confidence level upper limits on $g_{a \gamma \gamma}$ from the three background subtracted solar x-ray spectrum as shown in Fig. \ref{['fig:xsm_data']}. Also shown are the existing upper limits from the CAST experiment CAST:2017uph, the NuStar analysis of solar x-ray Ruz:2024gkl and the projected upper limits from IAXO Armengaud:2014geaIAXO:2019mpb and ALPS-II Bahre:2013ywa helioscope experiments.