Solar Axions from Nuclear Transitions

We investigate the possibility of detecting 14.4 keV and 9.4 keV solar axions and axion-like particles that could be produced in the M1 nuclear transitions of $^{57}$Fe and $^{83}$Kr, respectively. To do so, we used data from soft X-ray observations of the quiet Sun collected by the Solar X-ray Monitor (XSM) on board India's Chandrayaan-2 lunar mission. We observe that although the effective axion-nucleon couplings for $^{83}$Kr and $^{57}$Fe differ only slightly, their fluxes differ by nearly three orders of magnitude. Consequently, the limit on $|g_{aN}^{\rm eff} \times g_{a\gamma\gamma}|$ and only $g_{a\gamma\gamma}$ vs. $m_a$ provide more than an order-of-magnitude stronger constraint for Fe than for Kr.