Photo-production of axions in Supernovae

Compact stellar objects like supernovae and neutron stars are believed to cool by emitting axions predominantly via axion bremsstrahlung ($NN\to NNa$), pion conversion (${\pi }^{-}{p}^{+}\to Na$) and photo-production ($\gamma N\to Na$). In this paper, we study in detail the photo-production channel, from the unavoidable anomaly induced Wess-Zumino-Witten term $\propto {\epsilon }^{\mu \nu \alpha \beta }{F}_{\mu \nu }{\mathrm{\partial }}_{\alpha }a{\omega }_{\beta }$ in conjunction with the low energy pion photo-production data. We found that for heavier axions, i.e., $m_a\sim \mathcal{O}(100)$ MeV, photo-production processes can be dominant compared to the usual axion emission processes. In addition, the spectrum of axions emitted in the process is significantly harder than those originating from bremsstrahlung.