X-ray Searches for Axions from Nearby Isolated Neutron Stars

Axions may be produced thermally inside the cores of neutron stars (NSs), escape the stars due to their weak interactions with matter, and subsequently convert into X-rays in the magnetic fields surrounding the NSs. I will describe a hard X-ray search from 2 - 8 keV for X-rays arising from this emission mechanism from the nearby Magnificent Seven isolated NSs using archival XMM-Newton and Chandra data. This search leads to the strongest limits to-date on the product of the axion-photon and axion-nucleon couplings for axion masses below ~1e−4 eV. Moreover, I will show that an observed excess of hard X-rays from the Magnificent Seven may arise from axions. I will discuss near-term measurements and analyses to help rule out or confirm this possibility, including preliminary results from a search with NuSTAR data for axion-induced X-rays from nearby star clusters.