Constraining axions with ZTF J 1901 +1458

The axion-nucleon coupling enables the production of axions through the decay of excited ⁵⁷Fe nuclei, and axions produced in the Sun through this process are often a target of helioscope searches. We show for the first time that hot, highly magnetic white dwarfs such as ZTF J 1901 +1458 are a viable target to search for the x-ray signature of axions that were produced by the ⁵⁷Fe transition in the core and then converted to photons in the magnetosphere. We calculate that a 100 ks observation of ZTF J 1901 +1458 with NuSTAR would constrain the coupling of axions to nucleons and photons at a level below the bounds of both current and future planned helioscopes.