Emission of axions in strongly magnetized stars

We show that the axion decay constant does not receive any correction at any order from external magnetic fields. On the other hand, in the context of the Wilsonian effective action under external magnetic fields, the axial currents get a finite correction. We then calculate the effect of strong magnetic fields B>1018 G on the axion-nucleon coupling and find that magnetic fields enhance the axion emission rate. If B≳1020 G in strongly magnetized neutron or white dwarf stars, the effective four-quark interaction may become strong at low energy and enhance the emission rate by an order of magnitude.