We propose a new heating mechanism in magnetar crusts. Magnetars' crustalmagnetic fields are much stronger than their surface fields; therefore,magnetic pressure partially supports the crust against gravity. The crust losesmagnetic pressure support as the field decays and must compensate by increasingthe electron degeneracy pressure; the accompanying increase in the electronFermi energy induces nonequilibrium, exothermic electron captures. The totalheat released via field-decay electron captures is comparable to the totalmagnetic energy in the crust. Thus, field-decay electron captures are animportant, if not the primary, mechanism powering magnetars' soft X-rayemission.
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