Magnetic Field-Decay-Induced Electron Captures: a Strong Heat Source in Magnetar Crusts

We propose a new heating mechanism in magnetar crusts. Magnetars’ crustal magnetic fields are much stronger than their surface fields; therefore, magnetic pressure partially supports the crust against gravity. The crust loses magnetic pressure support as the field decays and must compensate by increasing the electron degeneracy pressure; the accompanying increase in the electron Fermi energy induces nonequilibrium, exothermic electron captures. The total heat released via field-decay electron captures is comparable to the total magnetic energy in the crust. Thus, field-decay electron captures are an important, if not the primary, mechanism powering magnetars’ soft X-ray emission.

💡 Research Summary

The paper introduces a novel heating mechanism for magnetar crusts that operates through magnetic‑field‑decay‑induced electron captures. Magnetars are known to possess interior magnetic fields that can be an order of magnitude larger than the surface dipole fields inferred from timing measurements. In the solid crust, the magnetic pressure (P_{\rm mag}=B^{2}/8\pi) contributes significantly to the hydrostatic support against gravity, sharing the load with electron degeneracy pressure (P_{\rm e}) and the pressure of the nuclear lattice. As the magnetic field decays over (\sim10^{4-5}) yr, the magnetic pressure component diminishes. To maintain equilibrium, the crust must increase its electron degeneracy pressure, which raises the electron Fermi energy (E_{\rm F}).

When (E_{\rm F}) exceeds the threshold for a particular nuclear species, the previously stable nucleus becomes susceptible to electron capture:
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