The Soft Gamma Repeaters as Very Strongly Magnetized Neutron Stars. II. Quiescent Neutrino, X‐Ray, and Alfven Wave Emission

TLDR

The study computes the quiescent X‑ray, neutrino, and Alfvén wave emission from highly magnetized neutron stars (B_dipole ≈10^14–10^15 G, B_interior ≈5–10×10^15 G) and compares the predictions with observed quiescent emission from soft gamma repeaters and anomalous X‑ray pulsars. The model attributes the quiescent emission to magnetic‑field decay, which drives crustal fracturing, core heating, and anisotropic thermal conduction, with the decay rate strongly temperature‑dependent and capable of powering both persistent X‑ray output and occasional soft‑gamma‑repeater bursts. The model predicts that these sources should exhibit no optical emission from X‑ray–heated accretion disks.

Abstract

We calculate the quiescent X-ray, neutrino, and Alfvén wave emission from a neutron star with a very strong magnetic field, Bdipole ~ 1014 − 1015 G and Binterior ~ (5–10) × 1015 G. These results are compared with observations of quiescent emission from the soft gamma repeaters and from a small class of anomalous X-ray pulsars that we have previously identified with such objects. The magnetic field, rather than rotation, provides the main source of free energy, and the decaying field is capable of powering the quiescent X-ray emission and particle emission observed from these sources. New features that are not present in the decay of the weaker fields associated with ordinary radio pulsars include fracturing of the neutron star crust, strong heating of its core, and effective suppression of thermal conduction perpendicular to the magnetic field. As the magnetic field is forced through the crust by diffusive motions in the core, multiple small-scale fractures are excited, as well as a few large fractures that can power soft gamma repeater bursts. The decay rate of the core field is a very strong function of temperature and therefore of the magnetic flux density. The strongest prediction of the model is that these sources will show no optical emissions associated with X-ray heating of an accretion disk.