A Necessary Condition for the Submergence of Proto-Neutron Star Magnetic Fields by Supernova Fallback

TL;DR

This paper investigates the conditions under which supernova fallback can submerge proto-neutron star magnetic fields, using simulations and semi-analytic models to identify a necessary criterion for magnetic field submergence.

Contribution

The study introduces a semi-analytic model linking fallback dynamics and shock stalling to establish a necessary condition for magnetic field submergence in proto-neutron stars.

Findings

Identified a critical fallback mass and timescale for magnetic field submergence.

Developed a semi-analytic model for shock stalling and crust formation.

Provided a criterion to predict magnetic field burial in neutron stars.

Abstract

Central compact objects (CCOs) are a subclass of neutron stars with a dipole magnetic field strength considerably weaker than those of radio pulsars and magnetars. One possible explanation for such weak magnetic fields in the CCOs is the hidden magnetic field scenario, in which supernova fallback submerges the magnetosphere of a proto-neutron star beneath a newly formed crust. However, the fallback mass and timescale required for this submergence process remain uncertain. We perform one-dimensional general relativistic magnetohydrodynamic simulations of the supernova fallback onto a magnetized proto-neutron star, while considering neutrino cooling. In our simulations, the infalling material compresses the magnetic field and drives a strong shock. The shock initially expands outward, but eventually stalls and recedes as neutrino cooling becomes significant. After the shock stalls, the gas density above the magnetosphere increases rapidly, potentially leading to the formation of a new crust. To understand the shock dynamics, we develop semi-analytic models that describe the resulting magnetospheric and shock radii when the shock stalls. By comparing the fallback time scale with the shock stalling time scale, corresponding to the waiting time for the new crust formation, we derive a necessary condition for the submergence of the PNS magnetic field. Our results will provide guidance for investigating the diversity of young isolated neutron stars through multidimensional simulations.