Are pulsars born with a hidden magnetic field?

TL;DR

This paper investigates how fallback accretion onto neutron stars can submerge their magnetic fields, explaining low observed magnetic fields in young neutron stars through a general relativistic model.

Contribution

It provides a detailed analysis of magnetic field burial in neutron stars due to accretion, showing that modest accreted mass can significantly weaken the surface magnetic field.

Findings

Magnetic fields of a few times 10^12 G can be buried by accreting 10^-3 to 10^-2 solar masses.

Weak magnetic fields in young neutron stars may be common due to accretion-induced burial.

The results support the idea that Central Compact Objects are not unusual, but rather a consequence of magnetic field submergence.

Abstract

The observation of several neutron stars in the center of supernova remnants and with significantly lower values of the dipolar magnetic field than the average radio-pulsar population has motivated a lively debate about their formation and origin, with controversial interpretations. A possible explanation requires the slow rotation of the proto-neutron star at birth, which is unable to amplify its magnetic field to typical pulsar levels. An alternative possibility, the hidden magnetic field scenario, considers the accretion of the fallback of the supernova debris onto the neutron star as responsible for the submergence (or screening) of the field and its apparently low value. In this paper we study under which conditions the magnetic field of a neutron star can be buried into the crust due to an accreting, conducting fluid. For this purpose, we consider a spherically symmetric calculation in general relativity to estimate the balance between the incoming accretion flow and the magnetosphere. Our study analyses several models with different specific entropy, composition, and neutron star masses. The main conclusion of our work is that typical magnetic fields of a few times 1e12 G can be buried by accreting only 1e-3 - 1e-2 solar mass, a relatively modest amount of mass. In view of this result, the Central Compact Object scenario should not be considered unusual, and we predict that anomalously weak magnetic fields should be common in very young (< few kyr) neutron stars.