Are pulsars born with a hidden magnetic field?

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 10 12 G can be buried by accreting only 10 −3 − 10 −2 M , 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.

Evolution of the surface magnetic field of rotating proto-neutron stars

“…We focus on both models that collapse to BHs and models where the PNS remains stable throughout the simulation. Although the simulations end too early for observing the crust formation [16] and the late time fall back of magnetized gas [10], we intend to get insights on the field structure of the very young neutron stars. To this end, we focus on the time evolution of the strengths of the poloidal and toroidal components of the surface field and analyze their multipole expansion in spherical harmonics in order to distinguish large-scale structures from the field fluctuating on small scales.…”

“…The time evolution of the rate and geometry of mass accretion and the magnetization of the accreted layers have an important impact on the accumulation of magnetic flux at the PNS surface. The picture thus generated may show a shell of enhanced magnetic field strength, but it is also possible for the magnetic field to be buried underneath additional layers of weakly magnetized gas [10]. These processes are complemented by the possible amplification of magnetic fields in the interior of the PNS, in particular if rotation and convection constitute a dynamo.…”

Determination of properties of protoneutron stars toward black hole formation via gravitational wave observations

“…For example, the recent three-dimensional (3D) numerical simulations for the core-collapse supernova show the existence of the gravitational wave signals associated with the oscillations of PNS (or core region) [15,18,19], whose frequency increases with time from a few hundred Hz up to ∼ kHz in the postbounce phase. This signal is considered either to arise from the fundamental (f -mode) oscillations of the PNS [20,21], the gravity (gmode) like oscillations of the whole region inside the shock radius [22,23], or to be the evidence of the Brunt-Väisälä frequency at the PNS surface (the so-called surface g-mode) [11,13]. Since the Brunt-Väisälä frequency is a local value, which is not associated with the global oscillations of the PNS, the gravitational wave signals in numerical simulations may be a result of the f -mode oscillations of the PNS or the g-mode like oscillations inside the shock radius.…”

“…Compared to the extensive studies of asteroseismology for cold neutron stars, the number of the studies of the PNS asteroseismology is relatively limited [20][21][22][23][36][37][38][39][40][41]. This may partially come from the difficulty for preparing the PNS model as a background for the linear analysis.…”

“…For the PNS asteroseismology in supernovae, two different approaches have been adopted so far. One approach is the way to construct the PNS model, whose surface is determined by a specific surface density [20,21,39,41], while another approach is that the whole region inside the shock radius is considered as a background [22,23]. For the first approach, one has to select the surface density and the eigenfrequencies of the PNS in the early phase (up to ∼ 500 ms) after core bounce depend on the selection of this density [20,21], but the boundary condition imposed at the surface is simple, i.e., the Lagrangian perturbation should be zero.…”