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On the origin of the strong internal magnetic fields of central compact objects

Kazım Yavuz Ekşi, İrem Bakır

Abstract

Central compact objects are radio-quite young neutron stars associated with supernova remnants. They have relatively small dipole fields, $B_{\rm p} \sim 10^{10}\,{\rm G}$ as inferred from their spin parameters. X-ray observations and theoretical arguments imply the presence of stronger internal magnetic fields. We argue that the dipole fields of these objects are very close to what they had inherited from the \textit{core} of the progenitor by flux conservation and their small initial rotation frequency does not allow for the $α$-process to enhance their poloidal fields. Although a full-fledged dynamo process can not proceed, relatively strong toroidal magnetic fields, $B_φ\sim 10^{13}\,{\rm G}$, can be generated from the seed poloidal fields via the $Ω$-effect in the proto-neutron star stage. We present a simplistic model for these processes and further speculate that the reason why these objects are born relatively slow-rotating is that they were not spun-up by acquiring angular momentum from the fallback matter.

On the origin of the strong internal magnetic fields of central compact objects

Abstract

Central compact objects are radio-quite young neutron stars associated with supernova remnants. They have relatively small dipole fields, as inferred from their spin parameters. X-ray observations and theoretical arguments imply the presence of stronger internal magnetic fields. We argue that the dipole fields of these objects are very close to what they had inherited from the \textit{core} of the progenitor by flux conservation and their small initial rotation frequency does not allow for the -process to enhance their poloidal fields. Although a full-fledged dynamo process can not proceed, relatively strong toroidal magnetic fields, , can be generated from the seed poloidal fields via the -effect in the proto-neutron star stage. We present a simplistic model for these processes and further speculate that the reason why these objects are born relatively slow-rotating is that they were not spun-up by acquiring angular momentum from the fallback matter.
Paper Structure (8 sections, 14 equations, 3 figures, 1 table)

This paper contains 8 sections, 14 equations, 3 figures, 1 table.

Table of Contents

  1. Introduction
  2. Model equations
  3. Results
  4. Discussion
  5. Implications for the putative neutron star in SN87A
  6. Implication for the CCO in Cassiopeia A
  7. Population-level implications
  8. Future observational tests

Figures (3)

  • Figure 1: Evolution of the magnetic fields, $B_{\rm p}$, $B_{\phi}$, and the shear rate, $q$. (Top panel) Results for the $P_{\infty}=105\,{\rm ms}$ pulsar PSR J1852+0040 with the initial conditions of $B_{\phi}(t=0)=B_{\rm p}(t=0)= 3.9\times 10^{9}\,{\rm G}$ and $P_{0}=1.2\,{\rm s}$. (Bottom panel) Results for the $P_{\infty}=424\,{\rm ms}$ pulsar PSR J1210-5226 with the initial conditions of $B_{\phi}(t=0)=B_{\rm p}(t=0)= 1.2\times 10^{10}\,{\rm G}$ and $P_{0}=5.0\,{\rm s}$. In all models, we assumed $q_0 = 0.2$ and $a=10^{3}$.
  • Figure 2: Effect of the final rotational period, $P_\infty$, on the saturation values of poloidal field, $B_{\rm p,\infty}$ and toroidal field $B_{\phi,\infty}$. Here, the initial fields are $B_\phi (t=0)=3.9\times10^{9}\,\rm G$ and $B_{\rm p} (t=0)=3.9\times10^{9}\,\rm G$.
  • Figure 3: (Left panel) Evolution of the angular velocity, $\Omega$, for PSR J1852+0040 and PSR J1210-5226. (Right panel) Evolution of mass, $M$, and radius, $R$, for three CCOs.