Investigating magnetically-induced distortions of neutron stars through gamma-ray burst X-ray plateaus

TL;DR

This paper proposes a method to estimate magnetic distortions in neutron stars associated with gamma-ray bursts, providing new constraints on their magnetic field structures and discussing potential gravitational-wave tests.

Contribution

It introduces a novel approach to estimate magnetically-induced neutron star distortions from GRB data and constrains internal magnetic field configurations.

Findings

Relation between ellipticity and magnetic field strength established

Constraints on internal magnetic field structure derived

Potential for gravitational-wave testing discussed

Abstract

Magnetic field may distort neutron stars (NSs), but its effect has not been robustly tested through gravitational-wave observation yet due to the absence of a fast rotating Galactic magnetar. Part of gamma-ray bursts (GRBs) are potential to investigate the magnetically-induced distortion since their central objects may be millisecond magnetars. In this paper, we propose a method to estimate the distortions of these possible magnetars under GRB magnetar scenario. According to the case study of GRB 070521, we find a relation between the effective magnetically-induced ellipticity, $\epsilon_{\rm B,eff}$, and the effective dipole magnetic field strength on NS surfaces, $B_{\rm eff}$, namely $\epsilon_{\rm B,eff}\sim 10^{-3}(B_{\rm eff}/10^{15}\rm G)^{2}$. Furthermore, we constrain the internal magnetic-field structure of the magnetar to be $B_{\rm eff}\sim 0.02 <B_{\rm t}>$ and $B_{\rm eff}\sim 0.1B_{\rm t}$, where $<B_{\rm t}>$ is the volume-averaged internal toroidal field. The constraint may be used as the initial condition in modeling the structure of NS magnetospheres. At last, the possibility of testing the method shown in this paper through gravitational-wave observations is discussed.