UNTWISTING MAGNETOSPHERES OF NEUTRON STARS

By A. Beloborodov. 2008

212 citations recorded by the source corpus.

Magnetospheres of neutron stars are anchored in the rigid crust and can be twisted by sudden crustal motions (“starquakes”). The twisted magnetosphere does not remain static and gradually untwists, dissipating magnetic energy and producing radiation. The equation describing this evolution is derived, and its solutions are presented. Two distinct regions coexist in untwisting magnetospheres: a potential region where ∇ × B = 0 (“cavity”) and a current-carrying bundle of field lines with ∇ × B ≠ 0 (“j-bundle”). The cavity has a sharp boundary, which expands with time and eventually erases all of the twist. In this process, the electric current of the j-bundle is sucked into the star. Observational appearance of the untwisting process is discussed. A hot spot forms at the footprints of the j-bundle. The spot shrinks with time toward the magnetic dipole axis, and its luminosity and temperature gradually decrease. As the j-bundle shrinks, the amplitude of its twist ψ can grow to the maximum possible value ψmax ∼ 1. The strong twist near the dipole axis increases the spindown rate of the star and can generate a broad beam of radio emission. The model explains the puzzling behavior of magnetar XTE J1810−197, a canonical example of magnetospheric evolution following a starquake. We also discuss implications for other magnetars. The untwisting theory suggests that the nonthermal radiation of magnetars is preferentially generated on a bundle of extended closed field lines near the dipole axis.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/2a2e11be2b10da78c47f7a4e78406050c89a1a46

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In Astronomy and Astrophysics

Citations in the corpus

NuSTAR DISCOVERY OF A 3.76 s TRANSIENT MAGNETAR NEAR SAGITTARIUS A* (2013, background): “5 s (Beloborodov 2009).”
THE McGILL MAGNETAR CATALOG (2013, background): “…vacuum dipole radiation and ignores the potentially important torques due to magnetospheric variability and the internal superfluid, both of which have been proposed to be relevant to magnetars (Kaspi et al. 2003; Dib et al. 2009; Archibald et al. 2013; Thompson et al. 2002; Beloborodov 2009).”
Unifying the observational diversity of isolated neutron stars via magneto-thermal evolution models. (2013, background): “Other configurations, qualitetively consisting of a confined bundle of currents with velocity of electrons decreasing outward, are expected (Beloborodov 2009).”