Bumpy Spin-Down of Anomalous X-Ray Pulsars: The Link with Magnetars
Abstract
The two anomalous X-ray pulsars (AXPs) with well-sampled timing histories, 1E 1048.1-5937 and 1E 2259+586, are known to spin down irregularly, with ``bumps'' superposed on an overall linear trend. Here we show that if AXPs are nonaccreting magnetars, i.e., isolated neutron stars with surface magnetic fields B0>~1010 T, then they spin down electromagnetically in exactly the manner observed, because of an effect called ``radiative precession.'' Internal hydromagnetic stresses deform the star, creating a fractional difference ɛ=(I3-I1)/I1~10-8 between the principal moments of inertia I1 and I3; the resulting Eulerian precession couples to an oscillating component of the electromagnetic torque associated with the near-zone radiation fields, and the star executes an anharmonic wobble with period τpr~2π/ɛΩ(t)~10 yr, where Ω(t) is the rotation frequency as a function of time t. We solve Euler's equations for a biaxial magnet rotating in vacuo, show that the computed Ω(t) matches the measured timing histories of 1E 1048.1-5937 and 1E 2259+586, predict Ω(t) for the next 20 years for both objects, predict a statistical relation between <dΩ/dt> and τpr, to be tested as the population of known AXPs grows, and hypothesize that radiative precession will be observed in future X-ray timing of soft gamma-ray repeaters.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- July 1999
- DOI:
- 10.1086/312104
- arXiv:
- arXiv:astro-ph/9904403
- Bibcode:
- 1999ApJ...519L..77M
- Keywords:
-
- STARS: PULSARS: INDIVIDUAL: ALPHANUMERIC: 1E 1048.1-5937;
- STARS: PULSARS: INDIVIDUAL: ALPHANUMERIC: 1E 2259+586;
- STARS: NEUTRON;
- STARS: ROTATION;
- X-RAYS: STARS;
- Stars: Pulsars: Individual: Alphanumeric: 1E 1048.1-5937;
- Stars: Pulsars: Individual: Alphanumeric: 1E 2259+586;
- Stars: Neutron;
- Stars: Rotation;
- X-Rays: Stars;
- Astrophysics
- E-Print:
- 9 pages, 2 figures, to be published in The Astrophysical Journal Letters