Possible Evidence for Free Precession of a Strongly Magnetized Neutron   Star in the Magnetar 4U 0142+61
  Magnetars are a special type of neutron stars, considered to have extreme dipole magnetic fields reaching ~1e+11 T. The magnetar 4U 0142+61, one of prototypes of this class, was studied in broadband X-rays (0.5-70 keV) with the Suzaku observatory. In hard X-rays (15-40 keV), its 8.69 sec pulsations suffered slow phase modulations by +/-0.7 sec, with a period of ~15 hours. When this effect is interpreted as free precession of the neutron star, the object is inferred to deviate from spherical symmetry by ~1.6e-4 in its moments of inertia. This deformation, when ascribed to magnetic pressure, suggests a strong toroidal magnetic field, ~1e+12 T, residing inside the object. This provides one of the first observational approaches towards toroidal magnetic fields of magnetars. 
Kazuo Makishima, Teruaki Enoto, Junko S. Hiraga, Toshio Nakano,   Kazuhiro Nakazawa, Soki Sakurai, Makoto Sasano, and Hiroak...
astro-ph.HE