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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.