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 ∼1⁰¹¹ T. The magnetar 4U 0142+61, one of the 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 h. When this effect is interpreted as free precession of the neutron star, the object is inferred to deviate from spherical symmetry by ∼1.6×1^0-4 in its moments of inertia. This deformation, when ascribed to magnetic pressure, suggests a strong toroidal magnetic field, ∼10¹² T, residing inside the object. This provides one of the first observational approaches towards toroidal magnetic fields of magnetars.