Observations of Magnetic Deformation of Magnetars

Through observations with the Suzaku and NuSTAR X-ray observatories, at least three magnetars have been found to show periodic phase modulations in their hard X-ray pulses, with a period T which is similar to 10(4) times longer than their pulse periods. Among multiple observation of each object. T was kept constant, but the modulation amplitude varied. This effect can be interpreted as a manifestation of free precession of neutron stars that are axially deformed by similar to 10(-4) in terms of the moment of inertia. Further interpreting this deformation as caused by internal magnetic stress, these neutron stars are inferred to harbor intense toroidal magnetic fields reaching similar to 10(16) G. These observational results thus provide valuable information on the magnetars' toroidal fields which are difficult to observe externally, and give clues to the X-ray emission mechanisms of magnetars.