paper

Possible Evidence for Free Precession of a Strongly Magnetized Neutron Star in the Magnetar 4U 0142+61

arXiv:1404.3705 · doi:10.1103/PhysRevLett.112.171102

Abstract

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.

4 pages, 3 figures, accepted for publication in Physical Review Letters. Corrected typos [revsion]; 1) abstract line 4, "~1.5 h" -> "~15 h"; 2) p.2 right col 3rd para, "Q=55 ksec" --> "T=55 ksec"; 3) p.3 left col 2nd para, "over of T=55 --> over T=55"

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