We present a unified explanation for the observed properties of the isolated neutron star RX J0720.4-3125 by obtaining a self-consistent model that accounts simultaneously for the observed X–ray spectrum and optical excess , the pulsed fraction , the observed spectral feature around 0.3 keV , and the long–term spectral evolution .
We show that all observed properties are consistent with a normal neutron star with a proper radius of about 12 km , a temperature at the magnetic pole of about 100 eV and a magnetic field strength of 2 \times 10 ^ { 13 } G , value inferred from the observed period decay .
The high magnetic field produces a strong anisotropy in the surface temperature distribution .
The observed variability of the effective temperature , strength of the spectral feature , and pulsed fraction are in good agreement with the predictions of our model in which the star is subject to free precession , producing changes in the angle between the magnetic field and the rotation axis of tens of degrees with a periodicity of 7 years , as pointed out by other authors on the basis of phenomenological models .
In addition to the evidence of internal toroidal components , we also find strong evidence of non-dipolar magnetic fields , since all spectral properties are better reproduced with models with strong quadrupolar components .