2006ApJ...644..439L

Observation of the isolated neutron star 1E 1207.4-5209 shows three distinct absorption features, regularly spaced at 0.7, 1.4, and 2.1 keV, which vary in phase with the star's rotation. In this paper, we attempt to clarify the understanding of these absorption lines by using the quantum theory of the cyclotron radiation under the quadrupole approximation, given in our previous papers. We found that these lines are more likely to arise from the electron-cyclotron absorption, rather than from proton-cyclotron. We explain why the observed fundamental and the second-harmonic cyclotron features with significantly different absorption coefficients have nearly the same strength. Assuming that the absorption features arise from an approximately uniform narrow column at the bottom of the pole region of neutron star, we complete a simplified model calculation to fit the observational data; thus, the included angle between the spinning axis and the observer's sight line, as well as that between the spinning and the magnetic axes of the neutron star, are determined by this method. The geometrical configuration inferred from the deduced directions of these axes is helpful to better understand the physics and some other relevant observation facts of star 1E 1207.4-5209.