Structures of cyclotron lines in the presence of non-dipole magnetic field near the surface of neutron star

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We calculate the cyclotron lines in neutron star atmosphere assuming the surface magnetic field to be the superposition of the star-centered global dipole d and the crust-anchored dipole moment m which was modeled by Gil et al. We study how the crust-anchored dipole moment influences the properties of the cyclotron resonant scattering lines. When the extra dipole makes the magnetic field strength increase with altitude in the line-forming region, i.e. the sign of m is the minus, the structures of the cyclotron resonant scattering lines indicate broader line at the first harmonic and the ratio of the second to the first at the centroid energies of the cyclotron line is less than 2. This is consistent with the features observed in some X-ray pulsars. Thus, this may suggest that the strength of the magnetic field increases with the height of slab in line-forming region. In the atmosphere with the emission region in the middle of that, the cyclotron feature has only one emission line in either the redwing or bluewing due to the distribution of different strength of the magnetic field, although it has two emission lines in both the redwing and bluewing in an atmosphere threaded by an uniform magnetic field.

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