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Journal Article

Cyclotron line formation by reflection on the surface of a magnetic neutron star


Trümper,  J. E.
High Energy Astrophysics, MPI for Extraterrestrial Physics, Max Planck Society;

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Kylafis, N. D., Trümper, J. E., & Loudas, N. A. (2021). Cyclotron line formation by reflection on the surface of a magnetic neutron star. Astronomy and Astrophysics, 655: A39. doi:10.1051/0004-6361/202039361.

Cite as: https://hdl.handle.net/21.11116/0000-0009-D77F-7
Context. Accretion onto magnetic neutron stars results in X-ray spectra that often exhibit a cyclotron resonance scattering feature (CRSF) and, sometimes, higher harmonics of it. Two places are suspect for the formation of a CRSF: the surface of the neutron star and the radiative shock in the accretion column.
Aims. Here we explore the first possibility: reflection at the neutron-star surface of the continuum produced at the radiative shock. It has been proposed that for high-luminosity sources, as the luminosity increases, the height of the radiative shock increases, thus a larger polar area is illuminated, and as a consequence the energy of the CRSF decreases because the dipole magnetic field decreases by a factor of two from the pole to the equator. This model has been specifically proposed to explain the observed anticorrelation of the cyclotron line energy and luminosity of the high-luminosity source V 0332+53.
Methods. We used a Monte Carlo code to compute the reflected spectrum from the atmosphere of a magnetic neutron star, when the incident spectrum is a power-law one. We restricted ourselves to cyclotron energies ≪mec2 and used polarization-dependent scattering cross sections, allowing for polarization mode change.
Results. As expected, a prominent CRSF is produced in the reflected spectra if the incident photons are in a pencil beam, which hits the neutron-star surface at a point with a well-defined magnetic field strength. However, the incident beam from the radiative shock has a finite width and thus various magnetic field strengths are sampled. As a result of overlap, the reflected spectra have a CRSF, which is close to that produced at the magnetic pole, independent of the height of the radiative shock.
Conclusions. Reflection at the surface of a magnetic neutron star cannot explain the observed decrease in the CRSF energy with luminosity in the high-luminosity X-ray pulsar V 0332+53. In addition, it produces absorption lines much shallower than the observed ones.