On the origin of cyclotron lines in the spectra of X-ray pulsars

¹ Astronomy Division, Department of Physics, PO Box 3000, FI-90014 University of Oulu, Finland
² Sobolev Astronomical Institute, Saint Petersburg State University, Saint-Petersburg 198504, Russia
³ Pulkovo Observatory of the Russian Academy of Sciences, Saint-Petersburg 196140, Russia
⁴ Institut für Astronomie und Astrophysik, Kepler Center for Astro and Particle Physics, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
⁵ Kazan (Volga region) Federal University, Kremlevskaja str., 18, Kazan 420008, Russia
⁶ Finnish Centre for Astronomy with ESO (FINCA), University of Turku, Väisäläntie 20, FI-21500 Piikkiö, Finland
⁷ Space Research Institute of the Russian Academy of Sciences, Profsoyuznaya Str. 84/32, Moscow 117997, Russia

^a e-mail: al.mushtukov@gmail.com
^b e-mail: juri.poutanen@oulu.fi

Published online: 8 January 2014

Abstract

Cyclotron resonance scattering features are observed in the spectra of some X-ray pulsars and show significant changes in the line energy with the pulsar luminosity. In a case of bright sources, the line centroid energy is anti-correlated with the luminosity. Such a behaviour is often associated with the onset and growth of the accretion column, which is believed to be the origin of the observed emission and the cyclotron lines. However, this scenario inevitably implies large gradient of the magnetic field strength within the line-forming region, and it makes the formation of the observed line-like features problematic. Moreover, the observed variation of the cyclotron line energy is much smaller than could be anticipated for the corresponding luminosity changes. We argue that a more physically realistic situation is that the cyclotron line forms when the radiation emitted by the accretion column is reflected from the neutron star surface. The idea is based on the facts that a substantial part of column luminosity is intercepted by the neutron star surface and the reflected radiation should contain absorption features. The reflection model is developed and applied to explain the observed variations of the cyclotron line energy in a bright X-ray pulsar V 0332+53 over a wide range of luminosities.