Influence of bulk motion of an infalling plasma in line-forming region on cyclotron line in accreting X-ray pulsars

Abstract

Abstract We examine the effect of the bulk motion of an infalling plasma on the properties (energy, width and depth) of a cyclotron line, including the effect of gravitational light bending as well as variation of the magnetic field. The cyclotron line energy decreases with increasing bulk velocity. Moreover, the dependence of the line energy on viewing angle becomes stronger as bulk velocity increases. As a result, the amount of a variation of the line energy with angle increases with increasing bulk velocity. On considering the two-dimensional structure of an accretion column, the bulk velocity in the line-forming region that is expected to be located around the walls of an accretion column can be considerably different from that in the continuum-formation region because of the decrease in the radiation energy density near the walls of the accretion column. In addition, the bulk velocity in the line-forming region can considerably depend on the beam pattern generated in the continuum-forming region because the bulk velocity is decelerated by photons propagating not across the magnetic field but along it. In the present paper, we demonstrate that the observed amount of a variation of the line energy with pulse phase can be explained by a variation of the bulk velocity with beam pattern, i.e., luminosity, using a cylindrical geometry with an accretion mound where a continuum spectrum is formed.