8th HEAD Meeting, 8-11 September, 2004
Session 17 Neutron Stars and X-ray Binaries
Poster, Thursday, September 9, 2004, 9:00am-10:00pm

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[17.19] On the Nature of the Accretion Columns in Her X-1

D.A. Leahy (University of Calgary)

RXTE observations of the occultation sequence of the accretion column of Her X-1 led to the picture that the observed pulse shape is due to a pencil beam from the near accretion pole and a fan beam from the far pole (Scott, Leahy & Wilson, 2000, ApJ, 539, 392). Those same observations give a pulse shape at a time when the pulsar is unobscured by the accretion disk. A simple geometry for an accretion pole which emits both a pencil beam and a fan beam and can qualitatively give the observed pulse shape of Her X-1 is a filled cone-shaped surface (Leahy, 2004, MNRAS, 348, 932). That work included gravitational effects and shadowing effects.

Here the modelling is generalized in a number of ways. More general forms for the emissivity and the shape of the accretion column are used. Five energy bands for the observed pulse shape are compared to the models instead of three. Least-squares fitting of the parametrized pulse shape model is used to obtain best-fit parameters. The parameters include: viewing geometry; the shape of the accretion column; the energy and angular dependence of the emissivity; and the mass-to-radius ratio of the neutron star. Several parameters are of interest to compare to current understanding of accretion columns and the mass-to-radius ratio helps constrain neutron star equations of state.

This work supported by the Natural Sciences and Engineering Research Council of Canada.

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