**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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## [17.20] Comparision of Methods of Calculating Pulse Shapes for Rotating Neutron Stars: I. Equatorial Photon Orbits

*D.A. Leahy (University of Calgary), C. Cadeau, S. Morsink (University of Alberta)*

A number of approximation methods have been used to
calculate pulse shapes from surface emission regions on
rotating neutron stars. Restricting ourselves to the case of
light emitted and received in the equatorial plane of a
rapidly-rotating neutron star, we show how pulse shapes can
be constructed using an exact spacetime metric and fully
accounting for time- delay effects. We compare this to a
method which approximates the exterior spacetime of the star
by the Schwarzschild metric, inserts special relativistic
effects by hand, and neglects time-delay effects. There are
significant differences between the exact and approximate
methods for some applications, such as pulse timing and
constraining the stellar radius. In the case of constraining
the stellar radius, we show that fitting the approximate
pulse shapes to the full calculation yields errors in the
fitted radius of as much as ~±10%. By carrying out
intermediate calculations with fewer approximations we also
show which part of the approximations are responsible for
the errors in fitted stellar radius. Not all applications of
pulse shape calculations suffer from significant errors: the
calculation of the soft-hard phase lag for a 1 keV blackbody
does not strongly depend on the method used.

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

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Bulletin of the American Astronomical Society, **36** #3

© 2004. The American Astronomical Soceity.