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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.