Effect of reflected X-rays on X-ray pulsar timing observations

As suggested by Milgrom and Avni (1976), X-rays that are reflected from the companion of an X-ray pulsar can render the X-ray pulse received at Earth different from the X-ray pulse emitted by the pulsar alone. This change can cause erroneous system parameters to be deduced from pulse timing measurements. A formalism is presented to calculate the effects of the reflected X-rays, and applications are made to several systems. For 3U 0900--40, the deduced orbital parameters are not significantly altered by this effect if the time delays are obtained through the use of sharp features in the X-ray pulse. However, time delays are obtained through the lowest periodicities in the X-ray pulse should be afected. Observations of the spurious time delyas might then constrain the size of the companion star and the mass and sense of rotation of the X-ray p pulsar.