Attenuation of Beaming Oscillations near Neutron Stars

Observations with RXTE have revealed kilohertz quasi-periodic brightnessoscillations (QPOs) from nearly twenty different neutron-star low-mass X-raybinaries (LMXBs). These frequencies often appear as a pair of kilohertz QPOs ina given power density spectrum. In many models the higher-frequency of theseQPOs is a beaming oscillation at the frequency of a nearly circular orbit atsome radius near the neutron star. In such models it is expected that therewill also be beaming oscillations at the stellar spin frequency and atovertones of the orbital frequency, but no strong QPOs have been detected atthese frequencies. We therefore examine the processes that can attenuatebeaming oscillations near neutron stars, and in doing so extend the work onthis subject that was initiated by the discovery of lower-frequency QPOs fromLMXBs. Among our main results are (1)in a spherical scattering cloud, allovertones of rotationally modulated beaming oscillations are attenuatedstrongly, not just the even harmonics, and (2)it is possible to have arelatively high-amplitude modulation near the star at, e.g., the stellar spinfrequency, even if no peak at that frequency is detectable in a power densityspectrum taken at infinity. We discuss the application of these results tomodeling of kilohertz QPOs.Comment: 19 pages including six figures, uses aaspp4, accepted by Ap