R-modes on rapidly rotating, relativistic stars - II. Blackbody emission

Rossby waves (or r-modes) in the surface layers of a neutron star have become a leading model for the oscillations observed during the tail of Type I X-ray bursts. Their frequency evolution matches well with the observed frequency drifts of the oscillations in the bursts and the burning appears to excite these waves quite naturally. This paper addresses the detailed shape of the expected flux profiles from r-modes on neutron stars as a function of energy. R-modes naturally account for both the small amplitude of the observed oscillations and their lack of harmonic content. However, the model predicts that the oscillation at higher energies leads the lower energy variation. The observed oscillations have the opposite trend, which possibly indicates that the higher energy photons are up-scattered in the plasma surrounding the neutron star and therefore delayed.