A Ginga hard X-ray search for 1-3 s quasi-periodic oscillations in AM Herculis systems

Abstract

We describe a search for the expected X -ray counterpart to the 1-3 s QPOs seen in the optical emission from a number of polars. The Ginga hard X-ray data of the three systems AM Her, EF Eri and V834 Cen have been examined for such QPOs. However, none has been detected, and 95 per cent upper limits to the rms amplitude of the QPOs of 4, 5 and 18 per cent, respectively, have been obtained. The broad-band optical rms values for the rapid QPOs are typically 1-3 per cent for EF Eri and V834 Cen, while no such QPO has been seen in the optical data of AM Her (with an upper limit < 0.25 per cent). The only viable models for the optical QPOs are the driven oscillating shock models. They predict approximately equal X-ray and optical modulation amplitudes, and are thus consistent with our results.