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H. C. Lee, R. Misra, R. E. Taam (Northwestern University)
An empirical Compton up-scattering model, successfully used to interpret the fractional RMS amplitude and the soft lags of the 830~Hz QPO in 4U1608-52, is tested against several additional kilohertz QPO, including those found in the atoll sources 4U1728-34, 4U1636-53, and 4U1702-43. If found consistent with the characteristics of the kilohertz QPO observed in these additional sources, the model will allow uniform interpretation of the data to yield estimates of physical parameters such as the characteristic size of the assumed hot Comptonizing region, as well as hint at the radiative processes operating in kilohertz QPO formation.
The model assumes that a hot, uniform corona is cooled by input photons, characterized by a Wien spectrum, from cooler, underlying material. Oscillations in the temperature of the corona, which also heats the underlying material, lead to oscillations in the input photon temperature. A combination of the two variations gives rise to the observed QPO, with their relative contributions determined by enforcing energy conservation. All input parameters to the model, save a characteristic coronal size, are derived from the photon energy spectrum of the QPO observation.