HEAD 2000, November 2000
Session 23. X-Ray Binaries
Display, Wednesday, November 8, 2000, 8:00am-6:00pm, Bora Bora Ballroom

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[23.08] A Compton Up-scattering Model for Soft Lags in Kilohertz QPO

H. C. Lee, R. Misra (Northwestern University)

An empirical Compton up-scattering model is described which reproduces both the fractional RMS vs. energy and the soft lags (of up to 60~\musec) in the 830~Hz QPO observed in 4U1608-52 on Mar.~6,~1996. These RXTE observations , which cover photon energies from 5~keV to 30~keV, contain the most constraining lag data publicly available.

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 Mar.~6 observation. Fits to the fractional RMS and phase lag data for this kilohertz QPO imply that the corona involved is less than 20~km in extent.

HL is supported by NASA GSRP grant NGT~5-50172.

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