Energy Spectra and High-Frequency Oscillations in 4U 0614+091

Abstract

We investigate the behavior of the high-frequency quasi-periodic oscillations (QPOs) in 4U 0614+091, combining timing and spectral analyses of RXTE observations. The energy spectra of the source can be described by a power law (alpha similar to 2.8) and a blackbody (kT similar to 1.5 keV), with the blackbody accounting for 10%-20% of the total energy flux. We find a robust correlation of the frequency, v, of the higher frequency QPO near 1 kHz with the flux of the blackbody, F-BB The slope of this correlation, d log v/d log F-BB,, is 0.27-0.37. The source follows the same relation even in observations separated by several months. The QPO frequency does not have a similarly unique correlation with the total flux or the flux of the power-law component. The rms fraction of the higher frequency QPO rises with energy from 6.8% +/- 1.5% (3-5 keV) to 21.3% +/- 4.0% (10-12 keV). For the lower frequency QPO, however, it is consistent with a constant value of 5.4% +/- 0.9%. The results may be interpreted in terms of a beat-frequency model for the production of the high-frequency QPOs.