Kilohertz Quasi-periodic Oscillation and Atoll Source States in 4U 0614+09

Abstract

We report three <i>RXTE</i>/PCA observations of the low-mass X-ray binary 4U 0614+09. They show strong (~30% rms) band-limited noise with a cutoff frequency varying between 0.7 and 15 Hz in correlation with the X-ray flux, fₓ. We observe two nonsimultaneous 11%-15% (rms) kHz peaks near 728 and 629 Hz in the power spectra of two of our observations when fₓ ~ 10^-9 ergs cm⁻² s⁻¹ (2-10 keV) but find no quasi-periodic oscillations (QPOs; <6% rms) when fₓ is half that. We suggest that count rate may not be a good measure for dot Ṁ even in sources as intrinsically weak as 4U 0614+09 and that QPO frequency and noise cutoff frequency track dot Ṁ more closely than count rate. The QPO increases in rms amplitude from 11% ± 1.3% at 3 keV to 37% ± 12% at 23 keV; the fractional amplitude of the band-limited noise is energy independent. This suggests different sites of origin for these two phenomena. The spectrum of the oscillating flux roughly corresponds to a black body with temperature 1.56 ± 0.2 keV and radius 500 ± 200 m (other models fit as well), which might indicate that the oscillations originate at a small region on the neutron star surface.