Kilohertz QPO and Atoll Source States in 4U 0614+09

Abstract

We report three RXTE/PCA observations of the low-mass X-ray binary 4U 0614+09. They show strong (~ 30% rms) band-limited noise with a cut-off frequency varying between 0.7 and 15 Hz in correlation with the X-ray flux. We observe two non-simultaneous 11-15% (rms) kHz peaks near 728 and 629 Hz in the power spectra of two of our observations when the X-ray flux is ~ 10^{-9} erg cm^{-2} s{-1} (2-10 keV), but find no QPO (< 6% rms) when the X-ray flux is half that. We suggest that count rate may not be a good measure for $\dot M$ even in sources as intrinsically weak as 4U 0614+09, and that QPO frequency and noise cutoff frequency track $\dot M$ more closely than count rate. The QPO increases in rms amplitude from ($11 \pm 1.3$) % at 3 keV to ($37 \pm 12$) % 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 ($1.56 \pm 0.2$) keV and radius ($ 500 \pm 200$) m (other models fit as well), which might indicate the oscillations originate at a small region on the neutron star surface.