Kilohertz Quasi-periodic Oscillation and Atoll Source States

We report three RXTEyPCA observations of the low-mass X-ray binary 4U 0614109. They show strong (130% 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 1 10 ergs cm s (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 Ṁ even in sources as intrinsically weak as 4U 0614109 and that QPO frequency and noise cutoff frequency track Ṁ more closely than count rate. The QPO increases in rms amplitude from 11% H 1.3% at 3 keV to 37% H 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 H 0.2 keV and radius 500 H 200 m (other models fit as well), which might indicate that the oscillations originate at a small region on the neutron star surface. Subject headings: accretion, accretion disks — stars: individual (4U 06141091) — stars: neutron — X-rays: stars