Discovery of a new, third kilohertz quasi-periodic oscillation in 4U 1608-52, 4U 1728-34, and 4U 1636-53. Sidebands to the lower kilohertz quasi-periodic oscillation?
Pg. Jonker et al., Discovery of a new, third kilohertz quasi-periodic oscillation in 4U 1608-52, 4U 1728-34, and 4U 1636-53. Sidebands to the lower kilohertz quasi-periodic oscillation?, ASTROPHYS J, 540(1), 2000, pp. L29-L32
We report the discovery of a third kilohertz quasi-periodic oscillation (kH
z QPO) in the power spectra of the low-mass X-ray binaries 4U 1608-52 (6.3
sigma), 4U 1728-34 (6.0 sigma), and 4U 1636-53 (3.7 sigma) which is present
simultaneously with the previously known kHz QPO pair. The new kHz QPO is
found at a frequency that is 52.8 +/- 0.9, 64 +/- 2, and 58.4 +/- 1.9 Hz hi
gher than the frequency of the lower kHz QPO in 4U 1608-52, 4U 1728-34, and
4U 1636-53, respectively. The difference between the frequency of the new
kHz QPO and the lower kHz QPO increased in 4U 1608-52 from 49.6 +/- 1.4 to
53.9 +/- 0.5 Hz when the frequency of the lower kHz QPO increased from 672
to 806 Hz. Simultaneously the difference between the frequency of the new k
Hz QPO and the upper kHz QPO increased by similar to 60 Hz, suggesting that
the new Wt QPO is unrelated to the upper kHz QPO. In 4U 1636-53 a fourth,
weaker, kHz QPO is simultaneously detected (3 sigma) at the same frequency
separation below the lower Wt QPO, suggesting that the new kHz QPOs are sid
ebands to the lower kHz QPO. We discuss the nature of this new kHz QPO and
its implications on the models for the kHz QPOs.