Discovery of a Kilohertz Quasi-periodic Oscillation in 2S 0918-549

Astronomy and Astrophysics – Astronomy

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Accretion, Accretion Disks, Stars: Binaries: Close, Stars: Individual (2S 0918-549), Stars: Neutron, Stars: Oscillations, X-Rays: Stars

Scientific paper

We report the discovery of a kilohertz quasi-periodic oscillation (kHz QPO) in the low-mass X-ray binary (LMXB) 2S 0918-549. The kHz QPO has a frequency of 1156+/-9 Hz, an FWHM of 70+/-20 Hz, and a fractional rms amplitude of 18%+/-2%. We also detected for the first time a type I X-ray burst in this source. The compact object must therefore be a neutron star. Finally, we were able to classify the source as an atoll source exhibiting the full complement of spectral/timing states of this class (island and banana). From the peak burst flux, an upper limit on the persistent luminosity can be derived of 0.5% of the Eddington luminosity, making 2S 0918-549 one of the least luminous LMXBs showing a kHz QPO. We compare the fractional rms amplitudes of the upper kHz QPO across the ensemble of LMXBs. We find a strong anticorrelation with luminosity. In LMXBs with luminosities ~100 times lower than those of Z sources, the fractional rms amplitude is a factor ~10 larger.

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