Discovery of a Khz Qpo in 2s 0918–549

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, a 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. Subject headings: accretion, accretion disks — stars: individual (2S 0918–549) — stars: neutron — X-rays: stars