Discovery of twin kHz quasi-periodic oscillations in the high galactic la titude X-ray transient XTE J2123--058

We report the discovery of twin kHz quasi-periodic oscillations (QPOs) in the persistent X-ray flux of the new X-ray transient XTE J2123-058. We detected the QPOs in data taken with the proportional counter array on board the Rossi X-ray Timing Explorer on 1998 July 14, when the source count rate was at about 50% of its outburst peak level. The frequencies of the QPOs were 853+/-4 Hz and 1129+/-8 Hz. The peaks had widths of 32 (+11/-8) Hz and 50 (+17/-13) Hz and the rms amplitudes were 6.1% (+0.7/-0.6) and 6.5% (+0.9/-0.8), respectively. The QPO frequencies increased marginally with count rate and the amplitudes showed a small increase with photon energy. On the basis of the appearance in the color-color and hardness-intensity diagrams, the low-frequency power spectrum, and the strength of the kHz QPOs, the system can be classified as an atoll source. The source showed five type I X-ray bursts. The distance to the source is estimated to be about 10 (5-15) kpc, which combined with the high Galactic latitude of 36.2 degrees indicates the source is a located in the Galactic halo.