XTE J2123−058: A New Neutron Star X‐Ray Transient

We report on optical and RXTE observations of a new high-latitude burstingX-ray transient, XTE J2123-058. We identified the optical counterpart, measuredthe 5.9573 hr orbital period and constrained the binary inclination and thesource distance. The distance lower limit indicates that the source is at least2.6 kpc from the Galactic plane, which is unusual for an LMXB. RXTEobservations were made between June and August 1998 during the first detectedoutburst from this source. We describe correlations between X-ray timing andspectral properties and discuss the possibility that the propeller mechanismturned on during the decay of the outburst. During one of the RXTEobservations, we detect a pair of high frequency QPOs at 847.1 +/- 5.5 Hz and1102 +/- 13 Hz simultaneously. According to the beat frequency model, the QPOseparation implies a neutron star spin period near 3.9 ms. As the X-ray fluxdecreases at the end of the outburst, the amplitude of the optical modulationincreases significantly. This behavior can be explained if the size of theaccretion disk decreases during the decay of the outburst.Comment: 24 pages, 9 figures, accepted by Ap