We report on optical and RXTE observations of a new high-latitude bursting X-ray transient , XTE J2123–058 .
We identified the optical counterpart and discovered a 5.9573 \pm 0.0016 hr periodic optical modulation , which was subsequently shown to be the same as the spectroscopic orbital period .
From the absence of orbital X-ray modulation and the presence of partial optical eclipses the binary inclination is between 55 ^ { \circ } and 73 ^ { \circ } .
From the optical magnitude in quiescence and from the X-ray flux of type I X-ray bursts , the source distance is between 4.5 and 15 kpc , which implies that the source is unusually far from the Galactic plane since b = -36.2 ^ { \circ } .
Optical bursts with properties consistent with being reprocessed X-ray bursts occurred .

We detect a pair of high frequency QPOs at 847.1 \pm 5.5 Hz and 1102 \pm 13 Hz simultaneously .
According to the beat frequency model , this QPO separation implies a neutron star spin period of 3.92 \pm 0.22 ms. A change in the energy spectrum occurred during the decay of the outburst , which may have been due to the onset of the propeller mechanism .
If so , then the neutron star magnetic field strength is between 2 and 8 \times 10 ^ { 8 } G for an assumed distance of 10 kpc .
However , the changes in the timing and spectral properties observed during the decay are typical of atoll sources , which may indicate that the changes are due solely to the dynamics of the accretion disk .

As the phase averaged V-band magnitude declined from 17.26 at the peak of the outburst to 19.24 , and the X-ray flux decreased from 9.6 \times 10 ^ { -10 } erg cm ^ { -2 } s ^ { -1 } to 7.3 \times 10 ^ { -11 } erg cm ^ { -2 } s ^ { -1 } , the peak-to-peak amplitude of the V-band modulation increased from 0.75 to 1.49 magnitudes .
This behavior can be explained if the size of the accretion disk decreases during the decay of the outburst .