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J. A. Tomsick, J. P. Halpern, J. Kemp (Columbia University), P. Kaaret (Harvard-Smithsonian CfA)
We report on optical and X-ray observations of the recently discovered high-latitude bursting X-ray transient, XTE J2123-058. We discovered a binary orbital period of 5.9573 hr, observed X-ray bursts with photospheric radius expansion, and detected kHz QPOs. During the decline from outburst to quiescence, we found an abrupt change in the X-ray spectrum which may be due to the onset of the propeller mechanism. From the orbital period, we constrain the spectral type of the optical companion and find a source distance between 4.5 and 9.1 kpc, indicating that the source is unusually far, at least 2.6 kpc, from the Galactic plane. Assuming a 1.4 solar mass neutron star, we derive a distance of 17 kpc from the radius expansion X-ray bursts. Our two distance estimates can be made to agree if the neutron star mass is significantly less than 1.4 solar masses, if the radius expansion X-ray bursts are significantly sub-Eddington, or if there is substantial light from the disk in quiescence.
The author(s) of this abstract have provided an email address for comments about the abstract: jat@astro.columbia.edu