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J.A. Tomsick, W.A. Heindl (CASS/UCSD), Z. Wang, D. Chakrabarty (MIT ), J.P. Halpern (Columbia University), P. Kaaret (Harvard-Smithsonian CfA)
We report on optical observations made at the Keck and Magellan Observatories of two neutron star X-ray transients in quiescence (XTE J2123-058 and SAX J1808.4-3658). For XTE J2123-058, our measurement of the companion's radial velocity curve reveals that its semi-amplitude of K2 = 298.5± 6.9 km s-1 is the highest value that has been measured for any neutron star LMXB. The high value for K2 is, in part, due to the high binary inclination of the system but may also indicate a high neutron star mass. The mass function of f2 = 0.684±0.047 solar masses, along with our constraints on the companion's spectral type (K5V-K9V) and previous constraints on the inclination, gives a likely range of neutron star masses from 1.2 to 1.8 solar masses. For SAX J1808.4-3658, we present results from optical spectra taken in 1998, 2000 and 2001.
JAT acknowledges support from a CASS Postdoctoral Fellowship.
The author(s) of this abstract have provided an email address for comments about the abstract: jtomsick@ucsd.edu