The low quiescent X-ray luminosity of the neutron star transient XTE J2123-058

We report on the first X-ray observations of the neutron star soft X-ray transient (SXT) XTE J2123-058 in quiescence made by the Chandra X-Ray Observatory and BeppoSAX, as well as contemporaneous optical observations. In 2002, the Chandra spectrum of XTE J2123-058 is consistent with a power-law model or the combination of a blackbody plus a power law, but it is not well described by a pure blackbody. Using the interstellar value of N-H, the power-law fit gives Gamma=3.1(-0.6)(+0.7) and indicates an 0.3-8 keV unabsorbed luminosity of 9(-3)(+4) x 10(31)(d/8.5 kpc)(2) ergs s(-1) (90% confidence errors). Fits with models consisting of thermal plus power-law components indicate that the upper limit on the temperature of a 1.4 M-circle dot, 10 km radius neutron star with a hydrogen atmosphere is kT(eff)<66 eV, and the upper limit on the unabsorbed, bolometric luminosity is L-∞<1.4x10(32) ergs s(-1), assuming d=8.5 kpc. Of the neutron star SXTs that exhibit short (<1 yr) outbursts, including Aql X-1, 4U 1608-522, Cen X-4, and SAX J1810.8-2609, the lowest temperatures and luminosities are found for XTE J2123-058 and SAX J1810.8-2609. From the BeppoSAX observation of XTE J2123-058 in 2000, we obtained an upper limit on the 1-10 keV unabsorbed luminosity of 9x10(32) ergs s(-1). Although this upper limit allows for the X-ray luminosity to have decreased between 2000 and 2002, that possibility is not supported by our contemporaneous R-band observations, which indicate that the optical flux increased significantly. Motivated by the theory of deep crustal heating by Brown and coworkers, we characterize the outburst histories of the five SXTs. The low quiescent luminosity for XTE J2123-058 is consistent with the theory of deep crustal heating without requiring enhanced neutron star cooling if the outburst recurrence time is greater than or similar to70 yr.