Uncovering the nature of the X-ray transient 4U 1730-22:: Discovery of X-ray emission from a neutron star in quiescence with Chandra

The X-ray transient 4U 1730 - 22 has not been detected in outburst since 1972, when a single similar to 200 day outburst was detected by the Uhuru satellite. This neutron star or black hole X-ray binary is presumably in quiescence now, and here we report on X-ray and optical observations of the 4U 1730 - 22 field designed to identify the system's quiescent counterpart. Using the Chandra X-ray Observatory, we have found a very likely counterpart. The candidate counterpart, CXOU J173357.5 - 220156, is close to the center of the Uhuru error region and has a thermal spectrum. The 0.3-8 keV spectrum is well described by a neutron star atmosphere model with an effective temperature of 131 +/- 21 eV. For a neutron star with a 10 km radius, the implied source distance is 10(-4)(+12) kpc, and the X-ray luminosity is 1.9; 10(33) (d/10 kpc)(2) ergs s(-1). Accretion from a companion star is likely required to maintain the temperature of this neutron star, which would imply that it is an X-ray binary, and therefore almost certainly the 4U 1730 - 22 counterpart. We do not detect an optical source at the position of the Chandra source down to R > 22.1, and this is consistent with the system being a low-mass X-ray binary at a distance greater than a few kpc. If our identification is correct, 4U 1730 - 22 is one of the 5 most luminous of the 20 neutron star transients that have quiescent X-ray luminosity measurements. We discuss the results in the context of neutron star cooling and the comparison between neutron stars and black holes in quiescence.