A Possible Transient Neutron Star in Quiescence in the Globular Cluster NGC 5139

Using the Chandra/ACIS-I detector, we have identified an X-ray source (CXOU 132619.7-472910.8) with a thermal spectrum consistent with that observed from transiently accreting neutron stars in quiescence at the distance of the globular cluster NGC 5139 (ω Cen). The absence of intensity variability on timescales as short as 4 s (less than 25% rms variability) and as long as 5 yr (less than 50% variability) supports the identification of this source as a neutron star, most likely maintained at a high effective temperature (≈10⁶ K) by transient accretion from a binary companion. An alternative hypothesis—that the source is an isolated neutron star accreting from the intracluster medium—could be excluded or confirmed by identifying the X-ray source's optical counterpart. The ability to spectrally identify quiescent neutron stars in globular clusters (where the distance and interstellar column densities are known) opens up new opportunities for precision neutron star radius measurements.