Discovery of a Second Eclipsing, ​​​​​Bursting Neutron Star Low-mass X-Ray Binary in the Globular Cluster Terzan 6

We have analyzed Chandra and Suzaku observations of the globular cluster Terzan 6, made when the recurrent transient GRS 1747-312 was in quiescence. Our analysis reveals the presence of a second eclipsing, bursting neutron star low-mass X-ray binary in the central regions of the cluster, in addition to GRS 1747-312. The new source, which we name Terzan 6 X2, is located only ∼0.″7 away from GRS 1747-312 in the 2021 Chandra images. The detection of a 5.14 ks long eclipse in the light curve of X2 at a time not predicted by the ephemeris of GRS 1747-312 confirms that it is an unrelated source. Using the Suzaku light curve from 2009, which, in addition to a type I X-ray burst, also showed an eclipse-like feature, we constrain the orbital period to be longer than 16.27 hr. The 0.5-10 keV luminosities of X2 vary in the range of ∼0.24-5.9 × 10³⁴ erg s⁻¹ on timescales of months to years. We have identified a plausible optical counterpart of X2 in Hubble Space Telescope F606W and F814W images. This star varied by 2.7 mag in V ₆₀₆ between epochs separated by years. In the cluster color-magnitude diagram, the variable counterpart lies in the blue-straggler region when it was optically bright, about 1.1-1.7 mag above the main-sequence turn-off. From the orbital period-density relation of Roche-lobe filling stars, we find the mass-donor radius to be ≳0.8