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arXiv · 2401.00727

Discovery of a second eclipsing, bursting neutron-star low-mass X-ray binary in the globular cluster Terzan 6

Maureen van den Berg·Jeroen Homan·Craig O. Heinke·David A. Pooley·Rudy Wijnands·Arash Bahramian·James C. A. Miller-Jones
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Abstract

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 arcsec 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 h. The 0.5-10 keV luminosities of X2 vary in the range of ~0.24-5.9x10^34 erg/s on time scales of months to years. We have identified a plausible optical counterpart of X2 in HST 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 Rsun.

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Maureen van den Berg, Jeroen Homan, Craig O. Heinke, David A. Pooley, Rudy Wijnands, Arash Bahramian, James C. A. Miller-Jones. 2024-01-01. Discovery of a second eclipsing, bursting neutron-star low-mass X-ray binary in the globular cluster Terzan 6. https://doi.org/10.3847/1538-4357%2Fad2f3d

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