The orbital period, black hole mass, and distance to the X-ray transient GRS 1716-249 (=N Oph 93)

We present evidence for a 0.278(8) d ( = 6.7 h) orbital period in the X-ray transient GRS 1716-249 ( = N Oph 93), based on a superhump modulation detected during the 1995 mini-outburst plus ellipsoidal variability in quiescence. With a quiescent magnitude of r = 23.19 ±0.15 N Oph 93 is too faint to warrant a full dynamical study through dedicated time-resolved spectroscopy. Instead, we apply the FWHM-K 2 correlation to the disc H αemission line detected in Gran Telescopio Canarias spectra and obtain K 2 = 521 ±52 km s −1. This leads to a mass function f ( M ) = 4.1 ±1.2 M ⊙, thus indicating the presence of a black hole in this historic X-ray transient. Furthermore, from the depth of the H αtrough and the quiescent light curve we constrain the binary inclination to i = 61 ±15 ◦, while the detection of superhumps sets an upper limit to the donor to compact star mass ratio q = M 2 / M 1 ≲ 0.25. Our de-reddened ( r −i ) colour is consistent with a ≈K6 main sequence star that fills its Roche lobe in a 0.278 d orbit. Using all this information we derive a compact object mass M 1 = 6. 4 + 3. 2 −2. 0 M ⊙at 68 per cent confidence. We also constrain the distance to GRS 1716-249 to 6.9 ±1.1 kpc, placing the binary ∼0.8 kpc abo v e the Galactic Plane, in support of a large natal kick. © 2023 The Author(s).