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

ABSTRACT 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–K2 correlation to the disc H α emission line detected in Gran Telescopio Canarias spectra and obtain K2 = 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 α trough 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 = M2/M1 ≲ 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 above the Galactic Plane, in support of a large natal kick.