Revised spin for the black hole in GRS 1716-249 given a new distance determination

¹ Key Laboratory of Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, China
² University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100049, China

^⋆ Corresponding authors; zhaoshujie@ihep.ac.cn; taolian@ihep.ac.cn; yinqq@ihep.ac.cn

Received: 17 February 2024
Accepted: 14 September 2024

Abstract

GRS 1716–249 is a stellar-mass black hole in a low-mass X-ray binary that underwent a giant outburst in 2016–17. In this paper, we use simultaneous observations from the Hard X-ray Modulation Telescope (Insight-HXMT) and the Nuclear Spectroscopic Telescope Array (NuSTAR) to determine its basic parameters. The observations were performed during the softest part of the outburst and the spectra show clear thermal disk emission and reflection features. We fit the X-ray energy spectra using the joint fitting method of the continuum and reflection components with the kerrbb2 + relxill model. Since there is a possibility that the distance to this source was previously underestimated, we used the latest distance parameter of 6.9 kpc in our study, in contrast to previous works, where the distance was set at 2.4 kpc. Through a spectral fitting of the black hole mass at 6.4 M_⊙, we observe a strong dependence of the derived spin on the distance: a_* = 0.972_−0.005^+0.004 at an assumed distance of 2.4 kpc and a_∗ = 0.464_−0.007^+0.016 at an assumed distance of 6.9 kpc, at a confidence level of 90%. When considering the uncertainties in the distance and black hole mass, there will be a wider range of spin with a_*< 0.78. The fitting results with the new distance indicate that GRS 1716–249 harbors a moderate spin black hole with an inclined (i ∼ 40 − 50°) accretion disk around it. Additionally, we have also found that solely using the method of reflection component fitting, while ignoring the constraints on the spin from the accretion disk component will result in an extremely high spin.