Spectral analysis of new black hole candidate AT2019wey observed by NuSTAR

AT2019wey is a new galactic X-ray binary that was first discovered as an optical transient by the the Asteroid Terrestrial-impact Last Alert System (ATLAS) on December 7, 2019. AT2019wey consists of a black hole candidate as well as a low-mass companion star (M_star ≲ 1.0M_◉) and is likely to have a short orbital period (P_orb ≲ 16 h). Although AT2019wey began activation in the X-ray band on March 8, 2020, it did not enter the soft state during almost the entire outburst. In this study, we present a detailed spectral analysis of AT2019wey in the low/hard state during its X-ray outburst on the basis of Nuclear Spectroscopic Telescope Array (NuS TAR) observations. We obtain tight constraints on several of its important physical parameters by applying the state of the art relxill relativistic reflection model family. In particular, we determine that the measured inner radius of the accretion disk is most likely to have extended to the innermost stable circular orbit (ISCO) radius, i.e., Rin=1.38‑0.16+0.23RISCO. Hence, assuming R_in=R_ISCO, we find the spin of AT2019wey to be a_* ∼ 0.97, which is close to the extreme and an inner disk inclination angle of i ∼ 22°. Additionally, according to our adopted models, AT2019wey tends to have a relatively high iron abundance of A_Fe ∼5A_Fe,◉ and a high disk ionization state of log ξ ∼ 3.4.