2026ApJ...999..255S

The black hole X-ray binary Swift J151857.0-572147 was discovered during its first outburst in 2024 March. We review the archive of NICER observations from this outburst, focusing on the soft states. We select spectra for which the disk-to-total flux ratio exceeds 0.8 and the coronal scattering fraction f_sc is less than 25%, conditions under which the accretion disk is expected to extend to the innermost stable circular orbit and remains geometrically thin. Through a continuum-fitting analysis, we explore the dependence of spin on mass, inclination, and distance. We constrain the spin within the parameter space typical of stellar-mass black holes, assuming a mass around 10 M_⊙, inclination angles between 20° and 60°, and distances between 4 and 16 kpc. For fiducial parameters, such as a mass of 10 M_⊙, a distance of 10 kpc, and an inclination angle of 40°, a moderate spin of approximately 0.7 is obtained. However, precise determination of the spin will require accurate measurements of these parameters. Our analysis provides a framework to infer the spin and estimate its uncertainties once more precise measurements of mass, distance, and inclination become available. As we demonstrate, lower inclination angles, greater distances, or larger black hole masses result in higher spin values.