Is the Spin of the Black Hole in GX 339–4 Negative?

Abstract We have studied the accreting black hole binary GX 339–4 using two highly accurate broadband X-ray data sets in very soft spectral states from simultaneous NICER and NuSTAR observations. Joint fitting of both data sets with relativistic models of the disk and its Comptonization and reflection allows us to relatively accurately determine the black hole mass, spin, distance, and inclination. However, we find that the measured values strongly depend on the used disk model. With widely used Kerr disk models treating departures from local blackbody spectra using color corrections, we find relatively low black hole masses and strongly negative spins (i.e., retrograde accretion). Then, models employing radiative transfer calculations of the disk atmosphere predict moderately positive spins and high masses. When adding a warm corona above the disk (as proposed before for both active galactic nuclei and accreting binaries), we find that the spin is weakly constrained but consistent with zero. In all cases, the fitted inclination is low, ≈30°–34°. For the spin axis aligned with the binary axis, the mass function for this source implies large values of the mass, consistent only with those obtained with either disk-atmosphere models or the presence of a warm corona. We also test different disk models for an assumed set of mass, distance, and inclination. We find that different models yield values of the spin parameter differing by up to ∼0.3. Our results confirm previously found strong model dependencies of the measured black hole spin, now by comparing different disk models and for a low-mass X-ray binary.