2021ApJ...907...31T

We analyze simultaneous observations with NuSTAR and Swift of the black hole binary GX 339-4 in which we clearly detect both a strong thermal component and prominent relativistic reflection features. We employ nkbb and relxill_nk, which are the state of the art of, respectively, the thermal and reflection models to test the Kerr black hole hypothesis. We obtain unprecedented precise measurements of the Johannsen deformation parameter α₁₃: α₁₃=-0.012_–0.039^+0.011 (lamppost coronal geometry) and α₁₃=-0.010_–0.018^+0.024 (broken power-law emissivity profile) at a 90% confidence level, where the Kerr metric corresponds to α₁₃ = 0. We investigate the systematic uncertainties by fitting the data with different models.