Testing the Kerr Black Hole Hypothesis with GX 339-4 by a combined analysis of its thermal spectrum and reflection features

Understand

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 $\alpha_{13}$: $\alpha_{13} = -0.012_{-0.039}^{+0.011}$ (lamppost coronal geometry) and $\alpha_{13} = -0.010_{-0.018}^{+0.024}$ (broken power-law emissivity profile) at a 90% confidence level, where the Kerr metric corresponds to $\alpha_{13} = 0$.
• We investigate the systematic uncertainties by fitting the data with different models.